installing generator on 400 amp panel.

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stephany

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nashville
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eletrician
I am installing a generator for a 400 amp service. I have installed generators for residential and commercial but never a single panel with 400 amps. We will be using a 400 amp transfer switch and a 30 Kw generator with SMM's for load shedding. I want to make sure I order the proper wire size. I remember using 250 kcmil for teh last commercial job I did but it was several years ago. The ones I have done recent used a 100 transfer switch with load shedding for Ac and so on. Would here be a reason to need 600 Kcmil since it has just the one panel?
 
I just looked at the transfer switch in the warehouse and spoke with the warehouse manager. He said this is the first one Generac has built in a 400 amp transfer switch. It took us almost a year to get it in.
 
Are you installing the ATS ahead of the main panel? From the sounds of it, everything is ordered and ready to be built, but would it have been more cost effective to provide a dedicated 100A generator panel?

Anyway, are you concerned with the conductor size to the generator? If that is so, 600kcmil is a tad over kill. Depending on your voltage drop calculation, I would refer you to 310.15(B)(16) or 310.16 depending on your code cycle.
 
Rock86 said:
Are you installing the ATS ahead of the main panel? From the sounds of it, everything is ordered and ready to be built, but would it have been more cost effective to provide a dedicated 100A generator panel?

Anyway, are you concerned with the conductor size to the generator? If that is so, 600kcmil is a tad over kill. Depending on your voltage drop calculation, I would refer you to 310.15(B)(16) or 310.16 depending on your code cycle.
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I believe the op is asking about the 400 amp side.
 
Ok to try and clarify this more. I work with a solar company and we sold a generator a year ago for a 400 amp service. Single panel with outside disconnect and meter traveling 120’ inside to a 400 amp panel. It has a 22kw generator. I misspoke earlier and said 30 kw. All the material was ordered prior to my start for the company. He wants the entire house backed up and I have already explained that 22kw will leave his heat, stove and dryer not working as they are electric and he has propane only for the generator. My concern was the 600 was to large. I was leaning on 250 kc mil in copper but i am not opposed to a 100 amp disconnect and then wiring that. I am kinda coming into this second hand and while I have worked generators from temp, permanent and the portable ones I have not installed 400 amp single panel for a residential house. The last commercial one I did was over four years ago and we did 500 kc mil and it was a big caterpillar for a medical building.

I thought it seemed to be over kill on the wire but it never hurts when you are unsure to ask for advice.
 
400A service coming into a house, - okay
22kW generator, - okay
disconnect and meter outside, - okay
disconnect to house panel = 120'?
wire size to the generator you are thinking 250 Cu? - Why are you considering this wire size? what does your math tell you?
 
Rock86 said:
400A service coming into a house, - okay
22kW generator, - okay
disconnect and meter outside, - okay
disconnect to house panel = 120'?
wire size to the generator you are thinking 250 Cu? - Why are you considering this wire size? what does your math tell you?
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The disconnect to the house is existing. I know there is voltage drop but I wasn’t the original installer for the service when the home was built. As for what my math says reading the wire charts in the 2020 nec 600 gives me over
qcroanoke said:
From generator output breaker to automatic transfer switch 2 awg would work. 1/0 if you want to consider voltage drop.
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thank you that’s what I was thinking but I was letting my self get thrown by the single 400 amp breaker for the main.
 
Thank you all for your input and answers. None of us can know everything and sometimes it helps to ask your peers when you are up against something you are not familiar with.
 
stephany said:
thank you that’s what I was thinking but I was letting my self get thrown by the single 400 amp breaker for the main.
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The 100A circuit breaker in the generator is providing fuse protection for the conductors; the 400A circuit breaker is just a switch at that point.

Those 400A transfer switches have been around for years; the covid shutdown has them backed up several months though. Its a massive enclosure if you haven't already seen it. It usually takes quite a bit of planning for me to get one installed properly at a residence due to the size of the switch and normal configuration of a 400A residential service.
 
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