Conductors in conduit, derates, uprates. Also array disconnects

[fastline]

Trying to get a PV array configured. We are designing for today and the future in mind. Array will be approx 200ft from use location. I am trying to optimize the use of conductors through use of the NEC tables and Vdrop calcs. What I am curious about is acceptable ambient temps for uprates when conduit is buried? Who makes that temp call? I know what it usually is. If I can use that, I can get an approx uprate of 25%, then a derate of 20% for 4-6 conductors, which means it becomes sort of a wash. Plus we will have to upsize conductors a bit anyway due to the distance.

As well, we are working to minimize parts/pieces/complexity. We are looking to run a single 2" conduit to serve all the needs as we will be running higher PV voltages at 500-600VDC. Should be able to serve a serious system with that.

What we have now is multiple 2" conduits coming in under the slab. I want to bring one of those up, then poke back outside the structure for an exterior disconnect. So this would technically have wire in the structure before the disconnect, but there will also be a fused disconnect at the array so it can get shut down. I am just trying to decide if and how we can add a second disconnect at the structure. I can load the system down and run a single disconnect, but rather have redundancy of two discrete PV arrays. It doubles the wire, disconnects, etc, but it allows service of one system while the other is operating.

One other thought here is "disconnection means", in whether disconnects truly need to be on the outside of the structure anyway if we have disconnects at the array? I am trying to look at this from a safety/service perspective as well as code.

Thoughts?

 

[jaggedben]

If the disconnect serves as a rapid shutdown switch it needs to be on the outside of the structure. Otherwise it doesn't. It's unclear from your description what requirement(s) the disconnect is fulfilling.

I've never 'uprated' conductors but maybe someone can enlighten me.

 

[ggunn]

I've never 'uprated' conductors but maybe someone can enlighten me.

Theoretically you can 'uprate' a 90 degree conductor by 15% if the maximum ambient temperature doesn't go above 50 degrees F. That's as much as Table 310.15(B)(1) allows for 90 degree conductors, although for 75 degree and 60 degree wires it allows a little more. I wouldn't do it myself, and I wish the OP luck in convincing an inspector that his calcs are legit. He also says that he will upsize for Vd, though, which could make the argument moot.