# Service Entrance Conductor Size



## Wiresmith (Feb 9, 2013)

is that with minimum 75 degree wire and terminations?


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## Paul S. (Sep 8, 2012)

Wiresmith said:


> is that with minimum 75 degree wire and terminations?


Yes, 4/O aluminum (usually URD/triplex/sweetbriar) is often used for residential 200A service here. Now I am wondering if the wire used is listed in the 90 degree column of our ampacity table, and, if that makes a difference here in how those two codes apply.


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## matt1124 (Aug 23, 2011)

They used to publish a table, now it’s math. Just go by the table for a dwelling.


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## Wiresmith (Feb 9, 2013)

read 240.4 (B) feeders and services unlike branches you can round up a ocpd size, i don't interpret 310.15(B(7) the same way as most people on here even though 240.4(B) got you all the same numbers on the old table except the last one. im going to give you another reference when i find it in a minute to consider though


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## Wiresmith (Feb 9, 2013)

110.13(C) you can't run your terminations hotter than what there rated (in my opinion 310.15(B)(7) does not absolve you of this, others will likely disagree with me on this)


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## Wiresmith (Feb 9, 2013)

i gotta go right now, i'll check back in on the posts that say i'm wrong later


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## Wiresmith (Feb 9, 2013)

another thing i forgot is read 310.15(A)(3), i'll post my argument why your not allowed to run a wire for an expected load higher than 310.15 tables later, i think your allowed to round up an ocpd but your still not allowed to use a wire knowing the intended load is over the table allowance


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## Paul S. (Sep 8, 2012)

Wiresmith said:


> read 240.4 (B) feeders and services unlike branches you can round up a ocpd size, i don't interpret 310.15(B(7) the same way as most people on here even though 240.4(B) got you all the same numbers on the old table except the last one. im going to give you another reference when i find it in a minute to consider though


240.4(B) refers to the OCPD. If the calculated load is 195, 4/O aluminum (using the 75 degree column) is only rated for 175. Maybe I am not following your logic here but I do not see how that answers my real question. Please do not interpret my reply here as being snarky or rude either, I really do not mean to be. I definitely appreciate your input.


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## Paul S. (Sep 8, 2012)

Wiresmith said:


> another thing i forgot is read 310.15(A)(3), i'll post my argument why your not allowed to run a wire for an expected load higher than 310.15 tables later, i think your allowed to round up an ocpd but your still not allowed to use a wire knowing the intended load is over the table allowance


I think I agree with you on this.


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## Paul S. (Sep 8, 2012)

Truthfully, I am not sure that my terminations are 75 degree. I am really just assuming they are. I have to look at the listing on the equipment out in my truck.


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## cabletie (Feb 12, 2011)

If this was a test question I would say you are good to go with 4/0. Even though they pulled the table from the section, it's still in the annex under the examples. 



In the real world you would probably want to go with a bigger service.


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## Dennis Alwon (May 9, 2009)

110.14 is the general rule but the 83% is the exception to the rule. Remember the calculated load is not the real load that the house will see. I would bet the home will never see anywhere near 180 amps anyway and if it did then it would not be for long. 

If you feel that is an issue then, by all means, use 250 kcm but IMO, 4/0 is sufficent for a residence


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## Paul S. (Sep 8, 2012)

Dennis Alwon said:


> 110.14 is the general rule but the 83% is the exception to the rule. Remember the calculated load is not the real load that the house will see. I would bet the home will never see anywhere near 180 amps anyway and if it did then it would not be for long.
> 
> If you feel that is an issue then, by all means, use 250 kcm but IMO, 4/0 is sufficent for a residence


I appreciate your input. The reason for my question is because I sometimes run into situations where I think it is questionable. Not too long ago, I went for an estimate; the guy had a 200-amp service on an approx 3500 square foot home (if I remember correctly), two pool pumps, maybe a pool heater, two heat pump units, and he may have had all electric heat and appliances. He also had an outbuilding or two, which to me seems like quite a bit on a 200-amp service. I told him I wanted to do a load calculation before I did any work because it looked to me like he had so much stuff on his existing service. What would you say in a situation like this? I would love to know better how to handle this type of situation.


