DC Source Output Conductors

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ggunn said:
A dead positive to negative short in that Romex would just arc and burn and never blow the fuse, and nothing the inverter could do would stop it.
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No, a dead short would result in no power as it would exceed the short circuit rating of the panels which will be less than the amperage rating of the conductors.

You could have a loose ground fault that would arc but you can have that with any branch circuit.
 
iwire said:
No, a dead short would result in no power as it would exceed the short circuit rating of the panels which will be less than the amperage rating of the conductors.

You could have a loose ground fault that would arc but you can have that with any branch circuit.
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I see what you mean; I should not have said dead short. But a loose wiring or pierced/worn insulation short between positive and negative will arc and fusing the lines won't stop it.
 
iwire said:
Strange, it is used for 480 volt 3 phase circuits but suddenly with DC it is a problem?

It seems a bit odd to me.
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Not strange. 480VAC 3 phase circuits are voltage sources with OCPD that will trip in the event of a short.
 
iwire said:
You could have a loose ground fault that would arc but you can have that with any branch circuit.
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In a branch circuit, an arc-fault breaker will detect the fault, open the circuit and eliminate the hazard.

In a PV circuit, an inverter with integrated dc arc-fault protection (per 7690.11) will shut down. If it's a series arc fault, then the hazard is mitigated. If it is a parallel arc fault (less common, but possible), then the arc will continue unabated.

PV source circuits present hazards that ac circuits do not. So it's perfectly reasonable to require additional physical protection for these circuits. The difference is not ac versus dc. The difference is the PV power source. With today's technology, if the sun shines, it's live.
 
SolarPro said:
In a branch circuit, an arc-fault breaker will detect the fault, open the circuit and eliminate the hazard.

In a PV circuit, an inverter with integrated dc arc-fault protection (per 7690.11) will shut down. If it's a series arc fault, then the hazard is mitigated. If it is a parallel arc fault (less common, but possible), then the arc will continue unabated.
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It seems to me that if the inverter were to detect a parallel arc fault between positive and negative and open the DC, it might make the arcing worse. If the inverter is operating, then the current from the array has two paths to follow, but if the inverter shuts down, all the current flows through the fault.
 
Yes, you are correct. It just makes it worse.

Fortunately, parallel arc faults are uncommon. But it's a lot easier to drive a nail through Romex and cause a parallel arcing fault than it is to do so by driving a nail through conduit and compromising two PV circuit conductors.
 
SolarPro said:
... it's a lot easier to drive a nail through Romex and cause a parallel arcing fault than it is to do so by driving a nail through conduit and compromising two PV circuit conductors.
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Agreed.
 
shortcircuit2 said:
Well, if the PV System is of the ungrounded type, the white wire in 2-wire non-metallic sheathed cable, if used for the output circuits would be in violation of the code.

.....re-identify the white wire.....

As for romex being bad practice as jaggedben said, IMO using romex for a high voltage DC run thru a dwelling is a disaster waiting to happen in that a lot of homeowners will attempt do their own electrical work. If renovations were done in the future and someone comes across that romex run, they just might try to add branch circuit wiring to that cable.

I see your point.....but I can not stop an unqualified homeowner from performing unlicensed/unpermitted electrical work. Especially if I comply with labeling and marking the DC source output conductors[/B


Your AHJ may have approved that installation. But if it were my installation, I would not be comfortable with the use of romex for the high voltage DC output circuits.

Yes....in compliance with NEC and local AHJ...

With that said, let this be a warning to you that using romex for the installation you have described could be hazardous to persons and property.
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There are no code violations.....it is a code compliant installation.....let it be a lesson to the home owner that does unpermitted electrical work......

Let me ask you this....how is that much more of a problem to me, when a homeowner puts a 20 or 30 amp breaker on number 14 wire???
 
Canton said:
There are no code violations.....it is a code compliant installation.....let it be a lesson to the home owner that does unpermitted electrical work......

Let me ask you this....how is that much more of a problem to me, when a homeowner puts a 20 or 30 amp breaker on number 14 wire???
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Your bold type notwithstanding, running high voltage DC though an attic not in conduit is not anything I would ever do, code compliant or not. Be advised that just because something is code compliant does not guarantee that it cannot be dangerous or that it is necessarily a good idea.
 
Canton said:
I will clarify...there is nothing that prohibits Romex from being used to supply DC power...as long as installed as I described....also I have received approval from the AHJ.
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Bob's your uncle, then.

Generally speaking, a dc disconnect in the vicinity of the array in a residential application isn't going to qualify as readily accessible. But there are exceptions to every rule. I'm still dubious that this installation meets all of the applicable Code requirements related to circuit routing and identification. But it looks great from my house, as they say.
 
ggunn said:
Not strange. 480VAC 3 phase circuits are voltage sources with OCPD that will trip in the event of a short.
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In either the 480 volt branch circuit or 450 volt DC PV circuit, a high resistance fault to ground or line to line can result in arcing fault.
 
SolarPro said:
In a branch circuit, an arc-fault breaker will detect the fault, open the circuit and eliminate the hazard.
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:D

AFCI effectiveness notwithstanding, NM is also used in areas that do not require AFCI breakers.






PV source circuits present hazards that ac circuits do not. So it's perfectly reasonable to require additional physical protection for these circuits.
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The only real difference is where it comes from, for the safety of firefighters it should be able to be easily disabled.

As far as NM vs raceway that is not about firefighters, they can cut through either just as quickly. That is aimed at helping the unqualified and I am not a big supporter of that.


The difference is not ac versus dc. The difference is the PV power source. With today's technology, if the sun shines, it's live.
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I think we all understand how it works. :D
 
As far as NM vs raceway that is not about firefighters, they can cut through either just as quickly.
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Right. 690.31(E)(1) is geared more to protecting these conductors from a saw blade. And so is 690.4(F). But these subsections all work together to achieve a common goal. And the way this installation has been described, it's unlikely that these goals are met.
 
Canton said:
There are no code violations.....it is a code compliant installation.....let it be a lesson to the home owner that does unpermitted electrical work......
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To repeat, the romex is not code compliant if it can't be de-energized by a readily accessible disconnect.


Let me ask you this....how is that much more of a problem to me, when a homeowner puts a 20 or 30 amp breaker on number 14 wire???
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It's pretty silly to suggest that a code violation is not a big deal because some other code violation is a bigger deal.

As far as homeowners being confused as to what is what - for that matter, the same goes for qualified electricians - all PV circuits must be marked no matter what the wiring method. Arguably it's a bit more difficult to label flexible wiring methods.
 
iwire said:
In either the 480 volt branch circuit or 450 volt DC PV circuit, a high resistance fault to ground or line to line can result in arcing fault.
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Of course, but an arcing fault is much more likely to be sustained in a DC PV combiner output circuit where nothing will stop it than in a CB or fuse protected AC circuit from the utility. Do whatever you feel comfortable with, but I am never going to put DC from a PV array in Romex in an attic.
 
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