Generator Sizing 702.4(b)

[JNMoffa]

Hello everyone,

I’m wondering if anybody has come across sizing a generator per NEC 702.4(b) and using “another approved method” to comply.

Our scenario is a 6500 sq. ft. custom home being built for which the customer would like a whole home generator backup. This house also contains a (5) car garage with (3) Level 2 EV chargers. My concern is over-sizing the NG generator since the simultaneous charging of all the vehicles, among all the other loads, is a highly unlikely case and possibly damaging to the generator.

Any insights you all could provide would be appreciated.
-J

 

[Russs57]

I would say I'd like to run at no less than 50% of rated KW for a natural gas generator. I don't know enough about code to help in any other way....other than to say a "load management" system can often be used to work around NEC requirements in a situation like yours.

 

[mopowr steve]

Hello everyone,

I’m wondering if anybody has come across sizing a generator per NEC 702.4(b) and using “another approved method” to comply.

If possible the other approved method is to retrieve the actual highest demand load over a course of a year, this could be obtained by calling the power company and asking them, if a smart meter has been installed on the residence.
Then try to add any possible KW of emergency heat or such that would be felt not to have ever been in that reading. And of course checking if amount of gas supply can be provided in addition to existing house load.

 

[JNMoffa]

Unless I'm missing something, load management for generators revolves around load-shedding in avoidance of overloading the capacity, which is the opposite issue we have here. Is that correct?

Unfortunately it's just a plot of dirt right now with no option for power logging or peak demand information from the utility provider. Either way those methods are covered under section 220 and 702.4(B) reads "The calculation of load on the standby source shall be made in accordance with Article 220 or by another approved method."

I find it strange that an optional standby system has to be fully sized in the first place.

 

[gadfly56]

If possible the other approved method is to retrieve the actual highest demand load over a course of a year, this could be obtained by calling the power company and asking them, if a smart meter has been installed on the residence.
Then try to add any possible KW of emergency heat or such that would be felt not to have ever been in that reading. And of course checking if amount of gas supply can be provided in addition to existing house load.

Yeah, that might work if the house actually existed yet...

 

[synchro]

Are the EV chargers already bought and paid for? Some EV chargers are available with power management that can reduce the peak load with multiple chargers. Chargepoint has some that are targeted mostly for commercial use, but they could possibly be a fit for the customer you described.

Charge More EVs with Power Management | ChargePoint

www.chargepoint.com

 

[mopowr steve]

Yeah, that might work if the house actually existed yet...

Ahhhh yes, I see now.

 

[JNMoffa]

Are the EV chargers already bought and paid for? Some EV chargers are available with power management that can reduce the peak load with multiple chargers. Chargepoint has some that are targeted mostly for commercial use, but they could possibly be a fit for the customer you described.

Charge More EVs with Power Management | ChargePoint

www.chargepoint.com

Thanks for bringing this up, it could prove helpful.

 

[steve66]

If its not built yet, can't you include some type of load management with the generator or ATS or distribution panel? I think that was the idea behind the original reply.

Also, why put both chargers on the Generator? Couldn't one charge either car? Or do they really have to charge both cars during a power outage?

If its a manual transfer switch, I wouldn't worry about the load on the generator. The owner can shut loads off with breakers before they start the generator, and I believe most generators will shut down or disconnect if they are overloaded.

 

[JNMoffa]

The customer requested a whole home generator, along with an ATS, with the understanding that he wouldn't be running (3) car chargers off of it simultaneously. I understand the distribution can be modified to lower the "calculation of load" by selective backing up of essential loads but was hoping that someone had come across "another approved method" since it is specifically mentioned in the relevant section of code.