rating and solar

[Ponchik]

We have a 200A OCPD with 4/0 aluminum feeder to an interior sub panel. The rating of XHHW-2 based on NEC table is 205Amps at 90C. The ahj is rating that cable based on 75C.

Our calculation for load side tap allows us to stay with a 200Amp OCPD and the 4/0 aluminum. However, the ahj is asking to reduce the OCPD to 175A because they are seeing the rating of the cable at 180A based on table and the 75C column.

your thoughts is appreciated.

Do I have a leg to stand on or the ahj does?

 

[Carultch]

We have a 200A OCPD with 4/0 aluminum feeder to an interior sub panel. The rating of XHHW-2 based on NEC table is 205Amps at 90C. The ahj is rating that cable based on 75C.

Our calculation for load side tap allows us to stay with a 200Amp OCPD and the 4/0 aluminum. However, the ahj is asking to reduce the OCPD to 175A because they are seeing the rating of the cable at 180A based on table and the 75C column.

your thoughts is appreciated.

Do I have a leg to stand on or the ahj does?

You rarely get to use the 90C column as the final ampacity answer. Usually, equipment terminations require the 75C. See 110.14(C) for more detail.

This is the issue of "service" conductors vs feeder conductors, and the fact that service conductors only require 83% of the ampacity you think they should have. (Why that is in the first place, I do not understand).

The 4/0AL is all that it would ordinarily have to be, for a 200A service. But it is not 200A worth of wire for a feeder. It is "protected" by a 200A OCPD per 240.4(B), provided that the operating current with the applicable 1.25 safety factor is 180A or less. Where it would not comply, is if it is a 240.21(B) tap instead of a feeder, where 240.4(B) does not apply.

 

[jaggedben]

Depending on the breaker brand, it may not be worth the trouble of arguing, even if you might be correct, which I think you are not. Seimens 175A breakers are available on Amazon for under $100, last I needed one.

 

[Ponchik]

You rarely get to use the 90C column as the final ampacity answer

. Usually, equipment terminations require the 75C. See 110.14(C) for more detail.

This is the issue of "service" conductors vs feeder conductors, and the fact that service conductors only require 83% of the ampacity you think they should have. (Why that is in the first place, I do not understand).

The 4/0AL is all that it would ordinarily have to be, for a 200A service. But it is not 200A worth of wire for a feeder. It is "protected" by a 200A OCPD per 240.4(B), provided that the operating current with the applicable 1.25 safety factor is 180A or less. Where it would not comply, is if it is a 240.21(B) tap instead of a feeder, where 240.4(B) does not apply.

I know the final answer is not based on 90C however, I can use the rating of the conductor based on 90 to do my calculations.

Based on 120% rule and 705.12(D)(2) it is allowing to use the rating of the buss bar or the feeder cable. So based on that info, the conductors are rated at 205A and 120% gives me 246A. So we can do a 40Amp load side tap.

But the AHJ uses the 75C column instead of 90C. So if we go with AHJ says then 180 x 120% = 216A we need to down size the breaker to 175A.

75C is not the "rating" of the conductor. At least I don't see it that way.

what say you??

 

[GoldDigger]

The higher (90C) temp rating can be used as the base for adjustments and corrections but the actual load current cannot be greater than the 75C figure. To me that means that the effective rating of the feeder is the 75C value.

 

[ggunn]

We have a 200A OCPD with 4/0 aluminum feeder to an interior sub panel. The rating of XHHW-2 based on NEC table is 205Amps at 90C. The ahj is rating that cable based on 75C.

Our calculation for load side tap allows us to stay with a 200Amp OCPD and the 4/0 aluminum. However, the ahj is asking to reduce the OCPD to 175A because they are seeing the rating of the cable at 180A based on table and the 75C column.

your thoughts is appreciated.

Do I have a leg to stand on or the ahj does?

The conductors must be protected by the OCPD, i.e., assuming you are using 90 degree wire the 90 degree ampacity of the conductors derated for conditions of use must be greater than the next standard size down OCPD from the one you are using (assuming the breaker is less than 800A). The 75 degree ampacity of the conductors doesn't matter to this calculation.

Where it matters is in calculating conductor sizing for inverter output; the 90 degree conditions of use derated ampacity and the 75 degree continuous use derated ampacity must both be greater than 125% of the inverter maximum current.

 

[jaggedben]

I think the AHJ is giving you the benefit of the doubt in the first place, by allowing this interpretation of the 120% rule for a load side tap under the 2011 NEC. He is not wrong about 75C rating either. So if it were me I don't think I would try to fight this.

 

[Ponchik]

Thank you guys. Appreciate your comments.