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I'm hoping somebody could look at these two diagrams to see if I'm going wrong anywhere. We're going to be adding a couple of keyless lamp holders (two or maybe three max) in a shed for incandescent / CFL lighting. We also want to add one GFCI outlet in the shed. The GFCI will not be used for power tools or anything else drawing even nearly as much.

An electrician will be removing an old 240V service panel located inside the house and installing a new service panel with breakers. We want to prepare the work inside the shed (including the 10/2 run to the house) and be certain the wiring is done properly before the work begins on the new service panel. The electrician will connect the 120V branch circuit to the 20 amp breaker.

The distance between the house where the panel is installed to where the 120V 20 A branch circuit will enter the shed is 140 ft. I estimate an additional 30 feet to the farthest keyless lamp holder plus an additional 12 feet coming out of the ground to the service panel. The voltage drop calculator I used for 10 AWG Cu with a 16A load on a 120V branch circuit comes to 114 V or ~ 5%. I'm planning to use quality gang boxes (probably weatherproof) and also run the 10/2 underground through PVC conduit

I need to be sure the wiring to the disconnect switch, light switch & GFCI is correct and whether 10/2 is okay to run for lighting Shed Diagram

Shed wiring

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    CFL & Incandescent? Are you a time traveller from 1999? Anyway, even if you are whipping out the 300Watt incandescents at 2.5A each, you're nowhere near 16A with your described loads. Additional light fixtures are NOT wired in series unless you like that dim orange glow from half-voltage. They are wired in parallel so each fixture gets full line voltage.
    – Ecnerwal
    Commented May 5, 2023 at 0:16
  • Okay. Light fixtures are not wired in series, but in parallel. But are the wiring connections shown in the diagram correct? And are the wires going to be connected correctly ? i.e., first to the disconnect switch, then the receptacle, then to the light switch, then to the light fixtures. This is what I need to know or if this is going against NEC. I'm in the U.S.. The house is actually a secondary residence (a cabin) and so incandescent bulbs or CFL's . . . it's whatever we find up there to buy in the boonies Commented May 5, 2023 at 1:09
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    The boonies have LEDs now, really. We're rapidly approaching the point where incandescents are 40W oven bulbs and not much else, even in the boonies, except for that guy who bought 200 "real" lightbulbs before they stopped making them, and the used bulbs at the fleamarket. Wiring critique below, in the "answer" space, not in comments.
    – Ecnerwal
    Commented May 5, 2023 at 1:16
  • @Ecnerwal and even the stroboscopic argument for incandescents rarely applies with high frequency LEDs (low-frequency fluorescents could make moving parts appear to stand still). It's getting to be a real pain for those of us who need cheap blackbody/spectrally smooth source in the lab, and a few other uses of light-emitting heaters.
    – Chris H
    Commented May 5, 2023 at 12:24
  • @ChrisH I believe incandescent lamps are usually permitted for specialty uses, just not general lighting - but you might have to buy them online like other specialty products. Commented May 5, 2023 at 18:01

3 Answers 3

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Series and parallel have very specific meaning in electrical work. All the lights should be wired in parallel. You can still run a cable from one to the next, but the wiring is, electrically, in parallel, not in series. The only thing in series in household wiring are switches, that are placed in series with a load they control. Every load on a circuit is in parallel between hot and neutral.

You don't run 10/2 cable in conduit unless you are intent on failing inspection, frustrating yourself immensely, or both. Common 10/2 cable is not wet rated, and wet-rated 10/2 cable requires a huge conduit. 3 10Ga THWN wires (that are wet-rated, as every wire or cable in an outside conduit must be) will fit with ease in a 1/2" conduit, and pull very easily. You could even add a 4th wire and have 2 circuits available, or use a 30A breaker at the house and have a subpanel for as many circuits as you like at the shed, with only 25% more wire cost. If you think you might have different and larger needs in the future, that's the best approach.

Then again, your loads (2-3 light bulbs + something that "doesn't draw nearly as much as a power tool") don't appear to add up to 16A as described, even with big honking incandescent bulbs, so perhaps you only need 12Ga wire. And again, for 25% more on wire, you can have two 20A circuits as a MultiWire Branch Circuit, so even if you used 16A, you could split it up so each circuit only saw 8A, and voltage drop would be around 5% on 12Ga copper wire.

