6/3 nm-b short run outside to spa box in conduit

Question

Our builder left about 100 ft. of 6/3 nm-b wire when our house was built. I hope I can use it to connect my basement panel to a spa box. The spa box will be on a suspended, fully covered deck about 14 ft off the ground.

I have read that you should never run nm outside and in conduit. However, I have also read that if the run from the dwelling to the outdoor panel is short, it can be acceptable in NC, where I live.

The thought is that the NM feeder, once outside, will be just under the deck and come up through the deck boards and into the spam box. The run will be less than 6 feet outside and will be run in 1" conduit to the box. From the spa box breaker to the hot tub, 6/3 THWN in conduit will be used.

Answer 1

There are two issues with NM cable in certain locations:

• Physical Protection - conduit can provide that.

• Wet Locations - conduit does not keep the wires or cable inside dry. In fact, conduit can actually collect water due to condensation.

So the big problem is wet location, and nearly every place outside is considered a wet location. There is, as I understand it, a bit of a debate about whether a cable going through a wall into the back of a box is even considered a wet location, but that seems to generally be allowed. But once you have cable actually outside the walls before it gets into a box, that is pretty clearly a problem, even inside conduit.

It may be worth checking with the local electrical inspector's office to find out whether this is considered acceptable. If not, or if you can't get a straight answer until the day of inspection, your best bet is to put a box inside the wall to transition from NM cable to THWN in conduit for the 6' run through the wall and outside.

Answer 2

NM cable is no good for any length outside - when the paper wadding gets wet, it wicks down the wire and the water rots out the insulation. We know that's real, because it trips GFCIs.

AHJs (inspectors) tend to turn a blind eye when the NM simply comes straight through the wall into a box mounted on the surface of the wall. GFCIs, however, will not be so forgiving.

PVC schedule 40 conduit does not provide physical protection. See NEC article 334.

When you have the wrong thing, use Craigslist etc. to sell/buy the right thing. Any novice doing an EV circuit will want 6/3 NM. Not really right for home charging, but they don't know that!