# Min size wire for a generac 20kw



## Salvatoreg02

Good morning all. Over the last several months many of us have installed many generators. Wire size for a 20kw has been the most popular. After reading many threads there seems to be a discrepancy on the correct wire size to use. I have stated that it's based on the output of the generator and also the type of fuel being used. The fuel plays a major factor simply because it then reduces or increase the generator output. 
IE. 20kw LP will produce 83.5amps
Where it's counter part natural gas will generator 18kw which will equate to 75amps maximum output. Using these numbers I will state that a 20kw will need a min size wire of #4cu or #2al
When natural gas is used the total output is 75amps where still number #4 can be used and now #3 alum. Based on table 310.16 
Additionally Generac could have chosen To use a 90amp OCPD but UL excepted installing a 100OCPD. 
220.40
225.5
240.4(b)
This will be very debatable. 
Please let's stick with topic. I know many of you are very intelligent. But, the code can be interpreted in many ways. 
Please do your HW properly. 
Rules no debating or quoting anyone, please state your facts and leave the forum.

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## McClary’s Electrical

Salvatoreg02 said:


> Good morning all. Over the last several months many of us have installed many generators. Wire size for a 20kw has been the most popular. After reading many threads there seems to be a discrepancy on the correct wire size to use. I have stated that it's based on the output of the generator and also the type of fuel being used. The fuel plays a major factor simply because it then reduces or increase the generator output.
> IE. 20kw LP will produce 83.5amps
> Where it's counter part natural gas will generator 18kw which will equate to 75amps maximum output. Using these numbers I will state that a 20kw will need a min size wire of #4cu or #2al
> When natural gas is used the total output is 75amps where still number #4 can be used and now #3 alum. Based on table 310.16
> Additionally Generac could have chosen To use a 90amp OCPD but UL excepted installing a 100OCPD.
> 220.40
> 225.5
> 240.4(b)
> This will be very debatable.
> Please let's stick with topic. I know many of you are very intelligent. But, the code can be interpreted in many ways.
> Please do your HW properly.
> Rules no debating or quoting anyone, please state your facts and leave the forum.
> 
> Sent from my iPhone using ET Forum


 

There is nothing debatable about it. You cannot change the wire size due to a fuel difference. The changeover from LP to gas takes very little time, and may be done at anytime in the future. The fuelt has absolutley nothing to do with sizing the wire.


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## Bulldog1

Salvatoreg02 said:


> Good morning all. Over the last several months many of us have installed many generators. Wire size for a 20kw has been the most popular. After reading many threads there seems to be a discrepancy on the correct wire size to use. I have stated that it's based on the output of the generator and also the type of fuel being used. The fuel plays a major factor simply because it then reduces or increase the generator output.
> IE. 20kw LP will produce 83.5amps
> Where it's counter part natural gas will generator 18kw which will equate to 75amps maximum output. Using these numbers I will state that a 20kw will need a min size wire of #4cu or #2al
> When natural gas is used the total output is 75amps where still number #4 can be used and now #3 alum. Based on table 310.16
> Additionally Generac could have chosen To use a 90amp OCPD but UL excepted installing a 100OCPD.
> 220.40
> 225.5
> 240.4(b)
> This will be very debatable.
> Please let's stick with topic. I know many of you are very intelligent. But, the code can be interpreted in many ways.
> Please do your HW properly.
> Rules no debating or quoting anyone, please state your facts and leave the forum.
> 
> Sent from my iPhone using ET Forum


I give up. Please post your company name and contact information as well as that inspector. I want a conference call with us you him and the state licensing board so they can straighten you two out.


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## Salvatoreg02

mcclary's electrical said:


> There is nothing debatable about it. You cannot change the wire size due to a fuel difference. The changeover from LP to gas takes very little time, and may be done at anytime in the future. The fuelt has absolutley nothing to do with sizing the wire.


I will make a exception this time but you don't follow rules very well. 
In some parts of the country NG is available to many home automatically via a gas conduit from your utility company. In my area many home don't have that option because it's not available. So LP IS THE ONLY OPTION. And yes it plays a critical part because LP BURNS HOTTER the NG which the generator can produce more power. Read.

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## Bulldog1

Salvatoreg02 said:


> I will make a exception this time but you don't follow rules very well.
> In some parts of the country NG is available to many home automatically via a gas conduit from your utility company. In my area many home don't have that option because it's not available. So LP IS THE ONLY OPTION. And yes it plays a critical part because LP BURNS HOTTER the NG which the generator can produce more power. Read.
> 
> Sent from my iPhone using ET Forum



You do know how many people are reading your posts right? I would logout and never come back. Even harry doesn't say crazy things like you have. Fuel type may increase output but It is regulated with either fuel type.


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## Salvatoreg02

Bulldog1 said:


> You do know how many people are reading your posts right? I would logout and never come back. Even harry doesn't say crazy things like you have. Fuel type may increase output but It is regulated with either fuel type.


What the hell are talking about!! Generac specifically states a 20kw model #5785 has an output of only 18kw when natural gas is used. Read the specs!!! 
LP on the other hand will produce a full 20kw.

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## McClary’s Electrical

Salvatoreg02 said:


> What the hell are talking about!! Generac specifically states a 20kw model #5785 has an output of only 18kw when natural gas is used. Read the specs!!!
> LP on the other hand will produce a full 20kw.
> 
> Sent from my iPhone using ET Forum


 

Duh, everybody knows that. You still can't size your wire by the fuel.


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## B4T

Just forget the fuel being used and run #3CU for any 20k genny.. problem solved..

I have never seen a FPN about conductor size and type of gas..


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## Salvatoreg02

mcclary's electrical said:


> Duh, everybody knows that. You still can't size your wire by the fuel.


