Sub panel conductor size

[MR.DEI]

I have a 200amp sub panel fed from a 200amp breaker with 2/0 THHN. Article 240.2(B) (2) indicates if it does not correspond to the standard amperage rating the conductor can be one size smaller. Article 240.6(A) table shows 200amp as standard rating, so if 200amp is a standard should the conductor be 3/0? In the past I have always use this rule without knowing the standard rating. Can anyone clarify this interpretation?

Thanks,

 

[infinity]

You need a minimum of #3/0 or change the OCPD to 175 amps.

 

[roger]

You were doing it wrong.

 

[MR.DEI]

Thanks, The table on 240.6(A) shows 400amp as a standard and we always use 500kcmil that is only rated at 380amps?

 

[Seven-Delta-FortyOne]

The only way that would be code compliant is if this a dwelling, and the sub panel is supplying the entire load of the dwelling, then 310.12 could come into play.

And it’s not reducing wire size, it’s a next size up rule for the OCPD.

 

[infinity]

Thanks, The table on 240.6(A) shows 400amp as a standard and we always use 500kcmil that is only rated at 380amps?

Yes 380 amps is not a standard size so you can go up to 400 amps. 175 amps (#2/0) is a standard size so you cannot go up to 200 amps.

 

[roger]

Thanks, The table on 240.6(A) shows 400amp as a standard and we always use 500kcmil that is only rated at 380amps?

As long as the connected load calculates to less than 380A that is fine

 

[MR.DEI]

Thanks everyone for the clarification!!

 

[wwhitney]

The only way that would be code compliant is if this a dwelling, and the sub panel is supplying the entire load of the dwelling, then 310.12 could come into play.

If it's a dwelling, and the service is 200A, then it's compliant for the feeder to be 200A and 2/0 Cu 75C, regardless of whether the subpanel supplies all or just part of the dwelling load. 310.12(C).

Cheers, Wayne

 

[infinity]

If it's a dwelling, and the service is 200A, then it's compliant for the feeder to be 200A and 2/0 Cu 75C, regardless of whether the subpanel supplies all or just part of the dwelling load. 310.12(C).

Cheers, Wayne

Correct. It's doubtful that this is in a dwelling with a 200 amp panel and a feeder from a 200 amp circuit breaker.

 

[Seven-Delta-FortyOne]

If it's a dwelling, and the service is 200A, then it's compliant for the feeder to be 200A and 2/0 Cu 75C, regardless of whether the subpanel supplies all or just part of the dwelling load. 310.12(C).

Cheers, Wayne

Not sure if I’m understanding you correctly.

If you had a 200A service, and 2 or more feeders to 2 or more sub panels, you could not use the 83% rule for the sub panel feeders. As the individual panels are not supplying the entire load.

 

[Eddie702]

Correct. It's doubtful that this is in a dwelling with a 200 amp panel and a feeder from a 200 amp circuit breaker.

 

[wwhitney]

If you had a 200A service, and 2 or more feeders to 2 or more sub panels, you could not use the 83% rule for the sub panel feeders. As the individual panels are not supplying the entire load.

You can, only for 200A feeders. It makes no sense to require 3/0 for the feeders, when they are supplied by 2/0 service conductors. Which is what 310.12(C) tells you.

Cheers, Wayne

 

[winnie]

What @wwhitney is describing is that when the 83% rule applies, downstream feeders need not be larger than the reduced size (service) conductors.

So if you have a 200A service that supplies the entire load of a dwelling, the 83% rule applies to those service conductors, and allows you to reduce their size. If you then have a 200A feeder that only carries a _portion_ of the load, then you can keep the reduced size conductors for that feeder.

Note: this is not applying the 83% rule to that feeder; because the 83% rule does not apply to a feeder that only carries a portion of the load. This is a separate rule that says the feeder conductors don't need to be larger than the service conductors. If you have a 100A feeder then this separate rule doesn't help you.