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I will be extending a GFCI-protected 15A, 120V circuit from my garage to a new plastic shed, just 6 ft away. I will be exiting the garage wall into a PVC service ell, down into the ground via 1/2" PVC conduit, and going underground to the shed via 14/2 UF cable direct-buried 12" deep (permitted by NEC 300.5 Column 4). I will enter the shed via similar means.

Inside the shed, the wiring needs are simple - a couple of outlets in convenient locations, one light, and a light switch. It will be used mostly for storage so I just need lighting and battery charging. All wiring and boxes will be surface mounted as there is no wall thickness to speak of.

Now the question. How should I wire inside the shed?

  • I know I cannot run exposed, surface mounted NM or UF. Can I simply use metal handy boxes and MC cable?
  • Or is the shed considered a damp location, and must I use all weatherproof boxes/switches, PVC jacketed MC cable, and threaded cord grips for all connections? (ref this question)

Photo of my shed and garage (new unconnected electrical service on the back of the garage is unrelated):

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  • Is there a reason you're laying UF to the shed instead of conduit? Is this all basically a temporary setup? Also, what does your local jurisdiction deem as "subject to physical damage" for cables in unfinished spaces? – ThreePhaseEel Sep 15 at 23:37
  • No, it’s permanent, but burying the cable instead of conduit is just slightly easier and I really don’t expect to need to run additional circuits in the future. Even if I do, it’s a short run that I can easily dig up. I haven’t had much interaction with my LAHJ so I don’t know their opinion on that. Regardless, I’m not really willing to run RMC in there... – Dan A. Sep 15 at 23:46
  • Who is your AHJ, even? – ThreePhaseEel Sep 15 at 23:49
  • St Paul, Minnesota – Dan A. Sep 15 at 23:50
  • What makes you believe you can't run surface-mounted NM or UF within the shed? Physical damage concerns, or...? – ThreePhaseEel Sep 15 at 23:53
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The provision you're using is a bit of a fool's errand for most detached structures

The provision in the NEC you're relying on (using GFCI protection at the source end of the buried run to allow for a 12" burial depth with UF cable) is a bit of a fool's errand when feeding a detached structure, as it turns out. Why? As it turns out, it's not terribly uncommon for things to develop ground faults (vs tripping breakers, even), and if something does trip the GFCI, you'd really rather not have it turn the lights out on you, especially if that something is a circular saw with a rapidly spinning blade on its business end that has a bit of momentum to it.

As a result, it's normally better to accept having to bury the cable or conduit deeper in exchange for having the lighting on a non-GFCI branch, with a GFCI receptacle or deadfront inside the shed providing protection to the receptacles there. If you're willing to live with that limitation for now, though, given that all you're dealing with are lights and the occasional battery charger, there still are a couple of other points you'll need to keep in mind.

You'll need some sort of disconnecting means at the structure

You'll need a disconnect where power comes into the shed. If you want an outdoor disconnect, the easiest and cheapest way to do that is using a non-fused pullout disconnect of the type used for air conditioner outdoor units; you could also use one of those inside the shed if you don't care about anyone ever finishing the inside, or you could use a 20A key-operated lightswitch such as a Leviton 1221-2WL in an ordinary box for an indoor disconnect instead since you are only dealing with a single branch circuit.

From there, you don't need to worry about damp

Since this shed has a reasonably functional roof on it, you don't need to treat it as a damp location per se, but there is some cause to be concerned about physical abuse given that there are no studs to speak of in such a shed, so it's much easier for boxes to pinch cables. As a result, using MC or EMT is not an unreasonable decision in your situation. Note that push-fit connectors are available for both wiring methods, so you don't have to worry about setscrews, clamps, compression nuts, or such; however, you will need to use metal boxes unless you want to do all the inter-connector bonding yourself. If you do go the cable route, by the way, look at MCI-A type cables (MCAP™ or equivalent) -- these combine the advantages of MC and AC cables in that they need neither the ground wire makeup required with traditional MC nor the "redhead" anti-short bushings required with AC cable; some products even manage to eliminate the normal assembly tape from the equation.

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  • Thanks! This raised some good points (GFCI, disconnect) that I wasn't aware of. I agree with your point on the GFCI, unfortunately I don't have much of a choice in this instance since it's not practical for me to run a non-GFCI circuit to my shed - the entire garage is already on GFCI and it's impractical to run another circuit from upstream. I appreciate the help! – Dan A. Sep 21 at 2:22

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