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## Dennis Alwon (May 9, 2009)

Paul S. said:


> I appreciate your input. The reason for my question is because I sometimes run into situations where I think it is questionable. Not too long ago, I went for an estimate; the guy had a 200-amp service on an approx 3500 square foot home (if I remember correctly), two pool pumps, maybe a pool heater, two heat pump units, and he may have had all electric heat and appliances. He also had an outbuilding or two, which to me seems like quite a bit on a 200-amp service. I told him I wanted to do a load calculation before I did any work because it looked to me like he had so much stuff on his existing service. What would you say in a situation like this? I would love to know better how to handle this type of situation.


I think you handled it fine. Suggest a load calculation before you go any further. If you are just adding a receptacle I wouldn't worry but if they wanted something like a car charge or hot tub I wouldn't hesitate to do a calculation.

In either case if the calc came to 200 amps I would have no trouble using 4/0


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## eddy current (Feb 28, 2009)

Dennis Alwon said:


> 110.14 is the general rule but the 83% is the exception to the rule. Remember the calculated load is not the real load that the house will see. I would bet the home will never see anywhere near 180 amps anyway and if it did then it would not be for long.
> 
> If you feel that is an issue then, by all means, use 250 kcm but IMO, 4/0 is sufficent for a residence


In the CEC the calculated load for a service is concidered a “non continuous load” and can be taken at 100%. For instance, If your calculated load was 99 amps you could put it on a 100 amp breaker. Does the NEC not have the same sort of rule for service calculations?

We also have a handy table for determining wire size for ressidential services, Table 39.

Our code (table 39) would only allow 4/0 aluminum for a calculated load up to 189 amps. Above 189 to 200 we would have to use 250 kcmil.


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## matt1124 (Aug 23, 2011)

What gets me about all this is the 320 amp meter cans. If I don't put the single wrap of white electrical tape on the neutral going into an *obvious* neutral bar that's riveted to the case, it's a failure. If I bring two runs of 2/0 copper out to 2 200A MCB panels, out comes the great job sticker from the city.


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## Wiresmith (Feb 9, 2013)

Paul S. said:


> 240.4(B) refers to the OCPD. If the calculated load is 195, 4/O aluminum (using the 75 degree column) is only rated for 175. Maybe I am not following your logic here but I do not see how that answers my real question. Please do not interpret my reply here as being snarky or rude either, I really do not mean to be. I definitely appreciate your input.


310.15(B)(16) 75 degree 4/0 aluminum= 180 amps

240.4(B)next higher standard ocpd can be used (if not a branch circuit and the other 2)

240.6(A) table goes from 175 to 200


i don't interpret it as snarky, i know i didn't explain it. i like to give people the code references more than a yes or know answer to avoid arguing. i appreciate you asking the question

i believe you are not allowed to use 75 degree aluminum on 195 amp calculated load, this is a subject i argue with others on here and i wanted to just give you code references to see what you come up with


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## Wiresmith (Feb 9, 2013)

my post disappeared? so did the Canadians i was about to respond to


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## matt1124 (Aug 23, 2011)

Wiresmith said:


> my post disappeared? so did the Canadians i was about to respond to


Yeah I posted something snarky about the sears panel in the other thread and its gone too.


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## Wiresmith (Feb 9, 2013)

Paul S. said:


> 240.4(B) refers to the OCPD. If the calculated load is 195, 4/O aluminum (using the 75 degree column) is only rated for 175. Maybe I am not following your logic here but I do not see how that answers my real question. Please do not interpret my reply here as being snarky or rude either, I really do not mean to be. I definitely appreciate your input.


310.15(B)(16) 75*aluminum 4/0 180A
240.4(B) you can round up a breaker for service and feeder
240.6(A) table goes from 175 to 200

i don't believe 310.15(B)(7) is an exception for 310.15(A)(3)

i think it is for when calculated load is low enough to still be compliant with 310.15(A)(3)

almost everyone else that will reply on this disagrees with me

i don't believe what you are asking is allowed, although i see good reason for the other interpretation although i respectfully disagree with it


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## Wiresmith (Feb 9, 2013)

matt1124 said:


> Yeah I posted something snarky about the sears panel in the other thread and its gone too.


mine wasnt snarky and neither was the canucks


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## Wiresmith (Feb 9, 2013)

eddy current said:


> In the CEC the calculated load for a service is concidered a “non continuous load” and can be taken at 100%. For instance, If your calculated load was 99 amps you could put it on a 100 amp breaker. Does the NEC not have the same sort of rule for service calculations?
> 
> We also have a handy table for determining wire size for ressidential services, Table 39.
> 
> Our code (table 39) would only allow 4/0 aluminum for a calculated load up to 189 amps. Above 189 to 200 we would have to use 250 kcmil.


nec allows service at 100% (non continuous) but ocpd has to be rated for 100% (like cec probably also has a provision for)

4/0 alum 75* is 180 in our 3 conductor in conduit or cable table


what dennis is talking about, and most others on here that will respond to this believe is you can actually size service wire at 120% of our standard conductor ampacity table


correct me if im wrong dennis, 83% is doing the same as how im saying 120% of 310.15(B)(16) right?


