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I want to run a window AC unit in Summer And an electric heater in winter. That for one 20Amp circuit. Other circuit for lighting and miscellaneous power tool operation. The 20A breakers square D Homeline type GFCI and CAFI with plug on neutral. Home Built in 85 with older Square D 100Amp panel. Do I need a sub panel and extra grounds being these are runs with ground back to house.

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    Are you trying to avoid putting a disconnecting means at the shed, or what's your rationale behind this setup? – ThreePhaseEel May 26 '20 at 4:08
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    I'm amazed at how far this question got with such a stubborn avoidance of verbs. They can really help clarify your question. – J... May 26 '20 at 12:38
  • Those materials I had lying around. Trying to avoid spending a lot of funds. If I sold that wire and bought 10/3 or better could I do the two separate 20Amp circuits. I believe that would suffice my needs. They make an inexpensive grounding rod now I guess I can use 2 of those with a sub panel. – TexasEagle May 26 '20 at 13:34
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    Don't direct bury UF-B to an outbuilding. You'll regret it. Run an excessively large 1.5" or larger conduit and future proof yourself. Buy once, cry once. – whatsisname May 26 '20 at 14:58
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    8/3? Sounds like you are plan on pulling UF in that in 1" conduit. You may have a fill capacity problem. – NoSparksPlease May 26 '20 at 23:52
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You can't run 2 separate circuits of the same type to an outbuilding.

If they are 120V circuits you could run the two as a multi-wire branch circuit, which counts as 1 circuit. You would need 10/3 cable for that.

Alternately, you could run a 10/3 or larger cable and feed that to a subpanel. That may be more of a production than you want to get into, because it'll mean selecting a subpanel with a main breaker*, as well as driving two grounding rods.

* Actually the purpose of the main breaker is to be a disconnect switch. You need one. Choosing a main-breaker panel is the cheapest way to obtain this. You don't care what the value on the main breaker is. It could be 200A for all you care lol.

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    They'll also need a disconnect at the shed (for your MWBC approach, a non-fused AC disconnect box is sufficient) – ThreePhaseEel May 26 '20 at 4:09
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    @ThreePhaseEel Even for a single circuit? – Harper - Reinstate Monica May 26 '20 at 13:31
  • Yes, even for a single circuit, see NEC 225.31. – ThreePhaseEel May 27 '20 at 0:04
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No you can't run two feeders from the same panel, the NEC only allows one feeder of a type to a building.

If everything in the building is on a single circuit there is an exception that allows you to not install ground rods, but if you install a panel with just two branch circuits you need to install ground rods (or other qualified grounding electrode).

Edit Re 1" conduit and 8/3: Don't use cable assemblies in conduit. Wet location cables don't slide. If you need only two 20A circuits you can run a Multiwire branch circuit on a two pole breaker and only need 3 conductors plus a ground. Use THWN conductors, 4 @ #10 will slide right in, you could even fit 4 @ #4's in a 1" conduit and you don't have issues stripping the jacket for termination.

If you've decided on using #8 for more capacity than you have already asked for realize 8/3 UF cable won't even fit. Multiconductor cable conduit fill is calculated major dimension as a round cable, maximum 53% fill. 53% capacity of 1" sch 40 PVC equals 0.441 in². Southwire 8/3 UF measures 1059X319 mils, besides being calculated as .880 in², the obvious problem without a calculator is 1.059" won't even fit in a 1" conduit. If you can find copper SEU it's square, not flat, measures 0.645", so it equals 0.332 in², it would fit legally, but still won't pull in without a big truck. You can't use NM (Romex) in outdoor conduit. 10/3 uf will fit in a 1" conduit (.727/2*3.14=.415, 53% fill max=.441) if you can find a truck big enough to pull it in. The stuff just drags, not made to slide.