10Ga is certainly fine for lighting, if you don't mind paying for it. The wiring in the shed should probably be 12Ga from the disconnect onward, though, because 10Ga is very hard to fit to fixtures made to connect to 12&14Ga wire, mostly, and also impacts your box fill requiring larger boxes when you have a few connections in a box.

Your diagram shows an improper routing of the grounding wires. The ground coming into each box connects to the box, first. Then anything else can be added to it. One method for this is a special green wirenut with a hole in it - another is to just leave the grounding wire free end long enough to connect to after it is connected to the grounding screw in the box. Your diagram shows it coming into a nut, then some other wire from the nut going to the box - that is not how it's done.

The switch and GFCI wiring shown seems otherwise correct. But:

GFCI protection at the house end will permit your 20A circuit to only be buried 12" to the top of the conduit, at which point you don't need an additional GFCI at the shed. If you don't provide a GFCI at the house end minimum burial depth to the top of the conduit is 18" (and that depth is required if opting for the 30A+sub-panel approach)

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  • Thank you Ecnerwal for this detailed critique. The only thing I need a a bit of clarification on is the part about The ground coming into each box that connects to the box, first. You said then anything else can be added to it. I'm not sure what you meant. Then you said, One method for this is a special green wirenut with a hole in it - another is to just leave the grounding wire free end long enough to connect to after it is connected to the grounding screw in the box. Could you explain a bit more what you mean? Thanks Commented May 5, 2023 at 1:50
  • . . since nobody asked, the service panel that's getting replaced is a ZINSCO. holy schnikeyes ! Commented May 5, 2023 at 2:55
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    For the green nut with a hole in it, the grounding wire comes into the box, goes through the hole in the nut, connects to the box screw, and all other grounding wires are connected to the green nut. For the other case, you wrap the grounding wire around the screw and leave 4-6 inches hanging out to connect other grounds to. In either case, you have the box grounded even if you are removing other things while doing work. Honestly, your (somewhat more explained) use case suggests a solar panel, batteries and DC lights as perhaps more cost effective than 140 feet of trench + wires + conduit.
    – Ecnerwal
    Commented May 5, 2023 at 12:20
  • I now understand what you were originally talking about. Thank you so much for explaining this in simple to understand words. This is not my shed. I can only offer suggestions to the owner. He's asked for a parts list with pricing. I'll suggest to him doing a cost comparison for a solar panel, batteries, and DC lighting. It'll ultimately be the owner's decision. This isn't a job I've been hired to do. I'm just a member of the family helping out. Thanks again Ecnerwal. You're great ! Commented May 5, 2023 at 14:00
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I can see stopping at #10 wire and putting up with 5% voltage drop, because copper is ridiculously expensive. But here's a secret: aluminum isn't, and it's perfectly fine for large feeders. Try 6-6-6 for 120V only, or 6-6-6-6 for potential future 120/240V at up to 50A.

You'll need to splice from aluminum to copper at the building, but you want a disconnect anyway: so pick any random $10 disconnect whose terminals are certified for #6 or #2 aluminum (typical disconnect terminals are made of aluminum, as that is the best material for a terminal.). A larger installation might use an Eaton "2-space subpanel" at $25, which will support up to 4 circuits and #2 wire - or go gold-plated and get a 6, 8 or 12 space subpanel for under $50. Spaces are cheap.

Note that burial depth for direct-burial cable is 24" of cover, unless a single 15-20A circuit is protected at the supply end by GFCI breaker or deadfront or other GFCI device, in which case 12" of cover. If you use conduit (a requirement for THHN/THWN wire) depth is 18" for plastic conduit or 6" (12" under vehicle pathways) for RMC or IMC metal conduit.

