Generator breaker

[kbrandt]

I am installing a 100kw 120/240 volt single phase generator to a transfer switch. It is NG and max output is 379 amps. Here's my question:

They have a 450 amp breaker on the generator (I ordered as a 400 amp generator), do I size for breaker or the 379 amps it puts out. I wanted to run parallel 3/0 cu to x-fer switch. Can I use the 90C degree column or have to use the 75C column and use 4/0 cu.

Thanks for any help.

 

[augie47]

IMO, unless the generator has an inherent means to prevent a greater current flow, the 450 amp breaker would be the determining device and the conductors will need to be rated accordingly.
If the device at either end are rated for 75?, you would be limited to the 75? rating.

 

[iceworm]

I am installing a 100kw 120/240 volt single phase generator to a transfer switch. It is NG and max output is 379 amps. Here's my question:

They have a 450 amp breaker on the generator (I ordered as a 400 amp generator), do I size for breaker or the 379 amps it puts out. I wanted to run parallel 3/0 cu to x-fer switch. Can I use the 90C degree column or have to use the 75C column and use 4/0 cu.

Thanks for any help.

This website is truly addictive. (sigh)

The conductors between the generator and the 450A CB, and the 450A CB are governed by art 445. The conductors are required to be 115% of the gen FLA (379 x 1.15 = 436A) The CB size is not exactly specified by the NEC. So one might consider the generator CB size set by design rather than by code. The designer selected athe next higher CB trip, 450A. You didn't exactly ask this, but it will help with the rest of the explanation.

After the first disconnect (this would be the generator CB), conductor protection is per the basic rule, art 240.4. The conductor ampacity would have to be more than 400A, and need not exceed 450A.

Auggie adequately covered the 75C issue

It does sound a bit silly, but in some cases, the conductors between the gen and the first OPCD (gen CB) may be smaller than the conductors between the gen CB and the next switchboard.

the worm

 

[ceb58]

I am installing a 100kw 120/240 volt single phase generator to a transfer switch. It is NG and max output is 379 amps. Here's my question:

They have a 450 amp breaker on the generator (I ordered as a 400 amp generator), do I size for breaker or the 379 amps it puts out. I wanted to run parallel 3/0 cu to x-fer switch. Can I use the 90C degree column or have to use the 75C column and use 4/0 cu.

Thanks for any help.

The thing to look at is with NG you will not have a 100kw unit it will only be around 91 kw. The 379 amp is probably a continuous rating and not the surge rating of a 100 kw unit. Unless the mfg. will swap out the 450 for a 400 you are stuck sizing to the 450.

The conductors between the generator and the 450A CB, and the 450A CB are governed by art 445. The conductors are required to be 115% of the gen FLA

If the breaker is factory installed the NEC has no control of the wire size on the line side of the breaker. The load side would be sized as normal with out additional %