Derating DC Conductors in Conduit

E

Elect117

Senior Member
Location
California
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Engineer E.E. P.E.
Someone asked me to look over a set of solar plans that has 6 DC circuits (12 total conductors + 1 EGC) per inverter in one conduit. I argued it was 12 CCCs and should be derated as such. They are saying it is 6 CCCs because the negative isn't considered a CCC per NEC 310.15.

Can someone chime in on if the "negative" is considered a CCC (like a neutral in a 120V circuit)?

I think the issue is terminology and I call it a negative and he calls it the grounded conductor. He isn't wrong, but I think that is why I am having a hard time getting him to understand why I think it is 12 CCCs. This code section calls out neutral conductor but does nothing to address grounded conductors.
 
Elect117 said:
Someone asked me to look over a set of solar plans that has 6 DC circuits (12 total conductors + 1 EGC) per inverter in one conduit. I argued it was 12 CCCs and should be derated as such. They are saying it is 6 CCCs because the negative isn't considered a CCC per NEC 310.15.

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🤣🤣🤣🤣🤣🤣🤣 lol lmao rotfl

Of course it's a CCC.
 
Elect117 said:
Someone asked me to look over a set of solar plans that has 6 DC circuits (12 total conductors + 1 EGC) per inverter in one conduit. I argued it was 12 CCCs and should be derated as such. They are saying it is 6 CCCs because the negative isn't considered a CCC per NEC 310.15.

Can someone chime in on if the "negative" is considered a CCC (like a neutral in a 120V circuit)?

I think the issue is terminology and I call it a negative and he calls it the grounded conductor. He isn't wrong, but I think that is why I am having a hard time getting him to understand why I think it is 12 CCCs. This code section calls out neutral conductor but does nothing to address grounded conductors.
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Your friend is mistaken; it's 12 CCCs. Even in the old days when the negative conductor was grounded, the bonding point was in the inverter, so the negative conductor carried the same amount of current as the positive one. On the, um, positive side, most of us use #10 wiring for home run DC circuits which often has twice or more the un-derated ampacity than is needed.

Incidentally, even in AC circuits if there is only one hot and a neutral, the neutral is a CCC.
 
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Thanks all for the replies.

Sometimes I have difficulty explaining things to others when it seems like common knowledge to me. And when I go to the NEC to clarify or just finish the discussion, it make me second guess what the code says vs what I know is true lol.

Obviously it is a current carrying conductor but I wanted the code to say something concrete about grounded conductors being CCC in all applications except where [ insert neutral conductor subsection ]. But there is no code to just point them to.

So now I have to explain it designers and salesmen that daylight as engineers. It can be a struggle.
 
Elect117 said:
Thanks all for the replies.

Sometimes I have difficulty explaining things to others when it seems like common knowledge to me. And when I go to the NEC to clarify or just finish the discussion, it make me second guess what the code says vs what I know is true lol.

Obviously it is a current carrying conductor but I wanted the code to say something concrete about grounded conductors being CCC in all applications except where [ insert neutral conductor subsection ]. But there is no code to just point them to.

So now I have to explain it designers and salesmen that daylight as engineers. It can be a struggle.
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Look at it like this. The rule in 310.15(C) is the main rule and applies to all conductors. The rule in 310.15(E) is a specific rule for neutral conductors that acts as an exception to 310.15(C).
 
jaggedben said:
And point them to the Article 100 definition of a neutral, which contains nothing about negative or grounded.
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And urge them to exercise critical thinking. If current is flowing through a conductor it is a current carrying conductor. After that, you can talk about how and why sometimes neutrals are the exception to this.
 
Now that the OP's question is answered, it brings to mind another question:

What the obstacles to taking the 6 DC 2-wire circuits and creating 3 DC 3-wire MWBCs by connecting the the positive of one circuit to the negative of another circuit, 3 times?

That would be actually be 6 CCCs, and only require 9 conductors.

Cheers, Wayne
 
wwhitney said:
Now that the OP's question is answered, it brings to mind another question:

What the obstacles to taking the 6 DC 2-wire circuits and creating 3 DC 3-wire MWBCs by connecting the the positive of one circuit to the negative of another circuit, 3 times?

That would be actually be 6 CCCs, and only require 9 conductors.

