Running DC in Flex Conduit through an attic

[Stevea]

If I'm running two strings from the array through an attic down to an inverter, can I run them in flex conduit if they are DC power? If so, what size flex conduit is the max I can use. My thoughts are that I would use two separate flex conduits with the 10 gauge wire already included in them by the manufacturer so I don't have to thread it in there.

I found some information that suggested, I might need to attach some strip guard to protect firefighters from cutting through them. Even f I meet that requirement, do I still run afoul of heat building up in the conduit if the conduit is too thin? Like say, less than 3/4 inch?

[solarix]

You can do that. There is a new code rule that makes you put it at least 10" below the roof deck, so that firefighter don't saw through it. We usually use 3/4" flex, secured every 4' and secured where it enters and exits the roof. Also needs warning stickers on it every 10' to make sure everyone is aware of the evil solar power flowing through it. We usually use weird colored wire (orange/brown, yellow/blue THHN) on the DC run (transformerless inverters with isolated dc side) to make sure if any electrician gets into it someday, they will think twice.

[foo1bar]

1> What type of flex conduit are you considering using?
LFMC? FMC? nonmetallic FC? (I hope not - but you weren't clear)

2> I think EMT is cheaper and what is widely used - so why not use that? You can probably do any shape you need in the attic with just fittings and straight pieces of EMT.

[Stevea]

Metallic flexible conduit. It is solid MC cable. 10 gauge. Each string in its own metallic MC cable run. What if it is less than 3/4 inch. It doesn't actually say on the MC cable packaging what the size of the cabling is, but I'm pretty sure it is thinner than 3/4. Am I getting into trouble here?

[foo1bar]

Metallic flexible conduit. It is solid MC cable. 10 gauge. Each string in its own metallic MC cable run.

Which?

MC Cable is different than Metallic Flexible Conduit.

Solarix is referring to 3/4" flexible metallic conduit with individual wires pulled through it. the "3/4" is the nominal size - like lumber you have a "2 by 4" that has actual dimensions of 1.375 inches and 3.375 inches.

I think MC Cable is allowed by code since it has the metal cladding - but I would think it will be more expensive.

[Stevea]

I don't know why I said "solid MC cable." It is flexible MC cable. It seems like it is maybe half the diameter of the 3/4 inch flex conduit...and it doesn't look like there would be any room for the wires to breath. But, then again, why put those wire in conduit (MC cable) if it wouldn't perform. But, then I got to thinking maybe it was designed only for AC electricity? But, you think the MC cables would be fine for DC even if only 3/8 inch or half inch? Is that what I'm hearing?

[foo1bar]

I think it is possible to use MC cable for attic DC wires. I'm not an electrician - but this says it can: http://www.solaredge.com/files/pdfs/...e_building.pdf

You will need to calculate whether the wires are sized correctly when derated because of the attic temperature.
And you will need to make sure you are properly terminating the cable. That you're using the right clamps to attach it to the boxes and that you are doing the proper thing for not having the wires potentially damaged by the metal cladding.

BTW: It's called MC cable - not flexible MC. It's Metal Clad (MC) not Metal Conduit; AFAIK Flexible is used when refering to conduit - not cable.

If it were me, I would not use MC cable.
I would use EMT.

[inetdog]

I don't know why I said "solid MC cable." It is flexible MC cable. It seems like it is maybe half the diameter of the 3/4 inch flex conduit...and it doesn't look like there would be any room for the wires to breath. But, then again, why put those wire in conduit (MC cable) if it wouldn't perform. But, then I got to thinking maybe it was designed only for AC electricity? But, you think the MC cables would be fine for DC even if only 3/8 inch or half inch? Is that what I'm hearing?

MC is a specific cable (not raceway) type recognized by the NEC based on UL standards.
The biggest thing you have to be aware of is that the spiral sheath of ordinary MC is NOT rated as a grounding conductor (EGC) so you have to have a green or bare wire inside the MC for that purpose. As long as you take that into account it is just as suitable for DC as for AC. You may need to tape the wire ends to make it conform to the appropriate color code for grounded or ungrounded DC as appropriate.
The wires do not need to "breathe" since all of the heat is going out through the metal shell anyway.