In this instance there is no change in the copper size but with alum it will decrease the size to #3 alum. 
I'm simply stating that wire size doesn't have to be greater then #4 cu. 
Have you ever realized that the #2AWG doesn't not fit under the neutral and grd detail in the Generac 20kw generstors.

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## Pete m.

If your generator has a 100 amp OCPD the fuel type and actual generator output has nothing to do with wire sizing as previously stated in all manner of different ways.

Pete


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## Hairbone

I think I know this guy and why he gets all of the bids….. He sizes his wires like everything is a dwelling unit


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## B4T

Salvatoreg02 said:


> In this instance there is no change in the copper size but with alum it will decrease the size to #3 alum.
> *I'm simply stating that wire size doesn't have to be greater then #4 cu.*
> Have you ever realized that the #2AWG doesn't not fit under the neutral and grd detail in the Generac 20kw generstors.
> 
> Sent from my iPhone using ET Forum


The OCP is 100 amp.. #4CU. is rated @ 85 amp... I don't see how that works..


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## Salvatoreg02

Pete m. said:


> If your generator has a 100 amp OCPD the fuel type and actual generator output has nothing to do with wire sizing as previously stated in all manner of different ways.
> 
> Pete


Sure it does thats the nameplate rating of the generator. They could have easily installed a 90amp breaker.

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## Pete m.

Salvatoreg02 said:


> Sure it does thats the nameplate rating of the generator. They could have easily installed a 90amp breaker.


Is there a 90 amp breaker installed or a 100? 

Using your logic they "could have installed" a 2000 amp GFP BPS.

What matters is what breaker is actually there!

Pete


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## Salvatoreg02

Pete m. said:


> If your generator has a 100 amp OCPD the fuel type and actual generator output has nothing to do with wire sizing as previously stated in all manner of different ways.
> 
> Pete


So let's say that the generator has an output of 95amps and there was a wire size for 95amps. 240.4b allows us to use the next breaker size up. You capesci now!!!!

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## Salvatoreg02

Table 310.15b6 allows #4 copper for a 100amp service 
Table 310.16 allow us to use #4 copper for 83.5 amps the generator produces based on 240.4b and all the others references I have provided.

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## Pete m.

Salvatoreg02 said:


> So let's say that the generator has an output of 95amps and there was a wire size for 95amps. 240.4b allows us to use the next breaker size up. You capesci now!!!!


Thanks for the code lesson... 

I know that very well but you are maintaining that you are permitted, in some other dimension of reality that I am unfamiliar with, to size your wire to the actual output of the generator based on the fuel type being employed and ignore whatever OCPD size is actually installed at the generator.

Can you back that with a code section that I could use to approve such an install?

Pete


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## Salvatoreg02

Pete m. said:


> Thanks for the code lesson...
> 
> I know that very well but you are maintaining that you are permitted, in some other dimension of reality that I am unfamiliar with, to size your wire to the actual output of the generator based on the fuel type being employed and ignore whatever OCPD size is actually installed at the generator.
> 
> Can you back that with a code section that I could use to approve such an install?
> 
> Pete


Check my opening statement as I created this thread. All code references are there and left for interrogation.

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## Pete m.

Salvatoreg02 said:


> Table 310.15b6 allows #4 copper for a 100amp service
> Table 310.16 allow us to use #4 copper for 83.5 amps the generator produces based on 240.4b and all the others references I have provided.
> 
> Sent from my iPhone using ET Forum


What size service is installed at the house (assuming residential since you chose T310.15(B)(6) as a reference)?

If it is a 200 amp service then you better go do some more homework. Table 310.15(B)(6) is only applicable where ALL loads for a dwelling are involved.

Pete


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## Amish Electrician

One of the most basic rules of the trade is that you size your wires according to the size of the breaker. 

Sure, there are various rules for changing the breaker size for various special loads, but generator output isn't one of the ten listed 'exceptions.' You need that wire to be able to handle whatever the breaker can feed it.

Otherwise, we might as well be choosing circuit wire based on what size light bulb we're using.

As an electrician, all I care about is what comes out of the generator. I care not what is inside. Gas, diesel, gerbils ... doesn't matter, all I see is the rating of that breaker.

Keep in mind that the generator also has a 'surge' rating, where it can be called upon to deliver much more than the 'rated' power, though for a very short time before stalling out. You NEED the bigger wire.

As for "following the rules," the OP can go chase cows. He wants my opinion, he takes it on my terms. He can feel free to ignore it, but I won't let him dictate it.


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## Salvatoreg02

Amish Electrician said:


> One of the most basic rules of the trade is that you size your wires according to the size of the breaker.
> 
> Sure, there are various rules for changing the breaker size for various special loads, but generator output isn't one of the ten listed 'exceptions.' You need that wire to be able to handle whatever the breaker can feed it.
> 
> Otherwise, we might as well be choosing circuit wire based on what size light bulb we're using.
> 
> As an electrician, all I care about is what comes out of the generator. I care not what is inside. Gas, diesel, gerbils ... doesn't matter, all I see is the rating of that breaker.
> 
> Keep in mind that the generator also has a 'surge' rating, where it can be called upon to deliver much more than the 'rated' power, though for a very short time before stalling out. You NEED the bigger wire.
> 
> As for "following the rules," the OP can go chase cows. He wants my opinion, he takes it on my terms. He can feel free to ignore it, but I won't let him dictate it.


