Generator Feeders

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Does the genny have an OCPD? See exception.

from the 08

445.13 Ampacity of Conductors.
The ampacity of the conductors from the generator terminals to the first distribution device(s) containing overcurrent protection shall not be less than 115 percent of the nameplate current rating of the generator. It shall be permitted to size the neutral conductors in accordance with 220.61. Conductors that must carry ground-fault currents shall not be smaller than required by 250.30(A). Neutral conductors of dc generators that must carry ground-fault currents shall not be smaller than the minimum required size of the largest conductor.
Exception: Where the design and operation of the generator prevent overloading, the ampacity of the conductors shall not be less than 100 percent of the nameplate current rating of the generator.
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To be clear that section applies only to the conductors between the windings and the first overcurrent device.

I have never wired a generator that did not already have an overcurrent device installed on it making that section meaningless.
 
Jerseydaze said:
Do you calculate gen feeders at 100% rating of generator or 125%
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How do you calculate a feeder supplied from a service?

Feeders shall have an ampacity not less than the noncontinuous loads plus 125 percent of the continuous loads. 215.2(A)(1)
 
Which way are we working here Bob? Sounds to me like OP has a generator already. Your sizing your feeders for the load which in turn would tell you the size of the OCP and gen-set.
 
chris kennedy said:
Which way are we working here Bob? Sounds to me like OP has a generator already. Your sizing your feeders for the load which in turn would tell you the size of the OCP and gen-set.
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You really lost me, I have no idea. :-?
 
Jerseydaze said:
Do you calculate gen feeders at 100% rating of generator or 125%
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Whaooo!

How could I be so confused by one sentence?(and the different times on my clocks and computer)

I'll try again later.:-?
 
iwire said:
How do you calculate a feeder supplied from a service?

Feeders shall have an ampacity not less than the noncontinuous loads plus 125 percent of the continuous loads. 215.2(A)(1)
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Ok let me put it this way the generator is going to be sized for future loads and is supply ed by HO it will be a 30KW or a 45KW id like to use SER

I know some of you think its helpfull to chastise guys for not knowing something obvious to you.I think its beter to ask something stupid then do something stupid.
 
Jerseydaze said:
I know some of you think its helpfull to chastise guys for not knowing something obvious to you.I think its beter to ask something stupid then do something stupid.
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Jerseydaze
I could not agree more with the above statement. I believe that Mike Holt set this site up just for that reason.



Also,
I think that Bob has answered your question about sizing of the conductors. It will be based on the size of the overcurrent device on located at the generator., if the device at the generator is not just a disconnecting means but also is an overcurrent device.
 
Pierre C Belarge said:
It will be based on the size of the overcurrent device on located at the generator., if the device at the generator is not just a disconnecting means but also is an overcurrent device.
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?

OK I have a generator with a 100 amp breaker, what is the smallest conductor the NEC will allow?
 
One thing that was not clear on my generator is that it is rated at 15KW, but it only has 50A built in main breaker. It has no provisions for hard wiring, only receptacles on it. All the smaller receptacles have their own breakers, but those breakers are fed from the load side of the 50A main breaker on the generator.

So is this 50A breaker 100% rated? You would think a 15KW generator would be good for 62.5 amps (and it even has a surge rating of 22 KW for motor/compressor startups). Lets derate that 62.5 amps some for continual use so 50A seems about right.

So can this generator run a 50A continuous load from its 50A receptacle which is protected by its 50A breaker? This breaker is small and part of the listed generator (it is not a typical molded case breaker for a loadcenter). It doesn't say 100% rated, but it could be. In fact, I am hoping it is. Otherwise, it seems silly to call this a 15 KW generator if I can only load it continuously to 80% of 50 amps.

If it can be loaded to a 50A continuous load, then the normal load calculations aren't right. Can a NEMA 14-50 be loaded to 50A continuously (I think it can if an individual branch circuit)?
 
Mark,

It is normal to only be able to derive 80% of the stated power from a generator. A 25KVA 3 phase will provide 3 legs of 120v at 50 amps for 18KVA usable (about 72%).

I'm not sure why the generator guys get to put a number on the genset higher than it will ever deliver, but it is something we plan for every time we use a generator at an event.

Tim.

Tim Cahall
The Main Event, Inc.
 
I agree that a generator should be loaded continuously to about 80% of its rated output. That comes to 50A on my generator. But is the 50A receptacle and 50A breaker rated to run at 50A for 100% of the time, or must another 80% derating occur making the total generator derating 64% of advertised?
 
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