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## Wiresmith (Feb 9, 2013)

Dennis Alwon said:


> 110.14 is the general rule but the 83% is the exception to the rule. Remember the calculated load is not the real load that the house will see. I would bet the home will never see anywhere near 180 amps anyway and if it did then it would not be for long.
> 
> If you feel that is an issue then, by all means, use 250 kcm but IMO, 4/0 is sufficent for a residence


i hope you don't take this as argumentative, if i'm wrong about this i would like to be corrected. but if your only going to see above 180 for short amount of time and rarely or likely never why not use 175A ocpd?


BTW did you catch the MH thread with rbalex? does he have a short fuse or am i really that thick sculled, i haven't been over there long and i'm not familiar with him


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## cabletie (Feb 12, 2011)

Residential services have a different set of rules/exceptions. You are using commercial rules. 

See the annex for examples on how to size service conductors for single family dwellings.


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## Paul S. (Sep 8, 2012)

I think you handled it fine. Suggest a load calculation before you go any further. If you are just adding a receptacle I wouldn't worry but if they wanted something like a car charge or hot tub I wouldn't hesitate to do a calculation.


That's part of what I was looking for, if you thought it might be a problem. I really appreciate your answer. I will probably try and speak with one of the lead inspectors in my area just to see what they say as well. (I expect they may give similar answers.) With that said, if I had a calculated load of 195 amps, I would probably use the 250 just to be on the safe side.


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## Wiresmith (Feb 9, 2013)

cabletie said:


> Residential services have a different set of rules/exceptions. You are using commercial rules.
> 
> See the annex for examples on how to size service conductors for single family dwellings.


i agree with the calculations in the annex, i do not see where the annex says you can use 4/0 aluminum 75 degree for a calculated load over 180 amps (or instructions that would lead you to believe that)

are you refering to d7?

i agree you can use 4/0 on 200 amp service and ocpd just not on a service where calculated load is over 180, 179 and lower yes


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## Wiresmith (Feb 9, 2013)

310.15(B)(7)

says "not less than 83%" it doesn't say it doesn't have to be larger than that


and i also believe you have to comply with all of the other de-rating requirements as well


and like i said earlier, i believe the other interpretation is far more common in the industry than mine


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## frenchelectrican (Mar 15, 2007)

Wiresmith said:


> 310.15(B)(7)
> 
> says "not less than 83%" it doesn't say it doesn't have to be larger than that
> 
> ...


I just dont want to point fingers or anything like that but how long ago did the art 310.15 (B) (7) did kick in effect ? 

I kinda got the grist of it but did that came in on 14 or 17 NEC cycle ?


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## Wiresmith (Feb 9, 2013)

frenchelectrican said:


> I just dont want to point fingers or anything like that but how long ago did the art 310.15 (B) (7) did kick in effect ?
> 
> I kinda got the grist of it but did that came in on 14 or 17 NEC cycle ?



they got rid of the table i think in 14' and replaced it with 83% in text, in 11' they had a table and no 83% rule but it is essentially the same thing

the reasoning in the code panel reasoning, is they got rid of table because people were miss using by not using de-rating (it was being interpreted as saying the conductors do not need to be larger than this instead of looking at it as a minimum)


there has been a residential load diversification table all the way back to 68' and i obviously have not turned over every stone but cannot see where it states what would explain(convince myself, no-one else needs explanation it makes sense to them) how most others interpret this section


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## frenchelectrican (Mar 15, 2007)

Wiresmith said:


> they got rid of the table i think in 14' and replaced it with 83% in text, in 11' they had a table and no 83% rule but it is essentially the same thing
> 
> the reasoning in the code panel reasoning, is they got rid of table because people were miss using by not using de-rating (it was being interpreted as saying the conductors do not need to be larger than this instead of looking at it as a minimum)
> 
> ...


