dc current conductors

[dcdave]

How many #10awg use-2 dc positive conductors can share a 1-1/4" emt conduit in a solar photovoltaic roof mount system? Does the 9 cunductor rule apply or can more be allowed because its dc electricity?

 

[coulter]

Re: dc current conductors

dc -

Originally posted by dcdave:
How many #10awg use-2 dc positive conductors can share a 1-1/4" emt conduit in a solar photovoltaic roof mount system?

Unless I'm missing something, this is a pretty standard conduit fill calc. I don't have a mfg cat handy, so don't have an accurate dimension for #10 USE

So, assuming stranded, using the conductor dim from CH 9, Table 8 and the insulation thickness from Table 310.13, the diameter is about .196 in. That puts the area at .029 in^2.

Per Ch 9, table 4, 1-1/4" EMT, at 40% fill, available cross sectional area is .589 in^2.

So you can pull 20 conductors and be below 40% fill.

Originally posted by dcdave:
Does the 9 cunductor rule apply or can more be allowed because its dc electricity?

Art 690.8 says to size conductors for 125%. So the NEC thinks the conductors could be under continuous load and get hot. And, DC current heats the wires just as well as AC. No reason why Table 310.15(B)(2)(a) wouldn't apply.

I did not see anything in Art. 690 that would give special forgiveness.

My response is pretty simplistic. Makes me think there may be more to your questions.

carl

[ January 09, 2005, 11:12 AM: Message edited by: coulter ]

 

[physis]

Re: dc current conductors

I don't think there's any distinction for DC.

I think the nine limit is based on practicle wire pulling limits.

 

[pierre]

Re: dc current conductors

If the conductors are THHN, Annex C provides the math for you and states 28 conductor (Table C1) Can be installed in 1 1/4.

I am not sure if this applies to DC as well as AC conductors.

Where did you find the "9 conductor" rule.

Pierre

 

[physis]

Re: dc current conductors

It was probably Table C-1 Annex C.

 

[coulter]

Re: dc current conductors

Originally posted by physis:
It was probably Table C-1 Annex C.

Sam - You're losing me. Table C1 doesn't have any limits on the number of conductors as long as they fit in the 40% fill.

As far as practicly pulling over 9, the Annex C listings for little wire and big conduit are practical.

And, Annex C wouldn't apply in the case anyway, since dc is asking about USE.

What am I missing?

carl

[ January 09, 2005, 12:07 PM: Message edited by: coulter ]

 

[russ]

Re: dc current conductors

I only know of one place with a 9 conductor rule, that's Chicago, not NEC.

 

[physis]

Re: dc current conductors

You're absolutely right. I'm not paying attention.

For some reason I'm thinking 12 gauge and EMT.

Just ignore me.

 

[don_resqcapt19]

Re: dc current conductors

Many people use a "9 rule" limit because at the tenth current carrying conductor you have a 50% derating. This often makes in impractical to install more than 9 current carrying conductors in a raceway.
Don

 

[petersonra]

Re: dc current conductors

just out of curiousity.

nfpa79 requires derating of current carrying conductors when more than three are in a raceway.

lets take a typical control panel. most of the wiring is typically routed inside plast wireway. i think this is a raceway. it seems to meet the definition of raceway.

has anyone ever seen even control panels that actually bother to derate wiring for having more then 3 current carrying conductors in a raceway?

i have had people claim that its free air because of the slots. I am not so sure.

 

[physis]

Re: dc current conductors

Do you mean derating conductors in a cabinet?

I don't think there's derating in boxes either.

Nope, I looked in Article 312 and it says to make sure they'll fit.

 

[steve66]

Re: dc current conductors

lets take a typical control panel. most of the wiring is typically routed inside plast wireway. i think this is a raceway. it seems to meet the definition of raceway

Only the power and lighting conductors have to be derated (310.15.2 Exception #1).