# Sizing generators



## B4T (Feb 10, 2009)

We need more info beside sq. ft of the house.. even the POCO requires a load letter to connect a new service.. :blink::blink:


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## Bulldog1 (Oct 21, 2011)

To do this correctly you calculate the loads by the NEC for a whole house set up. Use nameplates for all appliances and HVAC equipment. That is the only way to know what you really need for a whole house generator.
If you are not doing the whole house do an actual load calculation of the circuits you plan to back up.


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## RIVETER (Sep 26, 2009)

Salvatoreg02 said:


> Let's see how many different answers we get..
> Scenario 4000sq ft home.
> Use your experience from the past to assume what the calculate load will be determine what size generator is needed for whole house coverage.
> What is the min and max generator needed.
> ...


Why not just size it to the service capacity?


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## macmikeman (Jan 23, 2007)

On one hand I feel kinda lucky cause I've yet to have to produce load calcs to size services for dwellings, no real enforcement here on that requirement. (I don't do mega mansions, on those yeah, you have to produce something.) But on the other hand, I am kinda rusty at going thru the calcs every time when I do have to for a generator job. I bet the calcs will put you way over 20k, but the real load would fly with a 17k no problem in a place where you didn't have to go with calcs.


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## B4T (Feb 10, 2009)

RIVETER said:


> Why not just size it to the service capacity?


You always size the generator to the load served.. I could have a 200A service and use only 30A at any one time..


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## RIVETER (Sep 26, 2009)

B4T said:


> You always size the generator to the load served.. I could have a 200A service and use only 30A at any one time..


Okay, forget about the beer cooler...and the air conditioner...and the sump pump...etc.


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## Bulldog1 (Oct 21, 2011)

macmikeman said:


> On one hand I feel kinda lucky cause I've yet to have to produce load calcs to size services for dwellings, no real enforcement here on that requirement. (I don't do mega mansions, on those yeah, you have to produce something.) But on the other hand, I am kinda rusty at going thru the calcs every time when I do have to for a generator job. I bet the calcs will put you way over 20k, but the real load would fly with a 17k no problem in a place where you didn't have to go with calcs.



A 20kw gives you 83 amps.


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## Salvatoreg02 (Feb 26, 2011)

200amp service
2 -3ton units'
2-refridgerators
1-sump pump 1/3hp
1-well pump3/4hp
2-boilers
2-120volt air handlers
1-dishwasher
1-garbage disposal 1/2 hp
1-microwave not fastened in place "so does not count but can be added in equation"
add anything else


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## B4T (Feb 10, 2009)

Salvatoreg02 said:


> 200amp service
> 2 -3ton units'
> 2-refridgerators
> 1-sump pump 1/3hp
> ...


20KW with load sheding is my pick..


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## Bulldog1 (Oct 21, 2011)

Salvatoreg02 said:


> 200amp service
> 2 -3ton units'
> 2-refridgerators
> 1-sump pump 1/3hp
> ...


You need actual nameplate Amps on the appliances to size it correctly. You only have 83 amps with a 20kw.


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## macmikeman (Jan 23, 2007)

Sump pump, well pumps, and boilers....... You poor guys. You have to have all that stuff in your house? Money better spent on a Jet Ski......


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## JohnR (Apr 12, 2010)

macmikeman said:


> Sump pump, well pumps, and boilers....... You poor guys. You have to have all that stuff in your house? Money better spent on a Jet Ski......


 Yah know, keep talking like that and we will just stop liking you. 







:laughing:


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## chicken steve (Mar 22, 2011)

> 2-refridgerators
> 1-sump pump 1/3hp
> 1-well pump3/4hp
> 2-boilers


+








=









~CS~


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## electricmanscott (Feb 11, 2010)

B4T said:


> We need more info beside sq. ft of the house.. even the POCO requires a load letter to connect a new service.. :blink::blink:


Another factually incorrect statement.


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## HARRY304E (Sep 15, 2010)

electricmanscott said:


> Another factually incorrect statement.