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  • Thank you Harper - Reinstate Monica for those recommendations. However I added in a later comment some clarification that this shed is only going to be used for storage of kayaks, canoe, a riding mower, tools, etc. I've even talked to the owner about considering options of a multi-wire branch circuit or a sub panel. He is not interested. What I am still confused about is how to wire the grounding properly in the two gang boxes. Ecnerwal explained the two options but I've not been able to understand his wiring scheme Commented May 5, 2023 at 11:35
  • General note: If you don't already have a torque screwdriver, you should acquire one for aluminum wiring. Aluminum is much more sensitive to getting things accurately torqued than copper. You don't need to get the super-high-end ones, any cheap torque screwdriver can do the job, and it's something you should just have if you're working with aluminum wiring. (It's a good idea with copper as well, but not as critical.) Commented May 5, 2023 at 14:32
  • Yes. I believe I picked one up many years ago after learning other screwdrivers were tearing up aluminum Commented May 5, 2023 at 14:55
  • @DarrelHoffman I don't know if it's more sensitive, but it's certainly not more forgiving, and copper is pretty sensitive itself. Commented May 5, 2023 at 19:59
  • @A.G.Bell I hear you. You can price 10/2, which can go on an outlet directly, vs. 6-6-6 which will require adding a $10-ish disconnect. If you're taking money for electrical work, make sure you have appropriate licensure for the job. In most jurisdictions, work like this requires a licensed electrician, though a non-qualified person can certainly install empty conduit and pull strings so it's a 30-minute job for the electrician. Commented May 5, 2023 at 20:02
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  • When it is a separate building, you need a disconnect

Done. A simple switch works for that. So does an A/C disconnect. So does a main breaker on a panel.

  • When it is a separate building, you usually need ground rod(s)

Exact rules vary a bit by jurisdiction. Typical is one ground wire that goes to two ground rods. This is in addition to the ground wire going back to the feed panel. This may not be required when it is a single circuit, but I am not 100% sure.

  • With conduit use individual wires (THWN) and not cable (UF)

As already noted in another answer, running NM cable through underground conduit is not allowed (wet location) and running UF cable through conduit is hard to do.

  • Make your conduit large enough for the future

1/2" PVC Schedule 80 will handle 10 AWG copper x 3 (hot/neutral/ground) just fine. If you go up to 1" then you can put in, for example, 6 AWG aluminum x 4 (hot/hot/neutral/ground - may be able to use a smaller ground actually). That would get you up to 50A @ 240V (or 100A @ 120V).

You don't need that much power now, but if the conduit is large enough then you have options for the future.

  • Consider an MWBC

A Multi-Wire Branch Circuit is effectively two circuits in one. Two hots, one neutral, one ground instead of one hot, one neutral, one ground, but lets you run 2 x 120V of whatever you've got. So if you use 12 AWG or 10 AWG and a 20A breaker (double-breaker instead of single breaker) you can have a separate hot for lights vs. receptacles. An easy way to double the total power while still being just one circuit.

  • Consider a Subpanel

This is the big change. You can have a subpanel running even with just a 20A single breaker feed. But where it becomes very useful is as you use more power. A subpanel can easily include the necessary disconnect (either a "main breaker" or a backfed feed breaker), can handle larger wire and aluminum or copper - typical switches and receptacles and wire nuts can't properly handle combining aluminum and copper - with a breaker on a subpanel aluminum is perfectly acceptable. Most of all you get flexibility to add additional circuits - outdoor lighting, HVAC, tools, whatever.

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    Definitely need local grounding rod or rods if it's a sub-panel, and it's not a bad idea regardless, but I think for the "just a 20A circuit" (even an MWBC is one circuit for this purpose) case it's not required. But you can do things that are not required because you think they are a good idea...
    – Ecnerwal
    Commented May 5, 2023 at 1:56
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    Thank you manssekhatz-Moving 2 and Ecnerwal. I should have stated in my original post this shed is only to be used as a storage shed for kayaks, canoe, a riding mower, and things of that nature. I'm somewhat familiar with MWBC''s, and sub panels. But there's no need in this shed for more power other than some lighting or maybe for charging of a riding mower battery or something similar. Commented May 5, 2023 at 2:30
  • @Harper - Reinstate Monica commented that copper is ridiculously expensive. But aluminum isn't. My cost comparison isn't pricing 6-6-6 AL for the job (120V branch ckt) as less expensive than copper. I'm finding #10 Awg THHN/THWN-2 stranded copper building wire is coming up less expensive that 6-6-6 Aluminum SER cable. (Assuming that 6-6-6 SER cable is what Harper was referring to in this thread) Commented May 31, 2023 at 16:20

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