Generator Feed

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tryinghard

Senior Member
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California
NEC 2002, Article 445.13 specifies the ampacity of the conductors from a generator to the first OCPD not to be less than 115% of the nameplate current rating and 100% in exception.

240.21(2)(1) Ampacity of the tap conductors not less than 1/3 or .33 of the rating of the OCPD.

My question is can the tap rule be applied with a generator feed or does only 445 apply?
 

tryinghard

Senior Member
Location
California
Yep, and the generator in question does have a breaker - 400A - so I was curious about that myself. I did not understand the NEC to involve itself within appliances. 445.18 qualifies a disconnecting means so maybe this 400A is also the disconnecting means?

The problem I have is the 20KW 480V generator has a 400A circuit breaker on it. This generator is replacing an existing and its feed is 250MCM AL's 20' to a 225A breaker.

There is no way to overload the generator because the 225A but the 20' feeder is incorrectly protected, or is it?

Does the 25' tap rule apply?
 

hillbilly

Senior Member
I believe that the 25' tap rule [240.21(B)(2)] would apply to this situation.
The only problem I see is the ampacity of 250 KCM aluminum is only 205A at 75C.
IMO, the 225A breaker is too large for the 250MCM Aluminum, tap or no tap.
Just a opinion
steve
 

coulter

Senior Member
Guys -
With all due respect, I believe you are trying to use the NEC as a design manual.

JAO - 445 is written with the understanding that gen installations are not cookie cutter. They need to be designed. There are very few minimums. Most of it reads (paraphrased), "Put it in so it works, doesn't burn up and you can work on it safely." Having put in/designed a few (or more ;)), I don't remember a one that went in marginal - generally all they got is barely enough.

I remember a thread a few months ago where the OP was looking for reasons that he could cheap on the copper from the gens. As I recall, he was replacing burned up conductors, likely from overload. Hummmm - this reasoning eludes me.

tryinghard -
200kw, 480V, 3ph, .8pf (likely), gives 300A FLA (continuous). I would agree with previous posters, everything after the gen 400A CB is not under 445.

Most installations I deal with, the gen does not have a CB. So 445.13 applies to the conductors from the gen to the CB. I've always felt there was no magic occuring at the gen CB and one should install 125% ampacity from the gen to the first OCPD (JAO). Disenting thought: It's pretty hard to run a gen at 100% output, and hold freq and voltage, so maybe that was the reasoning for the 115% rule.

Looking at the 250AL conductors (205A per steve), no way can you use the continuous output of the gen (300A X 1.25 = 375A). If that is okay, good deal - but this is a design issue - not an NEC issue.

steve is correct that the 25' tap rule does not allow upsizing the CB. So you are stuck with a 200A CB at the end of the 250AL (240.21B).

The installation you have described is pretty marginal for a 200kw gen. You may wish to consider some redesign.

note to steve: Per 240.4B, I think one could use a 225A CB to feed 250AL. (but I'm thinking you already knew that:)

carl
 

tryinghard

Senior Member
Location
California
Yah-Howdy! I agree, generator applications always need design/engineering. In this particular case I am only an electrician working for a municipal power plant. There is actually an active contract including a contractor with an accepted submitted generator providing the installation.

This contractor?s scope includes removal and replacement of the generator only to an existing system without altering the other existing components. So I am a curious onlooker wondering if the contract will allow 250 MCM AL's to land on the feeding 400A breaker; but in the light of article 240.4(B) & 240.21(2): 400x.33=132A -- they could, although I wouldn't without engineered direction.
 
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