I have an outdoor shed that was powered with a single 15A circuit by the previous homeowner. I'm trying to rewire it to carry three 20A circuits. It's about a 100' run from my main panel via 1/2" schedule 40 PVC. I would like to run (7) #12 THHN for 3 hots, 3 neutrals, and 1 shared ground. I would terminate at the shed with a subpanel, three 20a breakers and a ground rod. From what I understand, seven #12 wires should work fine in 1/2 schedule 40 PVC without derating. Am I missing anything or good to proceed? Alternatively if I only wanted two 20amp circuits, could I run a multiwire branched circuit with 2 hots on opposite phases, a shared neutral, and shared ground to a disconnect at the shed without worrying about a ground rod?
You can't parallel wires, or run a bunch of circuits between two buildings
It's illegal to parallel wires skinnier than 1/0 (NEC 310.10(H)), and also illegal to run more than one of the same kind of circuit (same voltage, similar/same usage) to an outbuilding as that violates NEC 225.30. So, both your original plans are of no use; practically speaking, this means you need to run a feeder to the outbuilding, and fit a subpanel there, as well as install grounding electrodes.
However, a feeder is your best plan anyway
Your proposal gives you 60A at 120V, and no 240V, at the shed. However, with a feeder, you can fit 50A at 240V down the same 1/2" PVC, with fewer wires, and more room for expansion at the subpanel. In particular, your 1/2" Schedule 40 PVC has 74mm2 of usable fill area, which is enough room for 2 8AWG stranded copper THHN hot wires, a 10AWG stranded copper THHN neutral, and a 10AWG bare stranded copper ground, for a total of 66mm2 of fill used. While this setup requires care with balancing the loads at the subpanel to avoid overloading the undersized neutral, it's the largest feeder that can be practically crammed into a 1/2" conduit without exceeding fill limits.
As to subpanel selection...
If you don't want to be changing panels out later, your best bet is to get the correct subpanel now, and that means not skimping out. You'll want a 100A or 125A, main breaker (to provide a shutoff at the shed), 24 or 30 space (note: space, not circuit) panel for this job; this provides plenty of room for future expansion, including 240V circuits. In fact, you're best off running your shop receptacle circuit as a 20A multi-wire branch circuit using a two-pole ground fault breaker; this provides you with easy access to 240V for power tools that want it. On top of this, you'll have a 120V circuit for the lights and a second 120V circuit for the window unit, at minimum.
...and grounding this
You will also need to fit a grounding electrode system at the shed; typically, a pair of 8' ground rods driven 8' apart and connected to each other and the subpanel ground bar with 8AWG bare copper wire will do the trick. Don't forget to pull the green bonding screw out of the subpanel and fit dedicated grounding bars to it while you're at it, though!
TORQUE ALL LUGS TO SPEC
Last but not least, you'll want to use an inch-pound torque wrench or torque screwdriver to tighten all breaker, busbar, and panel lug screws to their labeled specification torques. Not only is this a requirement of the 2017 NEC, found in 110.14(D), it's a good idea anyway, lest your electrical system give you the loose lugnut!
The NEC only allows a single feeder to a building (NEC 225.30). If you want seven 20A circuits you would need to run a single set of 4 larger wires to an electrical panel, drive a ground rod (or rods depending on local interpretation) and break out your circuits from there. A good plan would be 3/#6 + 1/#8 ground being fed by a 60A breaker in 3/4" PVC.
If the single feeder consists of one "Multiwire Branch Circuit", being two 20A circuits being fed by 12/3+g UF (or 4 #12 THWN wires in a conduit) being fed by a two pole 20A breaker then you would still need a disconnecting switch where the feeder enters the building, but a single Multiwire Branch Circuit allows not driving a ground rod. NEC 250.32
Your "3 hots + 3 neutrals terminating in 1 subpanel" is paralleling. You can't do that, NEC 366.20, unless a bunch of things are true, starting with
- The wires must be 1/0 or greater (so 125A per parallel)
- The supply end of the cable must be special equipment rated by the manufacturer specifically for paralleling. Typically this special equipment will have overcurrent protection appropriate to the task; for instnace mine has six 400A fuses in it, each protecting a wire.
- etc. many more.
So that's Right Out.
You cannot have multiple circuits to the same destination, as I discuss here, except for certain exceptions which I also discuss, such as different voltage: For instance a plain 120V circuit and a 120/240 MWBC are acceptable. Or different uses: for instance an always-on 120V circuit, along with a switched-from-the-house 120V circuit (e.g. for lights; this can be 20A and have receptacles too, I won't tell ;)
That will get you (practically) four 120V/20A circuits, with the MWBC counting as two of them.
However, I strongly admonish you to "price it both ways":
- Three 20A circuits (3x hot + 2x neutral + 1x ground) versus
- a subpanel, ground rods and three #6 aluminum + 1 #8 aluminum ground
The latter will deliver 50A @ 240V, assuming the entire run is THWN-2 as you propose. The subpanel then has whatever circuits are needed, and has versatility for expansion as it provisions pretty much twice the power you're after today.