Okay, understood. So what size wire for 83.5 amps would you use to suffice bare min code. And the OCPD on the genny is 100

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## Salvatoreg02

Amish Electrician said:


> One of the most basic rules of the trade is that you size your wires according to the size of the breaker.
> 
> Sure, there are various rules for changing the breaker size for various special loads, but generator output isn't one of the ten listed 'exceptions.' You need that wire to be able to handle whatever the breaker can feed it.
> 
> Otherwise, we might as well be choosing circuit wire based on what size light bulb we're using.
> 
> As an electrician, all I care about is what comes out of the generator. I care not what is inside. Gas, diesel, gerbils ... doesn't matter, all I see is the rating of that breaker.
> 
> Keep in mind that the generator also has a 'surge' rating, where it can be called upon to deliver much more than the 'rated' power, though for a very short time before stalling out. You NEED the bigger wire.
> 
> As for "following the rules," the OP can go chase cows. He wants my opinion, he takes it on my terms. He can feel free to ignore it, but I won't let him dictate it.


Also, the surge rating of the generator only pertains to any motor loads that have LRA rating higher then the genny. The LRA on a 20kw Generac is 145 so all motors loads connected to it have to be less then 145. Wire size is not effected.

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## Pete m.

@ Salvatoreg02

What size service is at the dwelling?

Does your transfer switch put the ENTIRE load or only selected loads onto the generator?

Pete


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## Salvatoreg02

Pete m. said:


> @ Salvatoreg02
> 
> What size service is at the dwelling?
> 
> Does your transfer switch put the ENTIRE load or only selected loads onto the generator?
> 
> Pete


One of the two 209amp panels.

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## Pete m.

Salvatoreg02 said:


> One of the two 209amp panels.
> 
> Sent from my iPhone using ET Forum


Is this installation even at a dwelling (as defined by the NEC)?

Pete


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## Salvatoreg02

Pete m. said:


> Is this installation even at a dwelling (as defined by the NEC)?
> 
> Pete


Sure it's a 400amp service 8000 sqft home.

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## Pete m.

Salvatoreg02 said:


> Sure it's a 400amp service 8000 sqft home.
> 
> Sent from my iPhone using ET Forum


 
In my opinion if your generator does not feed the ENTIRE (400 amp) load T310.15(B)(6) cannot be utilized for sizing of your conductors.

That being said, if the generator has a 100 amp OCPD installed you must use a 3awg cu or 1 awg al feeder conductor assuming all termination points are rated for 75 degrees celcius.

Out of curiosity have you performed a load calculation for the 200 amp panel you are serving from the generator?

Pete


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## Salvatoreg02

Pete m. said:


> In my opinion if your generator does not feed the ENTIRE (400 amp) load T310.15(B)(6) cannot be utilized for sizing of your conductors.
> 
> That being said, if the generator has a 100 amp OCPD installed you must use a 3awg cu or 1 awg al feeder conductor assuming all termination points are rated for 75 degrees celcius.
> 
> Out of curiosity have you performed a load calculation for the 200 amp panel you are serving from the generator?
> 
> Pete


I know where your going with this, it's CT still in the 05'

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## joethemechanic

Salvatoreg02 said:


> LP BURNS HOTTER the NG which the generator can produce more power



Shall I pick it apart from a combustion standpoint?

NG and propane have almost identical combustion tempatures. BTU content is different.

And even though you are derating the engine by burning NG, you are not re-certifying the generator and changing the OPD


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## Salvatoreg02

@pete why do keep referring to table 310.15b6 u can't use that table to size generator wire size

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## Salvatoreg02

joethemechanic said:


> Shall I pick it apart from a combustion standpoint?
> 
> NG and propane have almost identical combustion tempatures. BTU content is different.
> 
> And even though you are derating the engine by burning NG, you are not re-certifying the generator and changing the OPD


The manufacture states that the output will be less when using natural gas. In this case then if someone install NG then the output is 75 amps and the OCOD is not sized properly. Manufacture mistake???

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## joethemechanic

Salvatoreg02 said:


> The manufacture states that the output will be less when using natural gas. In this case then if someone install NG then the output is 75 amps and the OCOD is not sized properly. Manufacture mistake???
> 
> Sent from my iPhone using ET Forum


If the overload at the generator is 75 amps, then you have to size conductors for 75 amps in order to comply with the NEC.

And don't forget, most of those machines are rated 0.8 PF so KW doesn't mean everything.

NEC could be concerned less about the brake HP of your prime mover


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## Bulldog1

Salvatoreg02 said:


> @pete why do keep referring to table 310.15b6 u can't use that table to size generator wire size
> 
> Sent from my iPhone using ET Forum



If the generator backs up all of the load you can use that table.


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## joethemechanic

18 KW by 0.8 PF 240 V = 93.75 amps (single phase)

20 KW by 0.8 PF 240V = 104.17 amps (single Phase)


20,000 / 240 X 1.25 = 104.1666666666

But even with a less powerful prime mover and a worse powerfactor (of loads) you can still overload your windings / feeders whatever. You won't be making the KW, but your VA can be beyond what everything was designed for.

If your loads have a 0.6 PF and you are pushing 20 KW at the gen set

20,000 / 240 X 1.67 = 139 amps single phase

And that is with no increase in engine power over the LPG rating


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## Salvatoreg02

Bulldog1 said:


> If the generator backs up all of the load you can use that table.


Sorry to say this but you can't use that table to size the conductors from the gen to the ATS, the service conductors yes.

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## Bulldog1

Salvatoreg02 said:


> Sorry to say this but you can't use that table to size the conductors from the gen to the ATS, the service conductors yes.
> 
> Sent from my iPhone using ET Forum



Yes you can if the generator transfers the whole load.


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## Pete m.

Salvatoreg02 said:


> Table 310.15b6 allows #4 copper for a 100amp service


I am not using T310.15(B)(6)... you keep referring to it.

Pete


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## Pete m.

Bulldog1 said:


> Yes you can if the generator transfers the whole load.


I agree.

Salvatoreg02... read the actual text of 310.15(B)(6) not just the table. 

Pete


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## Amish Electrician

Easy ... #2 Copper. Only because the supply house doesn't stock #3. 75 degree column because of the connections.


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## Pete m.

