2/0 for 200A Generator Transfer switch

[MJRobinson]

Hello,

We've been using 3/0 for our main service panel to transfer switch(200A), and the transfer switch to the critical loads' panel (200A).

I was asked questioned the other day about this and they wondered why most electrical panels only use 2/0 for 200A.

It looks like I can use the 310.15(b)(7) to get down to using 2/0 since the entire dwelling will be using that conductor.

Are there any exclusions for generators on 310.15(b)(7)?

Thanks,

Michael

 

[kwired]

Keep in mind that 310.15(B)(7) can only be used for a conductor supplying the entire load of a dwelling unit. If a dwelling isn't involved at all forget that section even exists.

Since you mentioned the transfer switch supplies "critical loads panel" I'm guessing you are not supplying the entire load to a dwelling in your application.

If it is a dwelling and your conductor in question supplies the entire dwelling load, then it doesn't matter what the source is, you can use 310.15(B)(7)

 

[infinity]

He did mention a dwelling in the OP.

 

[kwired]

He did mention a dwelling in the OP.

I did catch that, but he also said "critical loads panel" which seems to indicate there is more than just one 200 amp panel supplying this dwelling. If that is true none of his panels are supplying the entire dwelling load and only place he could possibly use 310.15(B)(7) would be for any common supply conductor that does supply the entire dwelling unit.

 

[MJRobinson]

Keep in mind that 310.15(B)(7) can only be used for a conductor supplying the entire load of a dwelling unit. If a dwelling isn't involved at all forget that section even exists.

Since you mentioned the transfer switch supplies "critical loads panel" I'm guessing you are not supplying the entire load to a dwelling in your application.

If it is a dwelling and your conductor in question supplies the entire dwelling load, then it doesn't matter what the source is, you can use 310.15(B)(7)

Thanks,

In most cases we are supplying the entire home with power, all of the circuits are being moved to a new sub-panel for generator support(except solar), I always call that panel the "critical loads panel", sorry for the confusion. We would be supplying the entire dwelling load.


Can you point me to the citation for my AHJ?

Now we just need to use up the 200ft of 3/0 we have.

Thanks

 

[kwired]

Thanks,

In most cases we are supplying the entire home with power, all of the circuits are being moved to a new sub-panel for generator support(except solar), I always call that panel the "critical loads panel", sorry for the confusion. We would be supplying the entire dwelling load.


Can you point me to the citation for my AHJ?

Now we just need to use up the 200ft of 3/0 we have.

Thanks

310.15(B)(7)

You won't find anything that specifically addresses a generator and this 83% allowance together. If the conductor in question supplies the entire dwelling load (no more no less)it can utilize this section for determining needed ampacity

 

[jaggedben]

This is similar to the question I asked here.

It seems to me that 310.15(B)(7)(3)* declares that one does not have to engage in the ridiculous exercise of making any feeder larger than the service conductors (at least, as long as the same design temperature applies). So if the service conductors are permitted to be 2/0 cu, then no feeder downstream would be required to be larger than 2/0 cu. I don't see how the insertion of an ATS changes this.

Your feedback on this question is welcome.

*2017 NEC reference. 2020 NEC reference would be 310.12(C).

 

[winnie]

IMHO you are correct and that code section is specifically to avoid having to increase the size of a conductor carrying less load.

Say you have a 200A service with 2/0 conductors, and a generator system that has 200A feeders but which involves some sort of load shedding. The generator circuits are explicitly not supplying the full load of the dwelling, because of the load shedding. However they still need not have an ampacity greater than that of the service conductors.

-Jon