[RE-wrenches] line-side tap in subpanel

[R Ray Walters]

It would depend on which side of the OCD the 380 amp rated cable was on. IF the 400 A OCD is integral to the panel, then it doesn't matter what the cable size is on the other side of the OCD, it's protected by the OCD, and the current can't exceed that. If the OCD is remote and the subpanel doesn't have a main OCD, then technically you'd have to do the calculations at 380 amps.
In most cases, though, the cable would never see additional current, unless a 2nd subpanel were being fed by it.
Basically look at the circuit, and see if there is any point that could be fed by more current than the OCD rating. If so, apply the 120% rule for that entire section ( buss bar & conductors). Most points in the circuit though, will actually see a reduction in current, thanks to the PV backfeed. 
Funny we get penalized for adding PV, but don't get any code benefits from reducing current at other points.

R. Walters
ray at solarray.com
Solar Engineer




On Jul 22, 2010, at 2:49 PM, Erika M. Weliczko wrote:

> Greetings all,
> When the subpanel is fed with 380A wire and the OCD is 400A and the panel bus is 400A, which number would be used for determining 120% capacity?
> I’m thinking the 400A because the bus would be overfed before the wire?
> So if we want to land 225A of OCD for solar output on the line terminals or a backfed breaker, we need to derate existing OCD to subpanel all the way to 255A, or realisitically 250A if the load calcs allow it.
> Or replace the feeders serving the subpanel. Yeah, right.
>  
> While for this one it does not end up mattering, it raises the question.
>  
> Thanks for input,
> Erika
>  
>  
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