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?
Electrical – Running multiple 20A circuits to a shed
electricalshedwiring
Related Solutions
- It is permissible to run more than one circuit in a conduit as long as the conduit is sized to carry all of the current carrying conductors. In your case a 1/2" conduit should do it, but I would go with a 3/4 for ease of pulling.
2(a). One of the advantages of pulling in one conduit is that you would only need a single grounding conductor sized to the largest circuit.The ground is attached to the receptacles and any metal enclosures and there are places marked for that purpose. You will attach the ground to the neutral bus in the main panel. You might check to see if there is a proper ground in the panel and make any necessary upgrades.
2(b). Most jurisdictions require any work to be permitted and installed by qualified personnel. We really can't be certain of every jurisdiction since codes vary by state and municipality. A quick call to your AHJ can solve that problem.
Good Luck.
There are so many mistakes here that I'm very concerned about the fact-finding process which led to them (or lack thereof). Because "writing a book" is not the SE format here, I'll be lightly touching on each.
But speaking of "books", and going back to that fact-finding process, I'd strongly advise hitting the library for some reasonably current books on electrical, and read them through - the goal being to get a well-rounded primer on the subject. Google gives you swiss-cheese knowledge because it only asks questions, and without a primer you don't know which questions to ask. Home-improvement store clerks know worse than nothing.
Anyway on to it.
Voltage drop - the first question on a 180' run. Here I deviate from stock advice, which is keep rigidly <3% of breaker trip. Actually, 3% isn't in Code anywhere, and it's a very coarse rule of thumb - 5% isn't wrong. And you should be computing voltage drop based on practical loads, i.e. what you expect to be actually pulling on a regular basis.
Aluminum wire - you didn't mention aluminum but you should be using it for large feeder >=4AWG. That means a size bump from copper. I often see people take dangerous shortcuts to save $30 on a project only to spend $800 more than they needed to because they're afraid of aluminum. Aluminum didn't work for small wires like #12 using alloys made for high tension lines. Feeders were always reliable in that alloy, and they've improved the alloy too.
The service panel. Everybody does it the first time. First get a huge panel - when you're buying a panel, spaces are cheap. Regrets, however, are expensive - and believe me when the power is there, you'll want to use it. 240V heaters, on-demand hot water (30A/240V is fine), electric car, prosumer shop tools that are 240V, these things chomp up breaker spaces 2 at a time. A 30-space breaker is only 15 loads. A 42-space is not uncalled for.
Honestly a 70A subpanel that's only 2 spaces makes no sense as a product. It could only serve as a shutoff switch for one single 30-70A machine. It is wholly inappropriate to power a variety of circuits, as it sounds like you want in this shop.
Second, you need a "main breaker" because that's your shutoff switch/disconnect (5). The most economical way to buy a disconnect is simply select a panel with a main breaker. Amp rating? Nobody cares, it's a disconnect. So feel free to get a subpanel with a 150/200A main breaker if that'll give you a sensible number of spaces.
For aesthetic reasons, I strongly prefer panels with an obvious, separate main breaker/disconnect, not one where the main is in the rows of regular breakers and only found with a "main" label. I want a visitor to be able to find the shutoff in a hurry, in half-light, in smoke.
Separate Ground bar - correct. Neutral-ground isolated - correct.
Ground rod (1) - unless you've done the arcane wizard measurement to affirm <25 ohms on the ground rod, you need two rods some distance (10'?) apart, unless your local code says otherwise. Code often has local amendments based on local conditions.
Ground wire - the only conflicting information there is "All the Codes say this is required" conflicting with "I don't want (you) to pay for it" (because I want to quote you a better price than my competitor, who is following the law). You need a wired ground wire back to the house. Period. Dirt doesn't conduct electricity well enough to serve as a substitute, if it did, we would just put THHN insulation jackets on dirt.
Ground size bump vs. voltage-drop size bump (3). When you make wires bigger for voltage drop, you must make the ground wire bigger in proportion to cross-section. That math sounds hard, until you realize wire gauge already works that way. So if you do a +2 AWG size bump for voltage drop, simply do a +2 AWG size bump for the ground also.
THHN wires inside conduit (2) - correct. You use THWN-2 wires inside conduit. (almost all THHN wire is dual-rated THWN-2, so people don't bother distinguishing them).
Don't push the limits of conduit fill. (4) Those limits are intended for pro electricians with experience, special pulling hardware, lube, and a winch on their truck. As a novice doing your own pull, be generous with conduit size, or else you could find yourself boxed into a corner and having to bring out an electrician to finish. In this economy electricians are not hungry, and won't return your calls unless you give them most of the job (and let them bill for that). Also if there are any defects in your work so far, they won't touch the job with a 10 foot pole unless you hire them to fix it all. So pencil in $1000+ for that.
No need for GFCIs downline of GFCIs (6) unless you want to play a "Yo Dawg" joke on yourself. There's no penalty for doing so; it's just wasteful.
Related Topic
- Electrical – Wiring Pool Subpanel for Pump and Receptacles
- Electrical – Conduit Fill and Derating for THHN Cables (outdoor run)
- Electrical – 10/3 Aluminum 20A with multiple electrical circuits? What is correct wiring
- Electrical – #8 THHN in Conduit for 50 amp circuits
- Electrical – Can 12 AWG THHN/THWN wire be placed in a buried conduit
- Wiring – How much 120 from 240 is too much
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- Electrical – Electricity to the shed
Best Answer
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!