sending electrical wiring underground from house to shed [duplicate]

Question

I have a shed about 60' from my home on my property in Northern CA. I'd like to send two 20 Amp circuits from the house to the shed via a buried 1" Schedule 40 PVC pipe.

I've read that I can't/shouldn't send non-metallic cable through the PVC but can send individual wires. Is this true?

If so, can I simply strip the yellow sheathing from the cable and send those wires through? Or do I need to use different type wires? If different what type of wires do I need?

Answer 1

1. If you're going to have 2 circuits in the conduit, one of them must be switched on the supply side (i.e. at the house).
   • You can run a MultiWire Branch Circuit (MWBC) composed of two hots (on adjacent breakers with common trip and handle ties). There are lots of questions here that will give you more info on MWBCs.
   • This would allow the two circuits to share a single neutral and avoid the "shut off in the house".
2. You can run cable through conduit, but it's more difficult to pull, therefore it's not recommended.
   • NM-B (or "Romex"), however, is not rated for wet areas and cannot be used in underground conduit. It could be used in indoor conduit, but, again, it's more bother than it's worth.
3. You can NOT strip the sheathing from your NM-B (Romex) cable and use the result in conduit. The individual wires in the cable aren't labeled for individual use.
4. You want THWN/THWN-2 wire. This is rated for wet locations and all buried conduit is automatically presumed to be filled with water.
   • I've seen it suggested (somewhat jokingly) that you just go ahead and fill the conduit with water once it's in the ground, just to get it over with. It's not necessary or suggested, but a good reminder that it's going to get wet in there.
   • You'll want a spool of Green for ground, White or Grey for neutral and any other color for hot. If you're running two circuits, White & Grey are good options.
   • You can share the grounds between circuits. If running a MWBC, you can share the neutral.
5. 1" conduit is more than sufficient for 6 wires and is a good idea because it gives you the option to upgrade later by simply pulling out your #12 and pulling through something bigger to run a subpanel in the shed so you can have more circuits.
6. Make sure you get it buried deep enough. There should be 18" of cover over the top of the conduit, except in certain situations, like passing under a driveway. If you're doing anything like that, ask a separate question about cover.

Answer 2

"Romex" (a brand name of a NM/B type cable) is not rated for wet locations.

You cannot strip the sheath because the wires inside the sheath are not rated at all without the marking on the sheath, and the sheath (jacket) needs to extend at least 1/4" into every junction box that a cable enters. And they were not rated for wet service in the first place.

So, every outside conduit is defined as (and usually is) a wet location. You need a wire type that is rated for a wet location - THWN, THW, XHHW, etc. The "W" is the relevant part. In smaller sizes THWN or THWN-2 are most common. Most "THHN" is also marked as THWN or THWN-2 (and possibly also MTW, and various other ratings)

For 20A at 60 feet a single Multi-Wire-Branch Circuit (MWBC) running on 4 12Ga Copper wires will do. One green or bare for ground, one white or gray for neutral, and two from one or more of the other colors for hots. They will connect to a two-pole 20A GFCI breaker in the house, and something to act as a local disconnect/shutoff at the shed.

If any of the conduit is above-ground, that part needs to be Schedule 80 for protection from damage. The top of the conduit (not the bottom of the trench) needs to be at least 18" below ground (more if a driveway is being crossed, less if protected by concrete...) Frankly I'd suggest just using schedule 80 for the whole run, it's not that much more expensive from a proper electrical supply (orange or blue boxes generally won't have any at all) and it's much sturdier.