400 AMP Service - Generator Size

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Jimmy7

Senior Member
Location
Boston, MA
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Electrician
Hello,

I just got these drawings for a small commercial building. Is a 45KW generator typically used with a 400AMP service?
Thank you1698669300723.png
 
I knew about the service size and the load the generator will serve, but the 400 AMP breaker at the generator was confusing me. I appreciate the feedback.
 
Jimmy7 said:
I knew about the service size and the load the generator will serve, but the 400 AMP breaker at the generator was confusing me. I appreciate the feedback.
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Yes that is strange given that the generator is only 45 kw. Also there should not be a #3 EGC from the pole to the meter main.
 
I suspect the engineer made a faux pas. Perhaps he wants a 400 amp generator since the wires seem to be sized that way.
 
Some future proofing to some extent?

Apparently not over 45kVA of load but that is getting close enough to 200 they did not want to design it with 200 amp gear just in case?

If more load isn't added for 5-10 years all that might need redone is to increase generator size, and it maybe already kind of paid for itself by then.
 
Maybe it was the intent that the generator be able to deliver 48kW however the loads were split between L1 and L2. If all the loads were 120V to neutral on L1 then it would need 400A OCPD and wiring, wouldn't it?
 
In my area, the AHJs do not require NEC load calcs for generators.

I do an on-site load Calc, and size the generator in accordance with manufacturer standards, and then list that size on the permit application.

If there are loads that can’t run, or that the owner doesn’t want to run, on gen power, I install a Power Management Module to deal with those.

I’ve seen some pretty poorly planned genny installs that were obviously done by someone with no knowledge of how it should be done.

I know that the Generac 48kW has a 200A OCPD. I’m guessing the draftsman/engineer just pulled stuff out of his butt on this design.
 
electrofelon said:
That's probably an appropriately sized generator for a 400 amp service from an actual realistic standpoint, although NEC calcs may show otherwise.
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For residential, offices, retail or similar yes. For industrial or industrial like applications depends on load served. A lot of the agriculture loads I deal with may be seasonal to some extent but when they are loaded they are loaded and for many hours at a time. Grain handling I have yet to see a standby generator or at least one that is intended to run the entire facility if needed. Livestock operations however can have a need to handle entire facility with a standby generator, and many them are not your grandparents farm anymore they are like mini industrial plants some even with 600 + amp 480/277 volt three phase services when it comes to electrical service and distribution.
 
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