Thanks for clearing this one up.

That part I am agree with you is derating conductors that what useally nail them pretty hard if they are not aware of it. ( it do happened from time to time over here as well )

BTW I am on modified 11 nec cycle in Philppines.


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## Dennis Alwon (May 9, 2009)

Wiresmith said:


> i hope you don't take this as argumentative, if i'm wrong about this i would like to be corrected. but if your only going to see above 180 for short amount of time and rarely or likely never why not use 175A ocpd?


I believe the code panel guys feel that if the load is calculated at 195 amps the reality is that you will not see more than 180 amps and if one did it would not be for very long. Calculated loads have a lot of leeway built into it so it is not an issue




> BTW did you catch the MH thread with rbalex? does he have a short fuse or am i really that thick sculled, i haven't been over there long and i'm not familiar with him


No, I did not see that thread and rbalex is generally not one to blow up at all. In fact, I had not seen him get angry at anyone


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## Wiresmith (Feb 9, 2013)

Dennis Alwon said:


> I believe the code panel guys feel that if the load is calculated at 195 amps the reality is that you will not see more than 180 amps and if one did it would not be for very long. Calculated loads have a lot of leeway built into it so it is not an issue


trip curves are part of my reasoning for my agreement with my interpretation, i'll try to explain my thought process.

http://download.schneider-electric....s&p_File_Id=8785982737&p_File_Name=736-06.pdf
QOM FAMILY MOLDED CASE CIRCUIT BREAKERS CHARACTERISTIC TRIP CURVE NO. 736-6
175A breaker at 180 amps will take 16 minutes to 2.7 hours to trip at 40*c ambient temp cold start


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## Wiresmith (Feb 9, 2013)

218 amps(175 x 1.25) is 5.8 minutes to 2.7 hours


i will say in my experience if a breaker is ran hot often through out its lifetime it does trip sooner than it used too (if it will trip). and old ones that haven't been exercised might take longer(or not trip) but there supposed to be exercised,(i am not using this reasoning for believing my interpretation)


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## Wiresmith (Feb 9, 2013)

by the way, i'm not saying he should use a 175a breaker in his situation, i believe he needs the larger wire and ocpd.

the reason i bring the trip curves up is because the common interpretation of the 83% rule to me is more in line with if the 83% rule would say you can lower your calculated load to 83% in a house because of diversification.

if you don't want to discuss this with me i understand, no one else does either (not because they think i might be right, but because they think i'm stubborn and completely wrong).


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## Dennis Alwon (May 9, 2009)

You are not listening to what is being written in previous posts. The code does not require a larger wire for a reason.... If you want to use a larger wire then you can but the NEC is clear that 4/0 aluminum is valid for a 200 amp service. Since a calculated load of 195 would require a 200 amp service then it follows that 200 amp service can be supplied with a 4/0 conductor.


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## cabletie (Feb 12, 2011)

Your way over complicating this. You don't have to do all the square rooting. The code already did it for you with sizing the service with the load calculations. 

Don't add anything else to the original question. The question was wire size, not if the service was large enough. 

They could have used a better example for D7 in the Annex. No one installs a 175A service on a single family dwelling. 

The load calc was done by the OP. A 200 amp service was determined. 

200 x .83 = 166
4/0 aluminum is good for 180amps. It's a done deal

It's done like that coast to coast. 

Any other question is outside the scope of the original question.


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## matt1124 (Aug 23, 2011)

Dennis Alwon said:


> I believe the code panel guys feel that if the load is calculated at 195 amps the reality is that you will not see more than 180 amps and if one did it would not be for very long. Calculated loads have a lot of leeway built into it so it is not an issue


Darn right there’s wiggle room. 

General lighing and recepts 3VA/sq ft. 

