ECN Forum Forums Code Related Discussion NEC & other Code issues Roof top disco for solar PV

Greg:
Yes, that's the Article.

I was going toward the following scenario:
Panels on roof (resi, SFD); DC thru roof into attic (accessable via pull down stairs); combiner box in attic; EMT thru second & first floor to basement; DC disco in basement w/inverter, net meter; POCO tie-in thru backfed CB in panel.

The DC conductors in the above scenario are unprotected (no OCP or disco) until they get to basement.

You still need the above defined disconnect.

They seem to assume PV sources are by nature current limiting.

I still believe the real answer is a gated output on each collector. They could just add a big FET power transistor on each output that would need to see the "gate" voltage to turn it on. Then all you need to do to disable each array is open the gate voltage circuit.

Greg,

Yeah, the disconnect is readily accessible in the attic with the pull down stairs, while the wire is sparking down in the basement. I guess that makes me feel safe.

Is it any less safe than having the utility service disconnect in the basement and the wire sparking in the attic?

Greg,

At least there is over current protection in the basement panel.

NFPA has said, unlike the utility, solar collectors are somewhat self protecting. If the conductors are sized to more than the maximum output of the collector the conductors are capable of handling anything the collector can throw at them.
Because this power is so expensive on a per watt basis they usually oversize for voltage drop to the point that overcurrent is seldom a problem.
I was always curious what happens when you have a bolted fault in a solar array.
If I have a 1kw array feeding a 1kw toaster wire heater it produces 3400 BTU of heat. (give or take)
If I have a 1kw array feeding a bolted fault where does the 3400 BTU go? Do the collectors burn up or does the imbalance of impedance cause the voltage to drop to close to zero, cutting off the current?

I thought that the DC was ungrounded so that a double fault would be required.

I thought that PV physics has current saturation issues such that bolted shorts are NOT the same as we see with batteries or infinite AC buses.

I didn't say a ground fault, only that it was bolted aka a dead short.

I suppose I was asking about the saturation issue.