Question about 445.13 Generator Load wire ampacity rating

[sfav8r]

I am a little confused about 445.13. It states that "The ampacity of the conductors from the generator to the first distribution device containing over current protection shall not be less than 115% of the nameplate rating of the generator.

I thought that this referred to the wiring from the actual generator lugs to the first breaker, which often is right on the generator. A recent discussion with an inspector is telling me otherwise. Basically saying that the load wires on a generator must be 115% of the generator nameplate rating. This doesn't make sense to me since they are protected by the 125A circuit breaker on the distribution side of the generator. Can someone tell me the reason for this code? Is seems to me that if the load wires are protected by a 125A breaker then the wire should need to be rated for 125 amps.

Also as a bonus question 310.15(b)6 allows 120/240 residential service and feeder to use the 310.15(b)6 table for ampacity. does this apply to the generator conductors as well if they are supplying the service to a residence?

 

[iceworm]

I am a little confused about 445.13. It states that "The ampacity of the conductors from the generator to the first distribution device containing over current protection shall not be less than 115% of the nameplate rating of the generator.

I thought that this referred to the wiring from the actual generator lugs to the first breaker, which often is right on the generator. A recent discussion with an inspector is telling me otherwise. ...

As you thought, 445.13 is for the conductors from the alternator terminals to the first disconnect. Since the generator came with a CB on the generator skid, that wiring was furnished by the gen MFG. The conductors from the gen CB to the premisis wiring is a article 240 issue. Calculate the load, size the conductors such that they will carry the load and are protected by the 125A CB

I'm guessing you have a 20kw - 25kw, 240V single phase gen. Nameplate FLA is 83A to 104A. With a 125A CB, you will size the conductors to the house at #2 CU. This is under the 125A CB, but over the next CB size down.

Then again, when responding to a DIY engineering by MH forum - I am often all wet.

... Also as a bonus question 310.15(b)6 allows 120/240 residential service and feeder to use the 310.15(b)6 table for ampacity. does this apply to the generator conductors as well if they are supplying the service to a residence?

I can't help you here. I do little residential generation. The few I have worked (DIY off-grid, generation, some with batteries/inverter), the question never came up. For one thing you don't have a service.

ice

 

[GoldDigger]

Also as a bonus question 310.15(b)6 allows 120/240 residential service and feeder to use the 310.15(b)6 table for ampacity. does this apply to the generator conductors as well if they are supplying the service to a residence?

This one is open to interpretation, since the section starts out with "services" and variations on that. If that were all that was there, it would not apply to a generator (even a prime mover for a building without utility service) since the definition of service requires a utility.
But then it goes and mentions feeders. It is an easy assumption to make that this applies only to feeders derived from a service, just after the service point, as in some multiple apartment buildings or condos.
But it does not explicitly say that. So it could be argued that once you pass the first OCPD (namely the one at the generator) you now have a feeder circuit.
But if the capacity of generator is notably lower than the capacity of the service it is backing up, you may find that an inspector is unwilling to allow this smaller size conductor. The basis would be that the engineering experience of POCOs in relating nominal service size and actual current consumption is not applicable to this case, even if you use load shedding in conjunction with the transfer.
Also, a purist could argue that the feeder is not really supplying the entire occupancy load, since it is too small to do that.
Again, it relates to the difference between calculated load and real load.

I can see this going different ways in different jurisdictions.