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I have an office / shed that I just built in my backyard. I just finished running wire out to the shed and I'm trying to figure out what size subpanel. I'll need. My plan is to have 3 lights (1 interior, 2 exterior), 4-6 outlets, and I will likely have a space heater for the winter months.

The dimensions of the shed are 12x10. 30 amps coming out of the house over 8AWG wire (2 hot, 1 ground, 1 neutral).

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  • What type of cable did you use? NM, UF, SER, MH? A cable is several wires wrapped in a sheath. I also assume copper since aluminum isn't really a player til #6 (because of that thing that happened). Aug 4 at 17:06
  • 8 Gauge Stranded Copper THHN Wire. I have 4 of them, 1 black, 1 red, 1 white, 1 green. Aug 4 at 17:12
  • Are the 4 THHNs in conduit? Are they also marked (they usually are) THWN or THWN-2?
    – Ecnerwal
    Aug 4 at 17:15
  • They are in conduit and are marked (had an electrician come out and confirm the hook up to the house was fine). I think it's just THHN (I bought them at HD) Aug 4 at 17:16
  • Virtually all THHN now sold is cross-listed THWN-2, so you should be OK for outdoor operation and 50A ampacity on those wires. You're allowed to use a 50A supply breaker. You're also allowed to use a 30A supply breaker since 30<50. By the way for 120V loads, that is 50A twice - so quite a lot of stuff actually. Aug 4 at 18:02

2 Answers 2

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Sub-panels are pretty inexpensive to start with, so go bigger than you think you'll ever need. I'd say at least a 6 space panel, 8 would be better. If, for whatever reason you needed to upgrade (maybe an EV charger?) later, a few dollars extra spent now for a decent sized sub-panel is way cheaper than upgrading a too small one later.

Again: rules for sub-panels:

  1. because it's a detached structure, it needs it's own ground rods.
  2. because it's a sub-panel, the neutral and ground must be isolated (not bonded, ie: not connected) The ground wire from the house (or wherever the main panel is located) will be connected to the ground bus bar in the sub panel, along with the locally installed ground rods.
  3. It's OK to have a sub-panel with a capacity greater than the feed to it.

There is a ton of good information regarding sub-panels here on SE. Do a few searches and you'll learn everything you need to know.

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  • George, this is such a helpful answer! Straightforward and clear. Thank you! Aug 4 at 17:24
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    6 spaces is nothing. As a detached outbuilding, a local shutoff will be required. They don’t make 6-space main breaker panels so two spaces will be eaten up by a backfed breaker to act as a shutoff (unless you spend more space and money on a separate disconnect). I wouldn’t consider anything less than 12, and you can often find these bundled with a couple breakers - cheaper than buying an empty 6 or 8 and adding breakers.
    – nobody
    Aug 4 at 20:16
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    Disregard the “X space / 2X circuit” labeling that’s still popular. Practically everything needs GFCI or AFCI now so using double-stuff breakers is largely a thing of the past.
    – nobody
    Aug 4 at 20:18
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    (Technically you may be able to avoid a local main disconnect via the “rule of six” - depending on your NEC version - but then you’re limited to… six handles. Just go big.)
    – nobody
    Aug 4 at 20:27
  • @nobody I did say "go bigger than you think you'll ever need", but to call out a 20 space panel for a 12' x 10' shed seems like massive over-kill. In retrospect, 6 space would be too small if there was a main disconnect involved. 8 would be the minimum and 12 would be better still. But 20 space panel for a small shed?! I can't agree with that. Aug 5 at 0:01
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What size subpanel for an office / shed

As in, physically?

Put a 20-slot out there.

Put lights on one circuit and put your outlets on alternating circuits so that you don't accidentally overload your space heater's circuit.

30 amps coming out of the house over 8AWG wire (2 hot, 1 ground, 1 neutral)

Is this the maximum calculated amperage given the distance and temperature conditions of the wire run?

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