Tell me what 3400 square foot house has 85 amps of lighting and flat screen TVs


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## Wiresmith (Feb 9, 2013)

cabletie said:


> Your way over complicating this. You don't have to do all the square rooting. The code already did it for you with sizing the service with the load calculations.
> 
> Don't add anything else to the original question. The question was wire size, not if the service was large enough.
> 
> ...



i disagree

i agree 4/0 is allowed for 200 amp service

but not over 180a calculated load

i don't see 310.15(B)(7) as an exception for the rest of the code, its just giving you the min wire size for the service, then you have to apply other parts of the code as well which may require an increase in sizes, such as de-rating


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## Wiresmith (Feb 9, 2013)

heres a new paragraph in 2017

310.15(B)(7) (last paragraph)
"where correction or adjustment factors are required by 310.15(B)(2) or (3), they shall be permitted to be applied to the ampacity associated with the temperature rating of the conductor"


you still have to de-rate, and you use the conductor ampacity (310.15(B)(16) or whichever)


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## Wiresmith (Feb 9, 2013)

Dennis Alwon said:


> You are not listening to what is being written in previous posts. The code does not require a larger wire for a reason.... If you want to use a larger wire then you can but the NEC is clear that 4/0 aluminum is valid for a 200 amp service. Since a calculated load of 195 would require a 200 amp service then it follows that 200 amp service can be supplied with a 4/0 conductor.


i will keep re-reading them


but do you understand what i'm getting at with the trip curves. and why i think the way you guys are interpreting it, that it would be more appropriate if it said "you can adjust your calculated load for a house to 83% of a standard calculated load"(or something to that affect)


i agree 4/0 is allowed on 200 amp service just not on calculated loads above 180(because of other code sections)


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## Wiresmith (Feb 9, 2013)

i see it says "...shall be permitted to be sized in accordance with...(83% of service)"

and "(1)...shall be permitted to have an ampacity not less than 83% of service rating"


but i also see the "not less than", and i don't see it saying anything that makes me think this is an exception to other parts of the code

the only way i can stretch the interpretation to accept it is by saying since (B)(7) is in 310.15 *ampacities for conductors* that for some reason service conductors (and i would also have to accept feeders because b7 allows the feeder to be sized in accordance with this) will run at lower temperatures than branch circuit conductors and other feeders(non just passed service disco feeder)


but i don't believe that's a correct interpretation
but i also know most others disagree with me, so i will keep looking


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## Wiresmith (Feb 9, 2013)

matt1124 said:


> Darn right there’s wiggle room.
> 
> General lighing and recepts 3VA/sq ft.
> 
> Tell me what 3400 square foot house has 85 amps of lighting and flat screen TVs


party house, serious gamer (they have liquid cooled processors)


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## cabletie (Feb 12, 2011)

Wiresmith said:


> heres a new paragraph in 2017
> 
> 310.15(B)(7) (last paragraph)
> "where correction or adjustment factors are required by 310.15(B)(2) or (3), they shall be permitted to be applied to the ampacity associated with the temperature rating of the conductor"
> ...


Your adding things to the original question. 

We are not figuring temperature adders because it's not in an attic or running across a roof. 

He's not running multiple 200 amp services in a 4".


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## Wiresmith (Feb 9, 2013)

cabletie said:


> Your adding things to the original question.
> 
> We are not figuring temperature adders because it's not in an attic or running across a roof.
> 
> He's not running multiple 200 amp services in a 4".


i agree, the original question alone does not require discussion on de-rating, i am only bringing this up to show you have to still comply with the rest of the code

i think people are looking at b7 like it allows 200 amp service 4/0 al and you don't have to consider anything else for sizing

i view it more as the same as every-other code section rather than an exception


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## cabletie (Feb 12, 2011)

99% of the time there is nothing else to consider. 

abalex called. He said he would like to continue this conversation, but I'm done.


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## Wiresmith (Feb 9, 2013)

cabletie said:


> 99% of the time there is nothing else to consider.
> 
> abalex called. He said he would like to continue this conversation, but I'm done.


i understand, i want to be done with it too. he won't talk to me either.


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## Dennis Alwon (May 9, 2009)

I am starting to see why rbalex got frustrated. If you want to continue with what you got in your head then go ahead. You are either trolling us...or you just don't want to believe what you are being told.


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## Wiresmith (Feb 9, 2013)

Dennis Alwon said:


> I am starting to see why rbalex got frustrated.


 its not the first time someone has, i understand. thanks for taking the time to explain things anyway.



Dennis Alwon said:


> If you want to continue with what you got in your head then go ahead.


i'm actually trying to find something that convinces me of what you all believe



Dennis Alwon said:


> You are either trolling us...or you just don't want to believe what you are being told.


i understand it may look like that and that it may be hard to see that i just don't see it when i look at everything i have found to consider

thanks for trying to help me though

all sincerely

BTW if i didn't respect your thoughts and opinions on this i wouldn't have been asking you about it, i do.


