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I'm running electricity to a 8’ x 12’ “backyard office” shed that I bought. The shed came pre-wired and finished with internal outlets and lights and wall paneling, and I need to hook it up. I will be heating in the winter with a 1500W space heater and cooling in summer with a 710W window AC unit. No power tools.

The shed is only 3 feet from the house, but far from the garage breaker panel. I plan to put in a new 20-amp breaker. The internal wiring in the shed is 12/2, but I’m going to use 10/2 wire along most of the 102-foot run from the panel to the shed.

I will need to run cable up to the attic from the panel, across (above) the garage, down to the (unfinished) basement, along the length of the basement, through to the outside of the house, and into PVC conduit, which will go down 18 inches below ground, and then up to the side of the shed to a new junction box. There’s currently a bare 12/2 NM-B cable coming out the bottom of the shed, through the spray foam insulation under the floor, which I’ll protect via conduit to my new junction box.

I plan to bury two separate conduits in the same trench so I can also run Ethernet to the shed. I’m planning to add a weatherproof switch on the side of the shed.

I do plan to get this city-inspected, even though everyone complains that they’re a pain to work with.

I have two main questions:

  1. Should I use THWN-2 wire the entire way, or should I use NM-B most of the way (90’), adding a junction box to switch to THWN-2 just before the wire exits the house?
  2. Confirming that I don’t need a GFCI breaker/outlet since there are no exterior outlets or lights on the circuit, and my conduit will be 18” deep. I’d much prefer no GFCI to so I have the option of using my walking treadmill desk. Even though its motor is not very powerful, it trips GFCIs.
  3. Is there anything I’m missing?

I don’t see any point in the added expense and complexity of adding a sub panel on the shed, given that the shed is already wired with 12/2 and I’m never going to need more than 20A out there.

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  • Most detached buildings require a disconnect/sub panel with main breaker and two ground rods. Most electrical devices newer than 08 should work on GFCIs, if not, it might have a problem, than blaming the gfci. Something to check.
    – crip659
    Commented Mar 3 at 20:53
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    Having a GFCI will not harm you, but it might prevent harm to you
    – Traveler
    Commented Mar 3 at 22:51
  • Just like you have a Disconnect for your outdoor AC you should have one for the shed.
    – Traveler
    Commented Mar 3 at 22:53
  • Your space heater will consume 12 Amp, so you will only have 8 Amps for everything else, which might be enough depending. Probably most important is the grounding to make it safe.
    – Traveler
    Commented Mar 3 at 22:59
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    @davidk sounds like your real problem here is something wiht the standing desk. Does it need more than a normal single phase connection (check the manual) or perhaps is it faulty?
    – Criggie
    Commented Mar 4 at 4:14

1 Answer 1

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In general, a shed needs GFCI. I've looked a little and it seems that the terminology is "accessory building". I have a feeling you will need GFCI protection on the receptacles in the building because of that.

But more importantly, as with any detached building, you will likely need:

  • Disconnect - this can be a simple switch (inexpensive but not normally rated for outdoor use), an actual disconnect (you can use an air conditioner disconnect - they are very inexpensive), the main breaker of a "main" panel or up to 6 breakers (technically, "6 throws", which can actually be more than 6 breakers) of a subpanel.
  • Ground rods

It all really depends on whether the inspector considers this "a single circuit going out from the house" (in which case it is like an outdoor lighting circuit - no disconnect or ground rods needed) or "a feeder to an outbuilding".

As far as wiring, you can use NM cable and transition to wires for conduit to the outside, or you can use wires and conduit for the entire distance. For the 3 feet of outside conduit, you can go with PVC at 18" or metal at 6" (and the metal also counts as ground if you do it right).

However, I am a bit concerned that it sounds like this building is wired to be entirely on one 20A circuit. That is NOT a good idea. A 1500W space heater will use up most of that circuit by itself, and you will also have:

  • Computer, printer, etc.
  • Lighting
  • Other equipment

I highly recommend a configuration with a minimum of 2 20A circuits. That can actually be a 20A MultiWire Branch Circuit wired up with 12/3 cable or equivalent wire (i.e., shared neutral until a split in the building). With that configuration you can be reasonably confident that the lights will stay on even if something trips the breaker (whether overload or GFCI).

As far as the problem with your desk on GFCI, that is an indication of a fault with the desk, not with the GFCI. With any reasonably well-designed equipment, I would expect more problems with AFCI (as it is detecting "patterns" rather than a simple current) than with GFCI. In general these problems (GFCI and AFCI) should be going away as manufacturers get used to designing equipment to avoid problems - nobody likes returns.

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  • This sounds more like a finished (habitable) space than the typical shed interior, so I suspect the GFCI requirements for accessory spaces don't apply to it Commented Mar 4 at 2:39
  • @ThreePhaseEel You may be right. On the other hand, if it really has Romex sticking out of foam on the bottom, that isn't exactly to code. Really should have the the cable terminate in a box securely attached above grade. Commented Mar 4 at 2:42
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    Thank you for your very detailed response! It sounds like I just need to call the city and try to talk to an inspector to figure out whether I need the GFCI or not. I was planning to put a switch on the outside, but thank you for the tip about the air conditioning disconnect. The cable coming out the bottom should be long enough to reach a junction box on the side I hope. I hear you about the 20A circuit, but I can't change that without tearing out the wood-paneled, spray-foam-insulated walls. And I really don't have a lot of power requirements other than the heating and cooling.
    – david k
    Commented Mar 6 at 1:27

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