46kva transformer and 400amp breaker.

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jbwhite

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We have a 46kva generator, 120/208 three phase.
factory installed in the generator is a 400amp breaker.

I am getting this information second hand and wont get to look till monday.

Can someone tell me why the factory breaker would be so large, and if we size the wire to the breaker or based on the 222 amp load or the 277 volt load (adding .25% for continous duty) or to the 400 amp breaker?

Since this serves a control tower under FAA supervision, I have to assume that this is a legaly required system.
 
Re: 46kva transformer and 400amp breaker.

A 46kVA generator is only good for about 128A at 208V, 3??

Have you reviewed Article 445 namely .12 and .13?
 
Re: 46kva transformer and 400amp breaker.

i am reading the 2005 code. where did you get the reduction in amps?
 
Re: 46kva transformer and 400amp breaker.

Originally posted by jbwhite: Can someone tell me why the factory breaker would be so large
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I can't. That is way too high a breaker rating for a generator that can only put out 128 amps. I can only hope that when you see the machine (Tuesday, perhaps, and not Monday the 1st?), you will discover that you had been given incorrect information.
Originally posted by jbwhite: . . . and if we size the wire . . . based on the 222 amp load or the 277 volt load (adding .25% for continuous duty). . . ?
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I see where you might have gotten the "222 amp" number. I suspect you divided 46 KVA by 208 volts, and forgot the square root of three. Bryan had the correct amp rating. One thing you can rely on in this business: If there is a three phase anything, then the square root of three is going to find its way into the calculations somehow.

But the "277 volt" number confuses me, especially with your mention of "continuous duty." I note that 222 times 125% does give 277, but that confuses amps with volts.

I would set the generator breaker at 115% of 128 amps, and pick a setting of 150 amps. Then I would select a conductor that can handle 150 amps.
 
Re: 46kva transformer and 400amp breaker.

Originally posted by jbwhite:
... Can someone tell me why the factory breaker would be so large ...
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445.12 Just tells you to protect the gen from overload - a few clues as to how, but no specifics.

445.13 says the conductors from the gen to the first ocpd need only be sized at 115% of the nameplate. The first ocpd is on the gen, so these conductors are already sized and installed.

As far as the 400A CB, maybe it has a rating plug - say 150A. But then again, the gen controls may protect it from overload and the 400A CB is just for short circuit protection.

If the installation literature doesn't shed enough light, I'd recommend a call to the mfg.

Originally posted by jbwhite:
... Since this serves a control tower under FAA supervision, I have to assume that this is a legaly required system.
Click to expand...
I wouldn't bet on this falling under the NEC - it may well be Corps of Engineers specs. Of course the NEC is still a good guide.


As to sizing conductors, everything after the first ocpd is a feeder. (And I knew you knew that :D

carl
 
Re: 46kva transformer and 400amp breaker.

LMAO charlie,... it is monday the second. and the legal holiday. it was the time that the govt would let us go look closer due to the risk that we may by accident cause a problem. they have to schedule other facilities to be on stand by just in case.

I work alot of holidays for this same reason.
 
Re: 46kva transformer and 400amp breaker.

Although a misapplication of the generator, I ran into a situation like this (assuming the information from the field is accurate) when the gen was serving a fire pump only. The breaker was oversized to handle locked rotor current, but the gen was sized to handle FLA/startup current.
Note that startup inrush is similar to locked rotor current.
 
Re: 46kva transformer and 400amp breaker.

I just got off the phone.

It is an 80 kva generator. we mistakenly figured 1.25 percent for continous duty instead of 1.15 percent for generators.

I still dont understand the 400 amp breaker thing.
 
Re: 46kva transformer and 400amp breaker.

coulter, the contract says to follow all state and local codes. the state i live in is under 2005 nec.

some of your contracts do say to follow army corp, but this is not the case on this one.
 
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