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I'm building a small detached office and have a few questions. More Details:

  • The office is about 100 ft from my main dwelling.
  • Plan is to share the current service from my main dwelling.
  • Only 120V is required, and 60A will be more than enough for my needs. (I can detail more if necessary, but this is already overkill).
  • I'd like to have a subpanel with 4-15A (120V) breakers in it. I'd also like to have either a main breaker or cutoff.
  • I plan to add a breaker to the main panel in my house to supply the office.
  • I really want to run conduit to the office in case I need to pull something in the future.
  • The office has a bond wire connected directly to the rebar in the concrete.

Ok, now for the questions:

  • Can I run 240V (2-hot, 1 ground, 1 neutral) and only size it for 30A? Then half of the circuits (2-15A) will run off of 1 hot and the other half off the other hot? Or am I missing something?
  • What kind of wire do I need to use in the conduit? Currently I'm thinking individual copper wire, 8-gauge in size.
  • Do I need to bring a ground wire to the office even though it will have its own ground?
  • For inspection, do they need to see the unburied cable? Where can I find local details (ex: backfilled with sand, 24" deep, warning tape, etc). I'm in FL.

Thanks!

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Fat conduit is your friend here

I would run 1.5" or 2" PVC conduit here, buried so that the top of the conduit is >18" deep in the ground. The requirements you mention (sand bedding/shading, extra burial depth beyond Code, and warning tape) are typically utility requirements for service conduits; out of these, only the warning tape is mentioned in the NEC, and only for service wires (300.5(D)(3)).

As to inspection, a typical requirement would be rough inspection of the trench and conduit job before backfill with the wires pulled in afterwards, but I would check with your local Code authority (same people you'll pull the permit from) for details.

Since this is an office, by the way, I would throw a second conduit into the trench while you have it open. This conduit will be left spare for now, but provides a place for communications lines to go, whether they be copper pairs, coax cables, or optical fibers. In fact, it's necessary if you want to run copper comms wiring to the office, as that can't share a conduit with the feeder; optical fiber can provided there aren't any conductive members in the fiber cable, but getting it in and out of a shared conduit is a bit tricky.

You can go down to 10AWG copper in the conduit here, but you do need 4 wires

For 240V/30A (your prospective load and then some, split evenly across the two legs of the service) and your given distance, 10AWG copper THHN is more than adequate within your conduit. You will need four wires here, namely two hots, a neutral, and a ground wire, as the ground wire to the main panel provides a path back to the utility for current that came from the utility, while the ground electrode at the shed provides a path back to Earth for current that came from the Earth.

Go big or go home!

One major mistake folks make when they go to put a subpanel in is skimping on spaces in the new panel. The cost difference between a barely-large-enough-for-the-job subpanel and an amply sized one is around $100, given that "amply sized" here is a 24- or 30-space, 125A, main breaker panel. Don't worry about the amp rating of the main breaker here, by the way, since all it is doing is acting as a local shutoff switch for power to the shed.

With a small feeder like yours, by the way, it's going to be fairly important to balance the actual loads across the two legs of the feeder in order to avoid creating a situation where one leg is overloaded by way of being asked to do more than its fair share of the work. This is mostly a matter of thinking about what's hooked up/expected to be plugged into the circuits in question and making sure it comes out mostly even in amps/watts across the two legs.

TORQUE ALL LUGS TO SPEC

One last thing that is required by current Code (2017 NEC 110.14(D)) is that the lugs on the breakers and loadcenter you are installing need to be torqued to the values specified on the label with a torque screwdriver or wrench (nominally, one that reads in inch-pounds, as that's what the labeling uses). Even if your local Code authorities have not adopted the 2017 NEC, it's still a good idea anyway, lest your electrical system do what Greg Biffle's infamous lugnuts did!

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A few things to add.

The shut off switch out at the sub panel is mandatory. It can be a circuit breaker, in fact that's usually the cheapest way to do it. The size of the circuit breaker does not matter since it's only being used as a shut off switch so if you really like a subpanel with a 200-amp main breaker, go for it.

Your idea of bringing two legs of 120 volts out to the sub panel is a good idea and actually is the usual way it's done. Yes, you can put two circuits on each side to balance. Your number 8 wire is a little bit overkill since you really don't need a wire size bump for only 100'. However, it will let you go to 40 amps per leg. On the other hand, if you bumped to number 6 wire, you will be cleared all the way to 60 amps per leg, which is double what you were planning now. Any larger than that, and we would want to look at aluminum wire, since running Copper in large sizes is generally not worth it.

Conduit is a good idea, but unless you plan to enlarge the electrical cables, there isn't anything else you could put in this conduit. You're not allowed to run two circuits unless it's something special like outside lights controlled from the house... and data cables are forbidden in a power conduit, unless they're fiber-optic, but I have never seen that.

  • Data cables in general, or just non-optical-fiber cables as ThreePhaseEel mentioned? – grawity Jun 11 at 5:22
  • @grawity edited. I have never seen a case of fiber optic in conduit. – Harper Jun 11 at 5:25
  • @Harper -- fiber in conduit is fairly common in the utility world, although the telecoms folks often call it "duct" instead – ThreePhaseEel Jun 11 at 11:44
  • What kind of wire is code in this situation? THWN? I'm using PVC conduit, and I've read that you can't use THHN in buried PVC. – Jon Wingfield Jun 11 at 13:26
  • @JonWingfield thWn is required, W for wet. Because all outdoor conduit is presumed to be 100% full of water 100% of the time. But fun fact, the cost difference among THHN, THWN and THWN-2 is so small that almost all wire is simply dual-rated THHN/THWN-2. It is colloquially referred to as THHN. – Harper Jun 11 at 15:48

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