Salvatoreg02 said:


> I know where your going with this, it's CT still in the 05'


I'm not sure you know where I'm going or the direction I'm trying to point you in either.

Perhaps I am communicating poorly or you are. 

Pete


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## Bulldog1

Amish Electrician said:


> Easy ... #2 Copper. Only because the supply house doesn't stock #3. 75 degree column because of the connections.



You need a better supply house. :laughing:


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## Bulldog1

Pete m. said:


> I'm not sure you know where I'm going or the direction I'm trying to point you in either.
> 
> Perhaps I am communicating poorly or you are.
> 
> Pete



The problem is he thinks he is correct. It appears nothing is going to change that.


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## Pete m.

Bulldog1 said:


> The problem is he thinks he is correct. It appears nothing is going to change that.


I get that feeling. 

I was only hoping that if he could grasp the concept he may be able to educate this inspector he keeps referring to because that same inspector sees and approves more installs than just his.

Pete


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## Salvatoreg02

Pete m. said:


> I agree.
> 
> Salvatoreg02... read the actual text of 310.15(B)(6) not just the table.
> 
> Pete


So if I agree with you then using #4copper is execrable like I first stated.

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## Bulldog1

Salvatoreg02 said:


> So if I agree with you then using #4copper is execrable like I first stated.
> 
> Sent from my iPhone using ET Forum



Only if the generator carries the whole load of the dwelling.


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## Pete m.

Salvatoreg02 said:


> So if I agree with you then using #4copper is execrable like I first stated.


If the 4 awg cu is connected to a 100 amp breaker at the generator then yes.


The largest breaker that the 4 awg cu conductor could be connected to is 90 amp. This is only if the calculated load on the 4 awg is 85 or less. If it is then you could apply 240.4(B) and round up to the next standard size found in 240.6

Pete


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## Bulldog1

Pete m. said:


> If the 4 awg cu is connected to a 100 amp breaker at the generator then yes.
> 
> 
> The largest breaker that the 4 awg cu conductor could be connected to is 90 amp. This is only if the calculated load on the 4 awg is 85 or less. If it is then you could apply 240.4(B) and round up to the next standard size found in 240.6
> 
> Pete



You can only use #4 to the generator when the whole load is transferred.


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## Salvatoreg02

Bulldog1 said:


> You can only use #4 to the generator when the whole load is transferred.


Code reference please?

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## Greg

First of let me state that I am a Generac Factory tech #434908. Do the math 83.5A x 125%(continuous load) = 104A, hence the 100A breaker on the unit. The breaker was sized for the rated output of the generator not the engine.

As everyone else has stated, you size the wire to the breaker size, 100A. Since you are at 100A you can use the 75C column if it was less than 100A use the 60C column, there are some exceptions. 

As for the reduced capacity depending on the fuel, it comes down to horsepower. The reduced horsepower means it cannot spin the generator at full load. At full load you have to think about counter EMF that the generator produces which in turn creates counter torque. The prime mover, in this case the engine, has to be able to overcome this effect. The fuel didn't change the maximum the generator can put out electrically, it changed the amount of horsepower the engine can produce.


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## dronai

And everyone continues to help this guy ?


Here was his question last week

"Converting KVA formula is KVA divided by voltage or a different formula. Single phase and 3 phase????"

Your profile says your a Master Electrician, but if I read all of your past threads, you are always asking basic how to questions !

This means you are a fake, and taking work from legit contractors.


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## McClary’s Electrical

Salvatoreg02 said:


> The manufacture states that the output will be less when using natural gas. In this case then if someone install NG then the output is 75 amps and the OCOD is not sized properly. Manufacture mistake???
> 
> Sent from my iPhone using ET Forum


So you think part of the conversion between lp or gas should be changing the main breaker?


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## B4T

Greg said:


> As everyone else has stated, you size the wire to the breaker size, 100A. Since you are at 100A you can use the 75C column if it was less than 100A* use the 60C column, there are some exceptions. *


I'm not getting this part.. why would you use a lower column.. :blink:


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## BBQ

B4T said:


> I'm not getting this part.. why would you use a lower column.. :blink:


Well .........

If it is NM you have to

If it is SE / SER you might have to

If the terminals are not marked 75 C you might have to


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## B4T

BBQ said:


> Well .........
> 
> If it is NM you have to
> 
> If it is SE / SER you might have to
> 
> If the terminals are not marked 75 C you might have to


Greg mentioned.. "less than 100A".. nothing about wiring method for the change in columns..


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## nrp3

#3cu into the house in pipe, #1 Al mc because I can't stay out of the insulation. 30 space subpanel for loads, 100a switch. A little conservative, but oh well.


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## Greg

B4T said:


> I'm not getting this part.. why would you use a lower column.. :blink:


2008 NEC 110.14 (C)(1)(a) and (b) Same in article in 2005 NEC


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## Salvatoreg02

mcclary's electrical said:


> So you think part of the conversion between lp or gas should be changing the main breaker?


Possibly, think about it. The next standard size breaker is 80.

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## Salvatoreg02

Greg said:


> First of let me state that I am a Generac Factory tech #434908. Do the math 83.5A x 125%(continuous load) = 104A, hence the 100A breaker on the unit. The breaker was sized for the rated output of the generator not the engine.
> 
> As everyone else has stated, you size the wire to the breaker size, 100A. Since you are at 100A you can use the 75C column if it was less than 100A use the 60C column, there are some exceptions.
> 
> As for the reduced capacity depending on the fuel, it comes down to horsepower. The reduced horsepower means it cannot spin the generator at full load. At full load you have to think about counter EMF that the generator produces which in turn creates counter torque. The prime mover, in this case the engine, has to be able to overcome this effect. The fuel didn't change the maximum the generator can put out electrically, it changed the amount of horsepower the engine can produce.


