I am wiring a 36' x45'shop that will store my RV and other equipment. I have 200 amp service already in. I want to put utilities in under the concrete pad before I have it poured. I would like to run a couple of circuits under floor so I won't have to use some much wire. I would like to run 6/3 & 12/2 romex inside 1 1/4" gray pvc. One circuit will be for my RV, one for a welder, one for a electric range. I'm wanting to add a 110 circuit at each of these locations is the reason for running 12/2 wire. I have already bought the wire and pvc for this project.
I have had a circuit run under my deck with 6/3 romex inside a 1" pvc pipe that worked fine for years. Some say you can run romex in pvc, some say you can't.

  • Do you already have the subpanel and feeder for the shop sorted? – ThreePhaseEel Jan 27 at 17:17
  • How much wire run are you saving by going under and not over/around ? I'd be leery of locking it away in the slab. Overhead is so much easier to access, and remember water goes down just about all the time. – Criggie Jan 28 at 7:05

NM in conduit under a deck (or a slab) is NFG

You cannot legally run NM cable in a wet location, and the inside of a conduit in a wet location (such as under your deck, or buried beneath a slab) is still a wet location. Don't believe me? Well, we start with NEC 300.5(B) for what's going on sub-slab:

(B) Wet Locations. The interior of enclosures or raceways installed underground shall be considered to be a wet location. Insulated conductors and cables installed in these enclosures or raceways in underground installations shall comply with 310.10(C)

and 300.9 for the under-the-deck case:

300.9 Raceways in Wet Locations Abovegrade. Where race‐ ways 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).

then move onto the fact that 334.12(B) point 4 prohibits the use of NM in wet or damp locations, as NM has a paper separator in it that will wick moisture up the cable, leading to failures.

What you should do instead

While you could use UF cable instead of the NM inside your conduits, that's not a good idea because cables:

  1. Gobble up astronomical amounts of conduit fill compared to individual wires -- the fill calculation for a cable in a conduit is run by treating the cable as a round cable with a diameter equal to the major axis dimension of the cable's cross section, as per Chapter 9 Note 9 in the NEC.
  2. Are quite stiff and hard to pull down a conduit compared to individual wires -- you'd rather not call an electrician in to bail you out of your conduit pulling now, would you?

What's done instead is to use individual wires (THHN/THWN-2) inside the conduit. Since you're using PVC, you'll need 12AWG and 6AWG THHN in black and white (twice as much black as white for the 6AWG, even), as well as 10AWG bare copper for ground wires (as table 250.122 lets you use 10AWG copper for grounds on circuits up to 60A).

The good news is that conservatively assuming your PVC is schedule 80, you have 178mm2 of fill available in the existing 1" PVC under the deck and 320mm2 of fill available in the 1.25" PVC you are putting in sub-slab. The three 6AWG THHNs for the 60A circuit use up just under 100mm2 of that fill, and the 10AWG EGC adds just over 5mm2 of fill, leaving just under 75mm2 of fill spare in the 1" conduit. In the 1.25" conduits, we add just over 17mm2 of fill for the 12AWG wires to this, leaving just under 200mm2 of fill spare, which should be plenty.

Furthermore, since we have a maximum of 4 current-carrying conductors in the 1.25" conduits (the neutral in the 3-wire 6AWG branch circuit does not count for this, as per 310.15(B)(5)(a)), and since THWN-2 wire (what most THHN is dual-rated at these days) is rated for 90°C in wet locations, we can use that as our starting point when applying the 80% ampacity derate from Table 310.15(B)(3)(a), yielding a derated ampacity of 60A, which is still in excess of the 55A that the 60°C rating on most receptacles limits us to. The 12AWG wires do not suffer either, as their derated ampacity is 24A, which is again above the 60°C ampacity limit for 12AWG (aka 20A), as well as the 240.4(D) small-conductor limit on 12AWG wire (again, 20A).

While you are pulling all this wire, I would replace the existing NM cable inside the 1" conduit under the deck with individual THHNs + bare ground of the appropriate size, by the way. That NM will only be fit for the scrapper, for that matter, since it will have suffered internal damage from the water exposure, which is why it shouldn't have been installed there in the first place.

  • 1
    Fully agree, I have seen NM wick water into a home a drip, drip drip inside the home in an electrical box is not a good thing.+ – Ed Beal Jan 27 at 18:42

Don't hold back on the conduit

Conduit under the slab is a great way to get wiring around your shop. Feel free to lay extra conduits to everywhere you wouldn't want to run cable in the future, certainly to every wall that's between doors.

The rules limit you to no more than 4 circuits in a conduit (that are practical), and in some cases you are limited to 3. So don't imagine one giant conduit with 20 circuits in it.

Cables in conduit, an understandable error

Generally, people who wire houses use a lot of cable such as "Romex", whose proper name is NM. So when they think about using conduit, their first impulse is to put the cable in conduit. "What else would you use?"

Well, a big part of cable is that plastic sheath, which you are familiar with. It bunches up several wires and protects them. But that's what conduit does, so a cable sheath is pointless. Also, when pulling wires in conduit, you want wires that are both flexible and slippery, and cables are the very opposite of that.

Also, cable requires must wildly oversizing the conduit, especially with 2 or more cables because they snag horribly on each other due to being both flat and inevitably twisted. You also hit thermal limits very quickly with NM cable.

There are other types of cable rated for direct burial (without conduit), those are also acceptable in conduit in wet locations. Cable in conduit isn't illegal, just wasteful and a lot of extra work.

The right wires (the only wires)

What's designed for conduit is individual wires. At first glance they look like a wire out of Romex, but they're not. The standard wire has a big name, THHN/THWN-2. That means it's dual-rated for both wet locations and high temperature, Romex is neither. The temp rating means you can run more wires in the same conduit, up to 4 full circuits without upsizing wire. THHN is also more flexible (quite flexible if you use stranded wire!) and it has a slippery nylon sheath.

You can change a circuit from THHN to cable inside any junction box.

As ThreePhaseEel discusses, Romex/NM is not allowed in wet locations, such as conduit in/under slab. THHN/THWN-2 is just fine (note the "W").

You might occasionally see a wire called XHHW/XHHW-2. That is overkill, the difference is a yet higher heat rating, and it is somewhat more flexible which may help on fat wires.

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