That is probably a requirement where he lives.


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## electricmanscott (Feb 11, 2010)

HARRY304E said:


> That is probably a requirement where he lives.


Which means that's the way everybody should do it.


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## Amish Electrician (Jan 2, 2010)

Generator sizing .... far more 'art' than 'science.'

First off, you need to size the transfer switch to service size (if it's feeding the whole house), or the breaker size (for individual circuits).

The generator operates best at something around 80% load -actual number varies by manufacturer- so there is no advantage to oversizing the genny. Bigger is NOT better. 

Load calculations are nice, but a better way is to actually better to monitor / log the actual demand over a period of time. Then you will be making your selection based upon what is really needed.


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## Magnettica (Jan 23, 2007)

RIVETER said:


> Why not just size it to the service capacity?


Ok, 200 amp service... 

200a x 240v = 48kw generator. 

No one wants that on the side of their house. Besides, it's not needed.


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## Salvatoreg02 (Feb 26, 2011)

Amish Electrician said:


> Generator sizing .... far more 'art' than 'science.'
> 
> First off, you need to size the transfer switch to service size (if it's feeding the whole house), or the breaker size (for individual circuits).
> 
> ...


What type of multi meter do you suggest?
And will the AHJ except that as the load calculation?

Sent from my iPhone using ET Forum


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## B4T (Feb 10, 2009)

HARRY304E said:


> That is probably a requirement where he lives.


Exactly.. the POCO wants that letter before they will install a meter..


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## user4818 (Jan 15, 2009)

B4T said:


> Exactly.. the POCO wants that letter before they will install a meter..


:sleep1:


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## electricmanscott (Feb 11, 2010)

Dear Poco,

Please connect my power. If you don't I will meet you at Home Depot to kick your ass. 

Love, B4T


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## Amish Electrician (Jan 2, 2010)

Our friends at Fieldpiece sell a datalogger - about $200 - that works with their usual meter 'heads.' That's one reason I like the line.

I've not had any AHJ issues regarding generator sizing- though, to be fair, all of the 'non-engineered' generator jobs were for purely optional, "homeowner convenience" installs. After all, the absolute worst thing that will happen if you overload the generator is that it will shut down.

Otherwise, I'm reading a bit of 'what if the generator isn't big enough' into your question .... which suggests a bias that it's better to be bigger than smaller. That's exactly the point I was trying to make when I said bigger is NOT better.

Most folks are not aware that running a generator with a small load is such a bad thing to do. Voltage fluctuations and mechanical damage to the generator are much greater problems when the generator is lightly loaded, or even unloaded. That's why every genny maker has a very narrow range where they want the genny to operate; as I mentioned, this figure is usually somewhere around 80% of the genny's rated full load.

Of course, if your genny is powering only motors, you won't have much choice but to massively oversize the genny, to allow for starting current. That's the exception to the rule.

Likewise, your periodic maintenance runs - I use monthly, though some makers say the interval can be longer- need to run the genny under load. 

It's pretty simple: size it too big, or skip the exercising, and the genny will break .... usually when you really need it. It's not like a fire extinguisher.

For larger, or mandated, generator jobs, the various generator distributors are much better at sizing the generator and selecting the other components than you are; it's best to work with them. Face reality: if you want a Cat, or Onan, etc.' you're going to be dealing with their rep anyway. Why not include him from the start?


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## ceb58 (Feb 14, 2009)

Salvatoreg02 said:


> What type of multi meter do you suggest?
> And will the AHJ except that as the load calculation?
> 
> Sent from my iPhone using ET Forum