Paragraph 1 wrong
Paragraph 2 wrong
Paragraph 3 wrong 
So why does Generac call there model #
5784 20/18kw???

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## Greg

Salvatoreg02 said:


> Paragraph 1 wrong
> Paragraph 2 wrong
> Paragraph 3 wrong
> So why does Generac call there model #
> 5784 20/18kw???
> 
> Sent from my iPhone using ET Forum


You sir are wrong. Read up on some generator theory and a code article I posted earlier.


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## Bulldog1

Salvatoreg02 said:


> Possibly, think about it. The next standard size breaker is 80.
> 
> Sent from my iPhone using ET Forum



The generator is UL listed. You can't modify it.......:no: If the instructions allow you to change the 100A breaker then you could do that.

A true professional would want to learn to do things correctly. Not try to argue with facts.


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## brian john

Bulldog1 said:


> The generator is UL listed. You can't modify it.......:no:
> 
> A true professional would want to learn to do things correctly. Not try to argue with facts.


Sal is either incredibly stupid.

Or

Sal likes to argue even when he knows durn well he is wrong

Or 

Sal is stupid and likes to argue.

But you have to admit his post keep the thread moving and us thinking.


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## Salvatoreg02

Bulldog1 said:


> The generator is UL listed. You can't modify it.......:no:
> 
> A true professional would want to learn to do things correctly. Not try to argue with facts.


I'm not arguing the facts. I understand it's UL. Who said I was doing things incorrectly. I wanna do things to bare min code and never exceed that expectation unless it's speced to a higher standard. 

Everyone in this conversation has stated there opinion and it's possible their right as well as myself. I have never had one generator install fail inspection to date. 

Every install I have done I have met with building inspectors for placement of generator location because we have many issues with wetlands. I have consulted with electrical inspectors on what they wound allow and not. 

We just finished up installing a 60kw generator that had a max output of 250 amps and the generator breaker was 300amps we ran 250mcm. It passed inspection. Do you think this last install is in violation???!

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## brian john

Is it possible you could take some time and read the NEC?

From an article on EC&M web

http://ecmweb.com/mag/electric_using_overcurrent_protection/

What does the NEC actually require in the way of overcurrent protection for conductors supplied by a generator?

One of the more confusing areas of the Code involves the what, where, and when of overcurrent protective devices protecting conductors originating at generator terminals. We recently received a question as to whether any over-current protection was required at all for these conductors, and if so, where it had to be located. The questioner noted that some generators, especially those with series-boost systems, can supply very large ground-fault or short-circuit currents. This question is not as simple as it looks.

The EC&M Panel's response

The place to begin is Sec. 240-3, *which requires conductors to be "protected against overcurrent in accordance with their ampacities as specified in Sec. 310-15, unless otherwise permitted or required in (a) through (m) below.*" In the common case where there is no such protection at the generator, this main rule cannot be complied with, and one of the subsections that follow must be applied. The reason for this is that overcurrent protection involves not just overload protection, which can be provided at the load end of a conductor, but short-circuit and ground-fault protection as well. This type of protection must always be ahead of a conductor.


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## Bulldog1

Salvatoreg02 said:


> I'm not arguing the facts. I understand it's UL. Who said I was doing things incorrectly. I wanna do things to bare min code and never exceed that expectation unless it's speced to a higher standard.
> 
> *Everyone in this conversation has stated there opinion *and it's possible their right as well as myself. I have never had one generator install fail inspection to date.
> 
> Every install I have done I have met with building inspectors for placement of generator location because we have many issues with wetlands. I have consulted with electrical inspectors on what they wound allow and not.
> 
> We just finished up installing a 60kw generator that had a max output of 250 amps and the generator breaker was 300amps we ran 250mcm. It passed inspection. Do you think this last install is in violation???!
> 
> Sent from my iPhone using ET Forum





:no:



No everyone has stated what the code actually says and allows. You and your inspector have stated opinions. We have the NEC to back us up you and him have nothing to back you up. From your posts we know you DO NOT do code compliant installs. As far as your 60kw not enough information to say if it was a code compliant installation. Keep in mind as I stated earlier just because your moron inspector passed it does not make it code compliant.


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## brian john

Bulldog1 said:


> :no:*No everyone has stated what the code actually says and allows. You and your inspector have stated opinions.* We have the NEC to back us up you and him have nothing to back you up. From your posts we know you DO NOT do code compliant installs. As far as your 60kw not enough information to say if it was a code compliant installation. Keep in mind as I stated earlier just because your moron inspector passed it does not make it code compliant.


And all but one person seems to be in agreement with the stated facts (supported by the NEC, a manufactures rep, licensed masters, and an electrical inspector).


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## Salvatoreg02

Bulldog1 said:


> :no:
> 
> No everyone has stated what the code actually says and allows. You and your inspector have stated opinions. We have the NEC to back us up you and him have nothing to back you up. From your posts we know you DO NOT do code compliant installs. As far as your 60kw not enough information to say if it was a code compliant installation. Keep in mind as I stated earlier just because your moron inspector passed it does not make it code compliant.


See, bulldog I gave you all the info what more do you need. I said the generator produced 250amps, 250mcm is rated for 255, we are allowed to use the next size breaker up which is 300amps which the manufacturer installed at the genny. I believe section 240.6 and 240.4b applies.

Sent from my iPhone using ET Forum


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## Salvatoreg02

brian john said:


> Sal is either incredibly stupid.
> 
> Or
> 
> Sal likes to argue even when he knows durn well he is wrong
> 
> Or
> 
> Sal is stupid and likes to argue.
> 
> But you have to admit his post keep the thread moving and us thinking.


Hehehe!!!

