Cummins 20 kW Generator

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prattz99

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Long Island NY
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Electrician
Hi, I'm bidding on a wiring job for a Cummins 20kW generator project for a single family residential home on Long Island. In my mind a #4 THHN wire would be sufficient because the generator output is only 83.3 amps. But a friend of mine who has wired a lot more generators with his boss said they use #2 THHN for a 20 kW. Now I'm second guessing myself and think I should quote #3 as I see a composite cable for that size generator is a number 3. Am I wrong to think #4 is correct?

Also, if anyone has any input for the control wiring that would be helpful. As I have only wired Kholer and Generac generators before.

Thank you,
K.R
 
The conductor must be sized to carry the load and then protected at it's ampacity. A #4 Cu has an ampacity of 85 amps, however, the 20kw generator likely has a 90 or 100 amp breaker which would call for your using a #3 Cu.
 
augie47 said:
The conductor must be sized to carry the load and then protected at it's ampacity. A #4 Cu has an ampacity of 85 amps, however, the 20kw generator likely has a 90 or 100 amp breaker which would call for your using a #3 Cu.
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Could the #4 AWG be permissible with a 90 amp breaker based on 240.4(B)?
 
rojay said:
Could the #4 AWG be permissible with a 90 amp breaker based on 240.4(B)?
Click to expand...

445.13 Ampacity of Conductors.
(A) General.
The ampacity of the conductors from the generator output 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.

I would say unlikely to accommodated either condition to allow under-sizing conductor to any degree.
Breakers integral to the generator is component part of the Genset combined listing, and changing to size different than provided would be a violation of the product listing.
 
Fred B said:
445.13 Ampacity of Conductors.
(A) General.
The ampacity of the conductors from the generator output 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.

I would say unlikely to accommodated either condition to allow under-sizing conductor to any degree.
Breakers integral to the generator is component part of the Genset combined listing, and changing to size different than provided would be a violation of the product listing.
Click to expand...
I didn’t think that 445.13 applied to feeder conductors supplied by a breaker at the genset? If the conductor size and the manufacturer’s breaker size line up, you’re usually good to go. I’m curious about the listing issue though. It’s interesting that a lot of generators have different outputs based on the fuel they use- LP vs. natural gas. The manufacturer breakers always seem to be sized based on the higher LP output.
 
rojay said:
I didn’t think that 445.13 applied to feeder conductors supplied by a breaker at the genset? If the conductor size and the manufacturer’s breaker size line up, you’re usually good to go. I’m curious about the listing issue though. It’s interesting that a lot of generators have different outputs based on the fuel they use- LP vs. natural gas. The manufacturer breakers always seem to be sized based on the higher LP output.
Click to expand...
Read it
445.13 Ampacity of Conductors.
(A) General.
The ampacity of the conductors from the generator output terminals (usually your generator breaker) to the first distribution device(s) (usually your ATS)
That is your feeder conductors.
rojay said:
It’s interesting that a lot of generators have different outputs based on the fuel they use- LP vs. natural gas.
Click to expand...
This is continuous load, peak or surge load can be up the breaker limit. Thus conductors should be able to handle these higher load limits.
rojay said:
I’m curious about the listing issue though.
Click to expand...
The mfg breaker installed in genset is part of the generator total listing. Altering a component of the listed product creates a violation of the code in that it violates the listing and it would then require a costly site visit to re-list the altered equipment state. Some parts replacing like for like are allowed as it is not altering the listing from mfg. If mfg specifically states an allowed alteration, usually within a set limitation, it then would not be a violation of the listing. Sort of like the many discussions of a ie: a SD Hom breaker in a Siemens panel is a listing violation.
 
rojay said:
I didn’t think that 445.13 applied to feeder conductors supplied by a breaker at the genset? If the conductor size and the manufacturer’s breaker size line up, you’re usually good to go. I’m curious about the listing issue though. It’s interesting that a lot of generators have different outputs based on the fuel they use- LP vs. natural gas. The manufacturer breakers always seem to be sized based on the higher LP output.
Click to expand...
LP has more btu content than natural gas, so you get more horsepower.
 
Fred B said:
Read it
445.13 Ampacity of Conductors.
(A) General.
The ampacity of the conductors from the generator output terminals (usually your generator breaker) to the first distribution device(s) (usually your ATS)
That is your feeder conductors.

This is continuous load, peak or surge load can be up the breaker limit. Thus conductors should be able to handle these higher load limits.

The mfg breaker installed in genset is part of the generator total listing. Altering a component of the listed product creates a violation of the code in that it violates the listing and it would then require a costly site visit to re-list the altered equipment state. Some parts replacing like for like are allowed as it is not altering the listing from mfg. If mfg specifically states an allowed alteration, usually within a set limitation, it then would not be a violation of the listing. Sort of like the many discussions of a ie: a SD Hom breaker in a Siemens panel is a listing violation.
Click to expand...
I’ve always thought that the first distribution device that contained overcurrent protection was the manufacturer’s breaker at the generator. For a typical residential install, I’m not used to seeing overcurrent protection for the generator feeder at the ATS. I thought the 115% rule would come into play when the genset was not designed to prevent overloading, and did not have a factory breaker installed.
 
rojay said:
I’ve always thought that the first distribution device that contained overcurrent protection was the manufacturer’s breaker at the generator. For a typical residential install, I’m not used to seeing overcurrent protection for the generator feeder at the ATS. I thought the 115% rule would come into play when the genset was not designed to prevent overloading, and did not have a factory breaker installed.
Click to expand...
I haven’t seen one that doesn’t have a breaker at the generator? Residential or commercial. Even the portable units have ocp. Usually the only ocp in the transferswitch is when it’s service rated, and only on the utility side of the switch. If it’s a motorized breaker transfer switch, only the service side has ocp, the generator side is just a molded case switch with no ocp. The breaker on the Cummins/Onan is 100 amp.
 
AC\DC said:
I’ll have to read more in gen section but is it really a continuous load. It probably is never going to run 3 hours at max
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I don't know this specific generator, but 20kw might not even be the continuous rating of the generator. Not sure of the fuel, but a fuel derating might also apply
 
hillbilly1 said:
The breaker on the Cummins/Onan is 100 amp.
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If the generator feeder is protected by a 100 amp breaker then I think the answer to the OP’s question is #3 AWG copper/ #1 aluminum (assuming 75 Celsius terminals at both ends). I don’t think #2 copper is necessary. I was looking at some different cut sheets for a Generac 20KW, some show a 90 amp breaker, others show 100.
 
All of the 20kw Onan’s I’ve put in, have 100’s. They are convertible between LP and natural gas. Usually the Generac’s are too, maybe one is air cooled, while the other is liquid cooled? And natural gas only?
 
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