genset sizing and the NEC

[Copperclad]

Hi guys,
Long story short: I have a local inspector telling me the NEC requires me to size a generator to the size of the service it will be feeding?
I cant find that requirement anywhere.
My customer had Ring Power "caterpillar" design the system for him. They ( ring power) go back and pull up a screenshot from the local utilitly company showing the demand of this particular bldg. over the last year, and size the generator from that particular document. Im not sure who is right or wrong. Any help would be appreciated!
The Generator is a 40KW 3phase 120/208 and it will be feeding a 400 amp ATS (and gear respectivly) the max kw demand from the past year was something like 32kw. Can a local inspector tell you what size genset to use? Ive done residential installs that use a 15 kw system to power a 200 amp service? Thats never been a problem? Any help would gratly appreciated!

 

[don_resqcapt19]

If this generator has an automatic transfer switch, see 702.4(B). Sizing based on demand would have to be "approved" by the AHJ. The actual demand is likely lower than what the Article 220 load calculations would show.

 

[ceb58]

I don't see what the problem is

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

So by what they did

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.
(2) The maximum demand at 125 percent plus the new
load does not exceed the ampacity of the feeder or
rating of the service.
(3) The feeder has overcurrent protection in accordance
with 240.4, and the service has overload protection in
accordance with 230.90.

 

[GoldDigger]

Hi guys,
Long story short: I have a local inspector telling me the NEC requires me to size a generator to the size of the service it will be feeding?
I cant find that requirement anywhere.
My customer had Ring Power "caterpillar" design the system for him. They ( ring power) go back and pull up a screenshot from the local utilitly company showing the demand of this particular bldg. over the last year, and size the generator from that particular document. Im not sure who is right or wrong. Any help would be appreciated!
The Generator is a 40KW 3phase 120/208 and it will be feeding a 400 amp ATS (and gear respectivly) the max kw demand from the past year was something like 32kw. Can a local inspector tell you what size genset to use? Ive done residential installs that use a 15 kw system to power a 200 amp service? Thats never been a problem? Any help would gratly appreciated!

If the inspector is hard to persuade, you may be able to get faster results by identifying any large non-essential loads (such as electric water heating) and connecting them them through a load shedding arrangement.

 

[qcroanoke]

If the inspector is hard to persuade, you may be able to get faster results by identifying any large non-essential loads (such as electric water heating) and connecting them them through a load shedding arrangement.

Why do that? If he can prove the Genset can handle the transferred load it is legal. no?

 

[Gregg Harris]

I would agree with 220.87 being applicable.

 

[Copperclad]

Thanks guys, once again!

Thanks guys, once again!

Sorry for the delayed response guys,
much thanks to all that helped. I have a much better understanding of the the "mechanics" of how generators are safely sized.
Now I will try to educate my bull headed boss!