Sent from my iPhone using ET Forum


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## Bulldog1

Salvatoreg02 said:


> See, bulldog I gave you all the info what more do you need. I said the generator produced 250amps, 250mcm is rated for 255, we are allowed to use the next size breaker up which is 300amps which the manufacturer installed at the genny. I believe section 240.6 and 240.4b applies.
> 
> Sent from my iPhone using ET Forum




I see no problem with your conductor size on that one but that doesn't mean the rest of your install is code compliant.


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## Salvatoreg02

Merry Christmas to all and to all a good night!!
Final conclusion. 
If table 310.15(b)(6) is permitted then #4 cu is Accetable?

Sent from my iPhone using ET Forum


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## Bulldog1

Salvatoreg02 said:


> Merry Christmas to all and to all a good night!!
> Final conclusion.
> If table 310.15(b)(6) is permitted then #4 cu is Accetable?
> 
> Sent from my iPhone using ET Forum



If you meet all the requirements of 310.15(b)(6) which would include transferring the complete service yes you can use #4 Copper from your generator to the panel. Merry Christmas to you too.


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## BBQ

Salvatoreg02 said:


> *If* table 310.15(b)(6) is permitted then #4 cu is Accetable?


Yes.


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## brian john

Salvatoreg02 said:


> Hehehe!!!
> 
> Sent from my iPhone using ET Forum


Sal is jerking our chains.........


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## Bulldog1

brian john said:


> Sal is jerking our chains.........



I get the feeling he isn't but I could be wrong....


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## BBQ

brian john said:


> Sal is jerking our chains.........


He could be, it would not be a first around here.

I am actually a 68 year old Walmart greeter, never a day in the electrical field.


But I think Sal might just be as screwed up as he seems.


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## Bulldog1

BBQ said:


> He could be, it would not be a first around here.
> 
> I am actually a 68 year old* POTHEAD* Walmart greeter, never a day in the electrical field.
> 
> 
> But I think Sal might just be as screwed up as he seems.



So you really are Cletis......:whistling2: :laughing:

And I fixed it for you.....


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## BBQ

Bulldog1 said:


> So you really are Cletis......:whistling2: :laughing:
> 
> And I fixed it for you.....


It is hard being a pothead on a greeters salary. :jester:


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## Bulldog1

BBQ said:


> It is hard being a pothead on a greeters salary. :jester:



Not when you grow you own......:whistling2:

Now we know why John asked about those special MH bulbs....


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## Salvatoreg02

Bulldog1 said:


> I see no problem with your conductor size on that one but that doesn't mean the rest of your install is code compliant.


Great!! So when I said the generator output of a 20kw was 83.5 amps and I used #4 copper, even though the next size OCPD is 90 and the generator was 100, I was wrong? honestly speaking now..also, the generator only Backed up one panel of the 2 200amp panels. I'm not trying to be controversial. And I didn't use table 310.15(b)(6). I used 310.16
Your honesty is not shunned upon.

Sent from my iPhone using ET Forum


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## Bulldog1

Salvatoreg02 said:


> Great!! So when I said the generator output of a 20kw was 83.5 amps and I used #4 copper, even though the next size OCPD is 90 and the generator was 100, I was wrong? honestly speaking now..also, the generator only Backed up one panel of the 2 200amp panels. I'm not trying to be controversial. And I didn't use table 310.15(b)(6). I used 310.16
> Your honesty is not shunned upon.
> 
> Sent from my iPhone using ET Forum



Sal forget the generator output. Look at the breaker installed and base your wire size off that. 

If you are transferring the whole house( as in every circuit ) you are allowed to use 310.15(b)(6) which allows #4 copper

If you are not transferring the whole load you must use 310.16 which requires #3 copper.

It is that simple. Don't make it so hard.


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## BBQ

Bulldog1 said:


> If you are transferring the whole house( as in every circuit ) you are allowed to use 310.15(b)(6) which allows #4 copper
> 
> If you are not transferring the whole load you must use 310.16 which requires #3 copper.
> 
> It is that simple. Don't make it so hard.


I agree, it is that simple.


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## Salvatoreg02

Bulldog1 said:


> Sal forget the generator output. Look at the breaker installed and base your wire size off that.
> 
> If you are transferring the whole house( as in every circuit ) you are allowed to use 310.15(b)(6) which allows #4 copper
> 
> If you are not transferring the whole load you must use 310.16 which requires #3 copper.
> 
> It is that simple. Don't make it so hard.


Bulldog you just contradicted yourself. In the previous thread I said the output of a 60kw was 250 and I sized the wire for 250 MCM. And the breaker was 300. So, your now saying don't base it on the output.

Sent from my iPhone using ET Forum


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## Greg

Salvatoreg02 said:


> Great!! So when I said the generator output of a 20kw was 83.5 amps and I used #4 copper, *even though the next size OCPD is 90* and the generator was 100, I was wrong? honestly speaking now..also, the generator only Backed up one panel of the 2 200amp panels. I'm not trying to be controversial. And I didn't use table 310.15(b)(6). I used 310.16
> Your honesty is not shunned upon.
> 
> Sent from my iPhone using ET Forum


The 90A OCPD doesn't mean anything the gen has a 100A OCPD because of continous loading, i.e. running more than 3hours, which they will do.


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## Salvatoreg02

Greg said:


> The 90A OCPD doesn't mean anything the gen has a 100A OCPD because of continous loading, i.e. running more than 3hours, which they will do.


It's for surges not continuos loads.

Sent from my iPhone using ET Forum


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## BBQ

Salvatoreg02 said:


> It's for surges not continuos loads.


:laughing::no:

It is for continuous loads.

If they provided a breaker that actually matched the generators rating you could not use the generator to it's full capacity with continuous loads.


(BTW the instantaneous trip is about the same between a 80 and 100 so both handle inrush currents the same)


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## Salvatoreg02

Welcome!!