Cheers, Wayne
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Well, you would double the string voltage and possibly exceed the max V of the inverter.
 
ggunn said:
Well, you would double the string voltage and possibly exceed the max V of the inverter.
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The idea would be to connect two inverters to this 3 wire DC circuit. If we call the 3 conductors +, 0, -, then one from + to 0 and the other from 0 to -. Not sure what "functionally grounded" actually means in terms of what the inverter is doing, but if each inverter functionally grounds one conductor, you might need to be able to configure one inverter to functionally ground its -, and the other to functionally ground its +.

You'd need the conductor insulation voltage rating to be at least twice the individual string voltage.

Cheers, Wayne
 
wwhitney said:
The idea would be to connect two inverters to this 3 wire DC circuit. If we call the 3 conductors +, 0, -, then one from + to 0 and the other from 0 to -. Not sure what "functionally grounded" actually means in terms of what the inverter is doing, but if each inverter functionally grounds one conductor, you might need to be able to configure one inverter to functionally ground its -, and the other to functionally ground its +.

You'd need the conductor insulation voltage rating to be at least twice the individual string voltage.

Cheers, Wayne
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Feel free to try it and tell me if it works. I'm not doing it. :D
 
Elect117 said:
Someone asked me to look over a set of solar plans that has 6 DC circuits (12 total conductors + 1 EGC) per inverter in one conduit. I argued it was 12 CCCs and should be derated as such. They are saying it is 6 CCCs because the negative isn't considered a CCC per NEC 310.15.

Can someone chime in on if the "negative" is considered a CCC (like a neutral in a 120V circuit)?

I think the issue is terminology and I call it a negative and he calls it the grounded conductor. He isn't wrong, but I think that is why I am having a hard time getting him to understand why I think it is 12 CCCs. This code section calls out neutral conductor but does nothing to address grounded conductors.
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Both polarities count. There's no other path for current to return under normal circumstances.

The fact that the conductor is grounded doesn't matter. The reason neutral conductors have a special case, has to do with the fact that Ia^2 + Ib^2 + Ic^2 + In^2 in an unbalanced case is not going to exceed Ia^2 + Ib^2 + Ic^2 for of a balanced load case at maximum capacity, no matter what the imbalance. The sum of squares matters, because total heat generated is proportional to it. The exception is harmonic-intensive loads, due to 180 Hz harmonic currents accumulating on the neutral instead of canceling.

In cases like single phase-to-neutral, and feeders supplying 2 out of 3 phases to a single phase panel (open wye as I like to call it), the neutral counts as a CCC. There'd be no other option than for current to return on the neutral.
 
Carultch said:
Both polarities count. There's no other path for current to return under normal circumstances.

The fact that the conductor is grounded doesn't matter. The reason neutral conductors have a special case, has to do with the fact that Ia^2 + Ib^2 + Ic^2 + In^2 in an unbalanced case is not going to exceed Ia^2 + Ib^2 + Ic^2 for of a balanced load case at maximum capacity, no matter what the imbalance. The sum of squares matters, because total heat generated is proportional to it. The exception is harmonic-intensive loads, due to 180 Hz harmonic currents accumulating on the neutral instead of canceling.

In cases like single phase-to-neutral, and feeders supplying 2 out of 3 phases to a single phase panel (open wye as I like to call it), the neutral counts as a CCC. There'd be no other option than for current to return on the neutral.
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You do understand that what he was asking about was two wire DC circuits, right?
 
wwhitney said:
The idea would be to connect two inverters to this 3 wire DC circuit. If we call the 3 conductors +, 0, -, then one from + to 0 and the other from 0 to -. Not sure what "functionally grounded" actually means in terms of what the inverter is doing, but if each inverter functionally grounds one conductor, you might need to be able to configure one inverter to functionally ground its -, and the other to functionally ground its +.

You'd need the conductor insulation voltage rating to be at least twice the individual string voltage.

Cheers, Wayne
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Wouldnt you need to upsize the 3rd or middle point conductor since they wouldn't phase cancel?

And I am pretty sure that is how some utilities do their DC. +125V, 0, -125V
 
There is, in fact, in inverter topology that uses bipolar arrays with a positive, negative, and a (grounded) neutral conductor. The purpose was essentially to have a 1200V array that didn't exceed 600V to ground. I think it became a failed idea economically and I haven't heard of it for a while, but I presume there are a number of installed systems still operating. In that case the neutral would fit the Article 100 definition and would not count as a CCC.
 
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