 
*
220.87 Determining Existing Loads.​*​​​​The calculation of a
feeder or service load for existing installations shall be
permitted to use actual maximum demand to determine the
existing load under all of the following conditions:
(1) The maximum demand data is available for a 1-year
period.​
_Exception: If the maximum demand data for a 1-year period
is not available, the calculated load shall be permitted
to be based on the maximum demand (measure of average
power demand over a 15-minute period) continuously recorded
over a minimum 30-day period using a recording
ammeter or power meter connected to the highest loaded
phase of the feeder or service, based on the initial loading
at the start of the recording. The recording shall reflect the
maximum demand of the feeder or service by being taken​when the building or space is occupied and shall include by
measurement or calculation the larger of the heating or
cooling equipment load, and other loads that may be periodic​in nature due to seasonal or similar conditions.
_


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## Bulldog1 (Oct 21, 2011)

ceb58 said:


> *
> 220.87 Determining Existing Loads.​*The calculation of a
> feeder or service load for existing installations shall be
> permitted to use actual maximum demand to determine the
> ...



They will accept a years worth of power bills here or a testing company has to do a 30 day recording with a recorder that has been calibrated as required.


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## Salvatoreg02 (Feb 26, 2011)

ceb58 said:


> 220.87 Determining Existing Loads. The calculation of a
> feeder or service load for existing installations shall be
> permitted to use actual maximum demand to determine the
> existing load under all of the following conditions:
> ...


That section is for an existing service. It's got nothing to do with sizing a generator. Unless, someone can prove me wrong!

Sent from my iPhone using ET Forum


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## Bulldog1 (Oct 21, 2011)

Salvatoreg02 said:


> That section is for an existing service. It's got nothing to do with sizing a generator. Unless, someone can prove me wrong!
> 
> Sent from my iPhone using ET Forum




A actual load calculation is the correct way to size a generator. Name plates of all appliances and hvac stuff. Anything that is on the service and is not included in your normal calculation.


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## Salvatoreg02 (Feb 26, 2011)

Bulldog1 said:


> They will accept a years worth of power bills here or a testing company has to do a 30 day recording with a recorder that has been calibrated as required.


Please explain the formula that is used based on my utility bill?

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## Bulldog1 (Oct 21, 2011)

Salvatoreg02 said:


> Please explain the formula that is used based on my utility bill?
> 
> Sent from my iPhone using ET Forum




They take the kw's you used each month and average them together to get an average of the kw's used per month. I prefer to do an actual load calculation.


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## Salvatoreg02 (Feb 26, 2011)

Bulldog1 said:


> They take the kw's you used each month and average them together to get an average of the kw's used per month. I prefer to do an actual load calculation.


So in my home I used 18000kwh for the entire year. What size generator would you guess I need.

Sent from my iPhone using ET Forum


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## Bulldog1 (Oct 21, 2011)

Salvatoreg02 said:


> So in my home I used 18000kwh for the entire year. What size generator would you guess I need.
> 
> Sent from my iPhone using ET Forum


I wouldn't guess. I would do what I posted. A load calculation with nameplates of all appliances etc.


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## dmxtothemax (Jun 15, 2010)

There are many things to consider here!
What exactily does the home owner expect to run
on his back up genny ?
Everything ? or will he accept less ?
This question will determine what you need.


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## ceb58 (Feb 14, 2009)

Salvatoreg02 said:


> That section is for an existing service. It's got nothing to do with sizing a generator. Unless, someone can prove me wrong!
> 
> Sent from my iPhone using ET Forum


 


*



702.4 Capacity and Rating.
(B) System Capacity. ​​​​The calculations of load on the
standby source shall be made in accordance with Article​
220 or by another approved method.


Click to expand...

Read the exception in 220.87
*


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## electriciansandy (Nov 17, 2011)

> There are many things to consider here!
> What exactily does the home owner expect to run
> on his back up genny ?
> Everything ? or will he accept less ?
> This question will determine what you need.


At least from my experience, most homes don't need all loads to have a generator backup, however I understand if this requirement can be different for some people, specially due to climate issues, the frequency and duration of power outages, etc. (for example, you probably don't want to spend a whole day without power if you are snowed in. My clients in Florida don't have this problem).

What I normally do is add a separate panel for important loads (some lighting, some outlets, home office, garage door opener, refrigerator) and use a smaller generator for these.


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