Sent from my iPhone using ET Forum


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## Bulldog1

Salvatoreg02 said:


> Bulldog you just contradicted yourself. In the previous thread I said the output of a 60kw was 250 and I sized the wire for 250 MCM. And the breaker was 300. So, your now saying don't base it on the output.
> 
> Sent from my iPhone using ET Forum



The NEC is full of contradictions. You listed the article so you know that is allowed. You could base your conductors off the 300 amp breaker but there is no need to do that. If you are going to base your calculation off the generator output add 125% and you will see the OCPD required. As Greg stated a 20kw has 100A OCPD for a reason.


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## mrcastrovinci

Greg said:


> The 90A OCPD doesn't mean anything the gen has a 100A OCPD because of continous loading, i.e. running more than 3hours, which they will do.


Pretty logical.....if you ran the genset at full capacity (say approx 80 amps) you would need the 125% which is the rule/reciprical of no more then 80% on a breaker

I always sized them in the past to the normal table and not the service entrance table, but after reading it, its pretty clear. Good savings in wire and conduit.:thumbup::clap::drink:


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## Speedy Petey

mrcastrovinci said:


> you would need the 125% which is the *rule/reciprical of no more then 80% on a breaker*


WHY is this myth so hard to kill???

Please tell me where to find this _"rule/reciprical of no more then 80% on a breaker"_.PLEASE.


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## mrcastrovinci

Speedy Petey said:


> WHY is this myth so hard to kill???
> 
> Please tell me where to find this _"rule/reciprical of no more then 80% on a breaker"_.PLEASE.


You would have to do the math....... Whats 1/80??

.0125 Which is 125%. Hence why you do continous loads that way. The math will never lie!


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## mrcastrovinci

mrcastrovinci said:


> You would have to do the math....... Whats 1/80??
> 
> .0125 Which is 125%. Hence why you do continous loads that way. The math will never lie!


.......And barring the breaker is not listed for 100% use.


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## hvacman

Woldn't it be much easier instead of arguing back and forth to just settle for the #4 from meter head to smart switch and also from smart switch to breaker box? Then 100 AMP service wire from the smart switch to the generator. Some of you are right on not having to size your wire to the fuel type...BUT...the gentlemen is right about LP burning hotter, and producing the total 20KW! Either way, the #4 copper will cover the load.


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## RICK BOYD

"One of the most basic rules of the trade is that you size your wires according to the size of the breaker. " I agree with this


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## YourBestFriend

RICK BOYD said:


> "One of the most basic rules of the trade is that you size your wires according to the size of the breaker. " I agree with this


That's not really true and we all know it. 

It's "basic" in that many people who don't know the code well follow it.


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## BBQ

RICK BOYD said:


> "One of the most basic rules of the trade is that you size your wires according to the size of the breaker. " I agree with this


But that is simply not true for almost anything except 15, 20 and 30 amp general purpose branch circuits.


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## YourBestFriend

BBQ said:


> But that is simply not true for almost anything except 15, 20 and 30 amp general purpose branch circuits.


I beat you Bob.


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## bsmars

*Wire Size needed for 20kw Generac*

There seems to be a lot of confusion in regards to the correct wire size needed to run from the ATS to a 20kw generator. The breaker in the generator is now a 90amp breaker (they used to come with 100 amp). I believe the correct wire size should be sized to the breaker (90A) and not the output of the generator (83A), therefore requiring a size of #3Cu or #2Al. The problem is the neutral bar in the Generator barely fits a #4Cu. I was also told by an authorized Generac Service Tech that #4Cu is sufficient. Can anyone clarify?


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## AmpsHertz

YourBestFriend said:


> I beat you Bob.


You're not only my best friend, you're my only friend.


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## RICK BOYD

why would this be wrong...article *310.15 Conductor Ampacity...

*That's not really true and we all know it. 

It's "basic" in that many people who don't know the code well follow it.[/quote]


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## 123electric

In conduit or mc cable use the 75c table for 310-16.


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## Meanie1500

We have started to use #2 AL XHHW for feeders and #4 AL XHHW for neutral and ground. We were using #2 SER but one of the inspectors turned us down. The reasoning is because he said that SEU and SER are only rated for 60c so #2 SER is not big enough. But #2 XHHW is fine when using the 75c column. It's all in how you interpret the code, or really how the inspector interprets the code.


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## RICK BOYD

inspectors turned us down. The reasoning is because he said that SEU and SER are only rated for 60c 
It's all in how you interpret the code, or really how the inspector interprets the code.

334.80
says nm/nmc/nms shall use only 60 degree chart but doesn't say about SER ,,, but cable, not single conductor in conduit is the reason for the derating I guess


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## Meanie1500

It's really weird since everyone has jumped on the generator bandwagon I don't think the inspectors have been able to keep up let alone interpret the code properly. We got turned down on an electrical inspection because the generator wasn't in a gravel bed or two slabs level on the ground. There was no way did up to do that so we dug down and also used lintels to level the slabs and had failed us. His supervisor had a talk with him ANC generac wasn't opposed to what we did.


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## Pete m.

Meanie1500 said:


> It's really weird since everyone has jumped on the generator bandwagon I don't think the inspectors have been able to keep up let alone interpret the code properly. We got turned down on an electrical inspection because the generator wasn't in a gravel bed or two slabs level on the ground. There was no way did up to do that so we dug down and also used lintels to level the slabs and had failed us. His supervisor had a talk with him ANC generac wasn't opposed to what we did.


What code book is your inspector using?

I looked through the entire NEC and couldn't find a requirement for gravel or concrete or lintels....