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## Wiresmith (Feb 9, 2013)

Paul S. said:


> 240.4(B) refers to the OCPD. If the calculated load is 195, 4/O aluminum (using the 75 degree column) is only rated for 175. Maybe I am not following your logic here but I do not see how that answers my real question. Please do not interpret my reply here as being snarky or rude either, I really do not mean to be. I definitely appreciate your input.


see why i only wanted to post code references?


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## Mulder (Sep 11, 2010)

Wiresmith said:


> i disagree
> 
> i agree 4/0 is allowed for 200 amp service
> 
> ...



The emphasis in mine below.
310.15
(B) Tables. Ampacities for conductors rated 0 to 2000 volts
shall be as specified in the Allowable Ampacity Table 310.15(B)
(16) through Table 310.15(B)(19), and Ampacity Table
310.15(B)(20) and Table 310.15(B)(21) _as modified by
310.15(B)(1) through (B)(7)._

The ampacities of service and feeder conductors for dwelling units are modified by 310.15(B)(7). The ampacity of 4/0 aluminum is 200 amperes unless other adjustment factors apply. The calculated load would have to be 200 amperes or less for 4/0 conductors. 

310.15(B)(16) does not apply to such conductors. 

Since 310.15(B)(7) states the the ampacity of 4/0 aluminum supplying a dwelling unit is 200 amperes, 240.4(B) never comes in to play as we are protecting the conductors at their ampacity.


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## farmantenna (Nov 22, 2012)

*by*



Paul S. said:


> Truthfully, I am not sure that my terminations are 75 degree. I am really just assuming they are. I have to look at the listing on the equipment out in my truck.


most likely are 75deg.


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## farmantenna (Nov 22, 2012)

that was educational.


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## Paul S. (Sep 8, 2012)

Wiresmith said:


> 310.15(B)(16) 75 degree 4/0 aluminum= 180 amps
> 
> 240.4(B)next higher standard ocpd can be used (if not a branch circuit and the other 2)
> 
> ...



Honestly, I thought you may be right on this; (not so sure after reading farmantenna's post.) - I really need to look at this more when I have more time. I also want to touch bases with one of the lead inspectors in my area to see what they might say about this. - I should have more time to do some digging later. - Thanks everyone for your input; I would also like to try and reply to some of the other comments.


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## Paul S. (Sep 8, 2012)

Mulder said:


> The emphasis in mine below.
> 310.15
> (B) Tables. Ampacities for conductors rated 0 to 2000 volts
> shall be as specified in the Allowable Ampacity Table 310.15(B)
> ...


Are saying that 4/O aluminum (rated at 75 degrees) is okay for a residential service calculated to be 195A? I just want to make sure I am reading what you wrote, or listed, correctly. If so, I see your point. Thank you!!


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## Paul S. (Sep 8, 2012)

matt1124 said:


> Darn right there’s wiggle room.
> 
> General lighing and recepts 3VA/sq ft.
> 
> Tell me what 3400 square foot house has 85 amps of lighting and flat screen TVs


Just to be clear on this, the first 3000 VA is calculated at 100%, then the remaining would be calculated at 35%. Therefore, only 5520 VA (46A) would go toward the final total calculated load. - Still possibly pretty heavy I suppose for an average household; just pointing this out to see if you still think there's so much wiggle room. (I hope my math is correct here... I did this quickly. I really need to go to bed. I have a service change to do in the morning!) Thanks for your comment here. I really do appreciate being able to hear from colleagues.


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## Mulder (Sep 11, 2010)

Paul S. said:


> Are saying that 4/O aluminum (rated at 75 degrees) is okay for a residential service calculated to be 195A? I just want to make sure I am reading what you wrote, or listed, correctly. If so, I see your point. Thank you!!


Yes, 4/0 would be compliant. (unless other adjustment factors apply)

The ampacity of 4/0 aluminum is 200 amps for a residential service or feeder if it meets the conditions in 310.15(B)(7). 

310.15(B)(16) does NOT apply if the conditions in 310.15(B)(7) are met.


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## Wiresmith (Feb 9, 2013)

Mulder said:


> The emphasis in mine below.
> 310.15
> (B) Tables. Ampacities for conductors rated 0 to 2000 volts
> shall be as specified in the Allowable Ampacity Table 310.15(B)
> ...


thanks

are you willing to have a conversation with me about this, i'm going to post some information that makes me think the way i do, i don't mean to argue, if i'm wrong i'm wrong. if you don't mind let me know what you think about what i post.


