Generator breaker size

O

olly

Senior Member
Location
Berthoud, Colorado
Occupation
Master Electrician
I am setting a 18KW generator - if it is rated for 75A, don't you add 125% since its for continuous loads over 4 hours. So that would be 93A. Round up to the next size up. So 100A correct?
 
Depends on the size of wire you intend to use, but installing everything as 90A to 100A is okay.

240.4(B).

445.13 requires it to be more than 115%. (Must be greater than 86A).

So you are okay with #2 and a 100A OCPD. Arguably okay with #3 as well. You will just have to make sure all terminations, breakers, etc. are 75°C rated (which is very common).
 
If it is a Generac, the 18kw units I've inspected have an 80 amp breaker as I recall
 
The 115% only applies if the generator has no breaker. Once you hit a breaker, it is like any other feeder.

The 125% only applies if it is continuous load, which it almost surely is not.

If it has a breaker, the wire gets sized per the actual load, so long as the breaker is appropriate for that size load.

If it is a generac 18, it comes with an 80 amp breaker, which is good for #3 aluminum at 75 amps.
 
i recently did a cummins 13kw and it had a 100 amp on it. Genset output max was 54 amps... after talking to a couple other electricians they said that breaker is basically there for short circuit interruption. not necessarily for overload. I was told even if it was a smaller breaker the generator would bog out before tripping a breaker. cummins resi series up to 20kw have a 100 amp breaker in it btw.
 
Look at the label.

Manufacturer, fuel source, and other factors may influence the actual output of your whatever-the-kW-is-listed generator you have. This is a nameplate off of a 2023 18kW Generac. The LP and NG output is not the same.

I, personally, would never use aluminum conductors for any generator wiring. Fear of material fatigue. FWIW
 

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Chamuit said:
Look at the label.

Manufacturer, fuel source, and other factors may influence the actual output of your whatever-the-kW-is-listed generator you have. This is a nameplate off of a 2023 18kW Generac. The LP and NG output is not the same.

I, personally, would never use aluminum conductors for any generator wiring. Fear of material fatigue. FWIW
Click to expand...

I once thought that way, but really, the breaker on modern generators is mounted to the tin, and the tin is mounted to the frame, and the frame is mounted to the concrete. There is no relative motion between the slab and the breaker, therefore aluminum won't know the difference. Copper is so blasted expensive now, it is a large factor in some jobs.
 
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