Pete


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## Bulldog

In some areas the inspetors overlap on inspection. NJ the building inspector is allowed to make sure the ufer ground is installed. The plumbing inspector can check for the bond wire on the hot water heater. 
So when the generator install does not need a building inspector, the other inspectors ie plumbing, electrical may have the authority to inspect that the generator was installed according to mfg specs.
Generac states the following 
The need for a concrete pad is dictated by your local building codes. If your local code authority does not require it, the generator can be placed on pea gravel using the included composite mounting pad. If required, then consult your local building codes to see if it must be anchored. 
If the local code requires the generator to be mounted or secured a certain way, than the electrical and or plumbing inspector may have authority to fail a noncompliant install.


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## Meanie1500

Pete m. said:


> What code book is your inspector using?
> 
> I looked through the entire NEC and couldn't find a requirement for gravel or concrete or lintels....
> 
> Pete


It's in the generator installation manual.


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## Meanie1500

Bulldog said:


> In some areas the inspetors overlap on inspection. NJ the building inspector is allowed to make sure the ufer ground is installed. The plumbing inspector can check for the bond wire on the hot water heater.
> So when the generator install does not need a building inspector, the other inspectors ie plumbing, electrical may have the authority to inspect that the generator was installed according to mfg specs.
> Generac states the following
> The need for a concrete pad is dictated by your local building codes. If your local code authority does not require it, the generator can be placed on pea gravel using the included composite mounting pad. If required, then consult your local building codes to see if it must be anchored.
> If the local code requires the generator to be mounted or secured a certain way, than the electrical and or plumbing inspector may have authority to fail a noncompliant install.


We always set them on concrete slabs because it looks better plus if it settles it will settle evenly. He was going by the the mfg installation guide.


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## Bulldog

I do also.


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## electricalwiz

Bulldog said:


> In some areas the inspetors overlap on inspection. NJ the building inspector is allowed to make sure the ufer ground is installed. The plumbing inspector can check for the bond wire on the hot water heater.
> So when the generator install does not need a building inspector, the other inspectors ie plumbing, electrical may have the authority to inspect that the generator was installed according to mfg specs.
> Generac states the following
> The need for a concrete pad is dictated by your local building codes. If your local code authority does not require it, the generator can be placed on pea gravel using the included composite mounting pad. If required, then consult your local building codes to see if it must be anchored.
> If the local code requires the generator to be mounted or secured a certain way, than the electrical and or plumbing inspector may have authority to fail a noncompliant install.


what ufer ground are you installing on a generator pad


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## Bulldog

It is not on the generator pad. My point was that when a ufer ground is installed the building inspector can inspect that it was installed during the foundation inspection.


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## Salvatoreg02

Greg said:


> The 90A OCPD doesn't mean anything the gen has a 100A OCPD because of continous loading, i.e. running more than 3hours, which they will do.


So, please tell me, why did Generac change the 100OCPD to 90OCPD?


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## therain

Did anyone look at art.445.13 I thought that was how you size conductors what am I missing ?:


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## Inspectorgadget

*Correct wire sizing for a generator*

How easily we digress. The original question was how to determine the correct wire size to be installed between a 20KW Generac and the transfer switch. If you read NEC 445.13 you will see that the conductors must have an ampacity of 115% of the nameplate rating of the generator. (Generac 20KW does not meet the requirements of the exception.)  The nameplate is 83.3A therefore the wire must have a minimum ampacity of 96A. Read the following carefully. The ampacity of a conductor can be limited by the ratings of the terminations 110.14(C) The circuit breaker in the generator has terminals that are rated 60/75degreeC and so become a determining factor in conductor size. You must use the 75degree column in table 310.16 even if you are installing 90 degree rated conductors. The correct size wire is #3Cu or#1Al as a minimum. This is a strict interpretation of what the National Electrical Code requires. As we all know, the Code is not always enforced. If you disagree with this interpretation, I welcome your comments. Please cite relevant Code references to back up your interpretations .


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## McClary’s Electrical

Inspectorgadget said:


> How easily we digress. The original question was how to determine the correct wire size to be installed between a 20KW Generac and the transfer switch. If you read NEC 445.13 you will see that the conductors must have an ampacity of 115% of the nameplate rating of the generator. (Generac 20KW does not meet the requirements of the exception.) The nameplate is 83.3A therefore the wire must have a minimum ampacity of 96A. Read the following carefully. The ampacity of a conductor can be limited by the ratings of the terminations 110.14(C) The circuit breaker in the generator has terminals that are rated 60/75degreeC and so become a determining factor in conductor size. You must use the 75degree column in table 310.16 even if you are installing 90 degree rated conductors. The correct size wire is #3Cu or#1Al as a minimum. This is a strict interpretation of what the National Electrical Code requires. As we all know, the Code is not always enforced. If you disagree with this interpretation, I welcome your comments. Please cite relevant Code references to back up your interpretations .


You only need 115% wire between the generator and the first breaker. The breaker is on the machine, so you can use 100% value and redo all you figures, not 115


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## McClary’s Electrical

Inspectorgadget said:


> How easily we digress. The original question was how to determine the correct wire size to be installed between a 20KW Generac and the transfer switch. If you read NEC 445.13 you will see that the conductors must have an ampacity of 115% of the nameplate rating of the generator. (Generac 20KW does not meet the requirements of the exception.) The nameplate is 83.3A therefore the wire must have a minimum ampacity of 96A. Read the following carefully. The ampacity of a conductor can be limited by the ratings of the terminations 110.14(C) The circuit breaker in the generator has terminals that are rated 60/75degreeC and so become a determining factor in conductor size. You must use the 75degree column in table 310.16 even if you are installing 90 degree rated conductors. The correct size wire is #3Cu or#1Al as a minimum. This is a strict interpretation of what the National Electrical Code requires. As we all know, the Code is not always enforced. If you disagree with this interpretation, I welcome your comments. Please cite relevant Code references to back up your interpretations .


Read the first sentence of 445.13 very closely, then read it again. Then come back with your adjusted figures and tell us how you are wrong. 

#2 al is fine on a20 kw.


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