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## Wiresmith (Feb 9, 2013)

Paul S. said:


> Honestly, I thought you may be right on this; (not so sure after reading farmantenna's post.) - I really need to look at this more when I have more time. I also want to touch bases with one of the lead inspectors in my area to see what they might say about this. - I should have more time to do some digging later. - Thanks everyone for your input; I would also like to try and reply to some of the other comments.


the way i read the code it's black and white, *BUT* all of these people sayings it's different makes me question it as well. i actually don't care too much about it other than it's just annoying, i often run larger conductors than the code requires in many different circumstances and i never use 240.4(B) either, so i actually don't care if it is allowed i wouldn't use it anyway, it's just the way i read everything i come up with it's not allowed.


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## Wiresmith (Feb 9, 2013)

301.15(B)

i can see reading it the way your reading it but other parts of the code make me think this _*may*_ be a misinterpretation. i may be wrong. i'll post my interpretation after we go through my reasoning's. i said *may* like i said earlier i know i might be wrong.


1-i don't see how (B) allows you to run your connections above there ratings

2-what would be the reasoning for letting you over-rate the service conductors?
the reason i have heard on here is that it will only go over-current for short duration's and only on rare occasion.
my rebuttle to this is my earlier post about trip curves



Wiresmith said:


> trip curves are part of my reasoning for my agreement with my interpretation, i'll try to explain my thought process.
> 
> http://download.schneider-electric....s&p_File_Id=8785982737&p_File_Name=736-06.pdf
> QOM FAMILY MOLDED CASE CIRCUIT BREAKERS CHARACTERISTIC TRIP CURVE NO. 736-6
> ...


by the logic of overrating because of this, wouldn't it make more sense to allow a reduction in load calc and still protect the conductors for overcurrent?

i.e. OP load calc was 195, why not let him do 85% load calc because of diversification, use 175A ocpd and 4/0

conductor is well protected and breaker won't trip because the people i have been arguing with says it will almost never see 175 and if it does it will be only a spike


i appreciate any consideration of the above and any respectful responses


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## Mulder (Sep 11, 2010)

Wiresmith said:


> 301.15(B)
> 
> 1-i don't see how (B) allows you to run your connections above there ratings
> 
> ...


The connections are not "above there ratings". 310.15(B)(7) simply allows the ampacity of dwelling unit service and feeder conductors to be 83% of the values listed in 310.15(B)(16) if no correction factors need to be applied. The columns for temperature ratings do not change.

I believe the reason the code allows smaller conductors for dwelling unit services and feeders is that for a typical house that a load calculation says needs a 200 amp supply will never see anywhere near 200 amps. The ampacities in the the table are very conservative IMO.


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## matt1124 (Aug 23, 2011)

Here’s what I think. If you really think that wire you’re needing every bit of 200 amps on a regular basis and shouldn’t bump down a size because you’ll overload the wire 17%, why the heck are you only putting a 200 amp can?

Run bigger wire than needed but push the jaws to the limit!


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## Wiresmith (Feb 9, 2013)

Mulder said:


> I believe the reason the code allows smaller conductors for dwelling unit services and feeders is that for a typical house that a load calculation says needs a 200 amp supply will never see anywhere near 200 amps. The ampacities in the the table are very conservative IMO.


my post #60 i gave my reason for why this doesn't seem like a logical reason to have 310.15(B)(7) say what you guys interpret


by the logic of overrating because of this, wouldn't it make more sense to allow a reduction in load calc and still protect the conductors for overcurrent?

for example; OP load calc was 195, why not let him do 85% load calc reduction because of diversification(single family dwelling), use 175A ocpd and 4/0 Al 75c

conductor is well protected and breaker won't trip because the people i have been arguing with says it will almost never see 175 and if it does it will be only a spike

and my post's(#33, #34) on trip curves 

what's your thought on this? thanks


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## Wiresmith (Feb 9, 2013)

matt1124 said:


> Here’s what I think. If you really think that wire you’re needing every bit of 200 amps on a regular basis and shouldn’t bump down a size because you’ll overload the wire 17%, why the heck are you only putting a 200 amp can?
> 
> Run bigger wire than needed but push the jaws to the limit!


by jaws do you mean ocpd?


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