6/3 NMB in non-metallic conduit?

Question

I’m running a 50A circuit using 6/3 NMB from my interior panel to a hot tub. I’ve got the required GFCI protected disconnect within direct line of sight of the tub (about 15ft from the tub). I have 3/4” LFNMC that is planned to use once the wire penetrates to the exterior of the crawl space. My question is....is it acceptable to run this cable through the conduit if I remove the romex jacket? It would need to be in the conduit for about 8ft from the crawl space to the disconnect and then about 25ft or so to get to the connection inside the hot tub. Thanks in advance for your help!

Answer 1

Don't make your life harder than it needs to be!

First off, shucking NM of its jacket is no good, because the wires inside NM cable aren't marked properly for individual use. Second, pulling NM through outdoor conduit is unacceptable because NM is no good in wet areas due to the paper separator layer between the jacket and the wires (which wicks moisture up the cable, leading to hard-to-troubleshoot failures later), and all outdoor conduit is considered a wet location by NEC 300.9:

300.9 Raceways in Wet Locations Abovegrade. Where raceways are installed in wet locations abovegrade, the interior of these raceways shall be considered to be a wet location. Insulated conductors and cables installed in raceways in wet locations abovegrade shall comply with 310.10(C).

Third, there's absolutely no reason to make your life this hard: pulling 4 6AWG THHN wires through your conduit (hot, hot, neutral, ground) will be far easier than trying to jam a cable through there. So, I would simply get an appropriate amount of 6AWG THHN (black, white, green is fine) and use that instead.

Answer 2

You can shuck it if and only if the internal wires have markings, and the markings are appropriate for this use. For instance if the interior wires said "THWN-2" that would be good enough. But they won't. Partly, THHN has a magical nylon outer coating which makes it slippery for pulling through conduit. Without that, good luck!

You're not allowed to assemble the conduit around the wire. So you can't slide the conduit on one stick at a time. You must finish the conduit pipe, then pull the wire through it in one shot.

NM will not fit in 3/4" conduit

Aside from the complete horrorshow of trying to drag stiff NM cable through conduit, it simply will not fit. You would need bare minimum 0.85" inside diameter, and that is not available from any 3/4" conduit of any kind.

And as ThreePhaseEel says, you can't use NM cable anywhere outdoors, so that's the end of that.

Outdoor-rated UF-B won't fit. Worse.

The outdoor rated version of NM is called UF-B cable. Aside from the horrorshow of pulling that through conduit, UF really, really won't fit. It needs a pipe minimum 1.68" inside diameter, and that means we need to kick up to 2" conduit. Are you kidding me?

Enough with the masochism. Back to the store the cable goes. Get yourself THWN-2 wire.

Or use cable as cable is meant to be.

If you're none too thrilled about returning already-cut NM cable, then it's a simple matter. (but next time, buy the wire last!)

You can transition from one wiring method to another in any (large enough) junction box. Run the NM cable as cable inside the house. When you get near the outdoor transition, go ahead and install a junction box indoors. Continue in conduit from that junction box. In that conduit (junction box through disconnect through tub), run THWN-2 type individual wires. The neutral must be white or gray. The ground must be bare, green, or yellow/green. The hot wires can be any other color(s) including black-black.

You will neeed 35 cubic inches in this box, so use a 4-11/16" deep square box. The junction box must remain accessible forever, so it'll need to be in an attic, basement or just an unsightly blank cover somewhere. You could put a recep in that cover, but you can't feed it from this #6 cable so you'd need to bring in a separate feed for the recep.

Liquidtight isn't

There simply is no such thing as water-tight conduit outdoors. All outdoor conduit pipe is presumed to be 100% full of water 100% of the time. So liquidtight doesn't buy you anything unless Code requires it, but it still is not counted on to keep out water. The moisture defense is in the skin of the wire itself. Further, by being a flexible conduit, you don't get to use it as a ground wire as you could with regular metal conduit.

The GFCI

There is no requirement to have the shutoff switch also be a GFCI. These are totally separate functions: the shutoff needs to be near the hot tub, and the GFCI needs to be anywhere it can protect the circuit. I for one am a fan of putting GFCI indoors (e.g. as a GFCI breaker in the panel) then use a plain disconnect switch. Being indoors is easier on the electronics.