electrical service to a shipping container workshop

Question

I am purchasing an 8x20ft shipping container ("conex") which I will be using as a toolshed and small workshop. I want to have some electrical service out there, for a compressor, fluorescent lighting, and some outlets for power tools etc; no HVAC or anything like that.

I'm a EE and have done plenty of wiring within the house, but want to make sure I've got the nuances of a sub-panel in a detached building correct, ask if the fact the building is all-metal suggests special considerations, and solicit comments on my whole plan. The work will not be subject to inspection. Here's my plan:

I'll run 10/3 (plus ground) UF direct-burial cable from the main panel (in my house) to the sub-panel in the container, protected by a double-pole 30 amp breaker in the main panel. We're talking maybe 75ft total wire run, so voltage drop will be a few volts max (10ga is about 1 ohm per 1000ft). Trench depth 2ft, with plastic conduit where the UF comes up out of the ground.

I will configure the sub-panel to keep ground and neutral separate (with "insulated ground bar" or "neutral bar" kit, assuming there's no subpanel boxes available with separate neutral and ground bars). No main breaker in the sub-panel. I'll have four single-pole/120vac branch circuits (probably two 20amp and two 15amp, since it's ok for the breakers in a panel to add up to more than the main breaker), wired with 12 and 14 gauge NM respectively.

I'll install an 8ft ground rod at the sub-panel, and bond the shipping container to it too. How exactly should I interconnect the ground rod to the container shell and to the ground wiring (the ground bar in the subpanel, which in turn is connected to the ground wire in the UF cable and to the ground wires in all the branch circuit Romex) ? Is this sufficient grounding ?

I'll probably drop 3/4" PEX in the trench too. No immediate needs for plumbing out there, but a faucet might be nice some day, and it's maybe $50 of materials.

Answer 1

It sounds like you have things well thought out here as far as electrical matters go. A couple of notes:

1. You may wish to bury a fat conduit (or two -- I'd use 2" or 3" Schedule 80 PVC for this) in the trench as well as the PEX line and the UF cable, or better yet, bury some conduits and use one for the feeder to the container-building. This way, you don't have to dig up or abandon the UF later if you need MOAR POWAH at your workshop.

2. You'll want to have your ground rod checked by an electrician to make sure it is less than 25 ohms to ground (for adequate lightning protection of the electrical system). If it isn't, you'll need to install a second one at least 6' away and bond them together with the Grounding Electrode Conductor.

3. You are correct in that the container should be earthed. If you don't plan to move it later, an exothermic welding kit, used properly, will make a permanent copper-to-steel bond between the bonding conductor to the container and the container chassis -- I would bond to a container frame rail to minimize the risk of the exothermic weld accidentally perforating the container wall sheets.

4. A strike terminator (lightning rod) is a good idea, especially if the container is the tallest thing in the area. Note that the lightning rod needs its own earthing electrode -- it cannot share an earthing path with the power system to avoid excessive common-mode voltages getting induced onto the power system and frying things.

5. The minimum size for Grounding Electrode Conductors in the NEC is 6AWG -- bare 6AWG copper wire is easy to find, and can be directly buried without issue. This wire needs to run between all the ground rods, the container chassis bond point, and the panel ground bar.

Answer 2

Since you're wiring an all-metal structure and have access to do so, I suggest the building interior wiring be done with EMT conduit. For one thing, you will find this easier to mount. For another, it will guarantee that the structure and electrical system are grounded to each other.

Also, try to make a point to not punch a bunch of holes in the building, as they and the screws punching through them would be ever-annoying. Nothing worse than a caulking-gun snipe hunt. The best way is weld tabs or spans as appropriate with metal thick enough to take a 10-32 or 1/4-28 thread. Though tapping is annoying, I prefer it to back-nuts because it provides much better electrical connection. Your call. Or you may find places already present which are suitable; i.e. the far side of the screw will still be indoors.

For grounding, I would bring it in with the conductors and just tie it into the panel. The EMT fixation would suffice, or I'd find one of those locations where the back of the bolt is still indoors, and drill/tap into the structure and attach a ground strap there.

Answer 3

To ground the conex just get a lay in lug and bolt directly to the conex. Use #6 from the ground bar to the lay in lug. Make sure to use a wire brush to get down to bare metal. Tap with a 10/32. That should suffice. Then the entire system would be grounded and bonded.