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  • Tip -- working conduit fill calculations in metric units makes the numbers much easier on the eyes :) – ThreePhaseEel May 27 '20 at 1:18
  • Could be, if Southwire cut sheets were in globalist units. mysouthwire.com/medias/… – NoSparksPlease May 27 '20 at 1:35
  • Yeah, I admittedly convert cable dimensions into metric when I do the fill calcs myownself – ThreePhaseEel May 27 '20 at 1:36
  • I'll give it a try next time, a 30 year habit is hard to break. – NoSparksPlease May 27 '20 at 6:49
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Fat conduit and individual wires are your friends here

First, I would get some 1.5" PVC conduit and drop it into that trench, alongside the 1" conduit you already have on hand. This will give you enough room for a 125A(!!!) feeder to the shed in the future, and make use of your existing 1" conduit to provide a communications duct to the shed for future TV/networking/telephone service.

Why? Because trenching is hard, and having to dig up existing wires to upgrade them is harder and more laborious yet. It is far better to provision ample space initially than to put in something that's merely adequate for now, only to have to rip it up completely when you need more power at the shed later.

As to what goes inside that conduit? Not UF, that's for sure!

Most people who have a basic knowledge of residential wiring try to jam NM or UF down conduits because that's the only kind of mains wire they know about. However, stuffing cables, especially flat ones, down conduits gobbles up valuable conduit fill atop being an absolute pain in the rear end for the conduit-stuffer, and as a result you never see the people who actually use conduit on a regular basis doing that. Instead, they use individual wires in conduit whenever possible; in particular, for mains use, the wire normally used is what's called THHN/THWN. These wires have a nylon coating on them that makes them much easier to pull through conduit, and also are available as stranded wire, which makes them far more flexible than the wires in NM cable.

For your application, in your PVC conduit, I would pull 4 10AWG stranded THHN wires (black/black/white/green or black/red/white/green are the obvious choices, but the hots can be any color besides green, grey, or white, and don't have to be distinguished from each other either) to get you your 30A feeder.

GO BIG OR GO HOME

The other mistake people setting up outbuildings make is provisioning an undersized panel. Spaces are cheap now, compared to ripping out and replacing an electrical panel, so there's no reason to be stingy! I'd get a 24-space or 30-space, 125A, main breaker panel and a set of accessory ground bars, if need be, if I were in your shoes; this provides the Code-required disconnecting means at the shed while leaving you plenty of space for whatever future expansion is desired.

With that panel, you'll need to run some 6AWG bare copper down to a pair of 8' ground rods driven a minimum of 6-8' apart. This provides a grounding electrode system for your shed, so that wayward natural electricity can get returned to terra firma before it messes up anything else. This bare copper connects to one of the panel's grounding bars, along with the incoming grounding wire from the feeder. Alongside this, the neutral-ground bonding screw or strap is removed (or simply left uninstalled) in the shed's panel, so that wayward utility electricity takes the correct route back to the utility, namely via the equipment grounding system.

Sidebar: If you really can't afford a panel...

If putting a subpanel in the shed is not in the cards, since you only need a single multiwire branch circuit, you can omit the grounding electrode system described above (it's easy to put in ground rods later); however, you still will need a disconnecting means for the shed. The easiest way to deal with that issue is to use a non-fusible pullout disconect box of the type commonly used for air conditioners; these cost all of $10, although you will need a conduit reducer fitting (swedged reducer) if you are trying to hang one off a 1.5" conduit.

Inside this pullout disconnect box, the incoming hots go to the line terminals, your branch circuit hots go to the load terminals, the grounds land on the grounding (bonded) bar, and the neutrals are simply wirenutted together, as there's no isolated bar available for them, but NEC 312.8 permits such a splice in a cabinet or cutout box anyway.

You don't need to get fancy here, though

The good news, however, is you don't need a bunch of fancy breakers for this job; a detached shed does not need AFCI protection, for one. Furthermore, you're better off GFCI-protecting the shed receptacles specifically using a single GFCI receptacle at the shed (hint: this is why GFCI's have LOAD terminals), lest a GFCI trip put you in a dark and scary place close to a still-spinning saw blade!

As a result, you can feed the run to the shed from a single two-pole, 20A breaker if you're using the multiwire-branch-circuit + disconnect box approach, or whatever size is appropriate to protect your wires if you're using a subpanel at the shed. If your breaker box is short space, by the way, post another question on these boards with photos of it and we should be able to help you with your space crisis.

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