DC Derating

[mimo]

for derating of dc feeders or branch circuit ampacity, should I consider the (-) as current carrying conductor?

 

[charlie b]

That is the same situation as a single phase AC circuit, and the same question as asking whether the ungrounded (netrual) wire counts as a current-carrying conductor. Yes, in both cases. Any current leaving the source on the (+) wire (or the ungrounded/phase wire) will return on the (-) wire (or neutral).

 

[mimo]

I agree about what you said, However the inspector has another openion that the (-) wire from a PV module is not considered as a CCC.

 

[raider1]

I agree about what you said, However the inspector has another openion that the (-) wire from a PV module is not considered as a CCC.

The inspector is incorrect.

Chris

 

[iwire]

All two wire circuits have two current carrying conductors. Once you get too three or more circuit conductors one may not have to be considered a current carrying conductor.

 

[Jraef]

If you are considering the ampacity of the conductors, then yes, you consider it current carrying. But if the context of the question was related to switching devices, then it was a trick question. You cannot switch the grounded circuit of a DC system in the US, which also must be the negative in a solar PV system. So if you asked him about derating of devices, he was tripping you up because the negative is not allowed to be switched!

 

[iwire]

You cannot switch the grounded circuit of a DC system in the US, which also must be the negative in a solar PV system.

The NEC does not specify if you use negative or positive as the grounded conductor.

One of the leading module manufacturers requires positive grounding for their modules.

 

[brian john]

. You cannot switch the grounded circuit of a DC system in the US, which also must be the negative in a solar PV system.

Why is this?

 

[Jraef]

The NEC does not specify if you use negative or positive as the grounded conductor.

One of the leading module manufacturers requires positive grounding for their modules.

You're right, I was reading too much into it.
Section 690.5(A), only the current-carrying ungrounded conductor should be switched. It doesn't specify which one, only that only the ungrounded one is the switched one.

 

[dereckbc]

You're right, I was reading too much into it.
Section 690.5(A), only the current-carrying ungrounded conductor should be switched. It doesn't specify which one, only that only the ungrounded one is the switched one.

Jaref do not feel bad,you just fell into a trap into thinking for what ever reason the negative conductor is grounded. You can choose to ground the positive, negative, and none of them. This is how we keep auto mechanics and DIY out of our club.

There is only one good safety reason we reference an electrical system to a body like earth, or a frame of a vehicle like a car or plane: To make over current protection simple and cost effective.