Generator breaker sizing

Merry Christmas

hhsting

Senior Member
Attached shows 400kw, 208v, three phase generator. Their are two breakers 150A and 600A shown. From one of the breakers it feeds distribution switchboard and another breaker feeds fire pump.


Would this sort of arrangement be allowed by NEC 2017 Section 700 or not for emergency generator?


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roger

Moderator
Staff member
Location
Fl
Occupation
Electrician
Before you waste everyone's time here answer Gus's question. Now, considering PE' s have some level of technical writing skills you really need to learn the difference between there and their.

Roger
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
The basic concept, a Generator set as emergency power, is, of course allowed by Art 700.
The engineer has his work set out for him, tom's point + assuring generator OCP (445.13), and most lilely some load shedding especially with a fire pump involved.
 

hillbilly1

Senior Member
Location
Atlanta,Ga
Occupation
Field coordinator/ technical support
Although most engineers tap the fire pump ahead of the main on the generator, and call for load shedding as Augie mentioned, I had a fire inspector say the generator is not held to the same standard as utility, and does not have to supply locked rotor or oversize the breaker. He said it only had to start and run the pump along with any other loads that were not shed. He gave me the section number, and I looked it up, and he appeared to be correct. But danged if I can remember the number.
 

JoeStillman

Senior Member
Location
West Chester, PA
Nothing wrong with that design. I would have done it that way myself (mostly). I agree with hillbilly's inspector; generators don't have to carry fire pump locked-rotor current like the utility service does.

Also, in their design there, they're leaving a lot of capacity on the generator. 400 kW @ 80% pf is 500 kVA, which at 208v can put out 1390A. The generator may be oversized if they selected it to carry the fire pump locked-rotor. All the major generator manufacturers offer sizing software and they all know how to handle a fire pump. It won't sustain the locked rotor amps, but it does provide enough voltage during starting - even without load shedding if you size it right.
 
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