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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.

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  • Thanks for moving this. I wasn't sure, but thought I;d seen other questions about home electrical wiring in the EE forum. – RustyShackleford Feb 26 '17 at 19:02
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    You might want to think about adding a lightning rod to it. – Andrew Morton Feb 26 '17 at 19:11
  • Will you be cutting in windows and installing a ventilation system? It can get pretty stuffy inside a container that you only access from one end. This can be a particular issue in hot sun at the height of summer. – Michael Karas Feb 26 '17 at 19:25
  • I'm going to have a firewood rack along the 20ft side that faces the house, in order to hide it from view. I'll build a simple gable roof that covers the firewood rack and container - so the ridge of the roof is along one of the container's 20ft edges. Though the container is waterproof, this should help with heat buildup, and it's also a heavily-forested lot. It should also make it look better, I'm hoping. – RustyShackleford Feb 26 '17 at 19:31
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    But yeah, there's not much ventilation - just 4 small passive vents along each side. I might want to add something, to avoid recreating scenes from Bridge on the River Kwai. – RustyShackleford Feb 27 '17 at 6:16
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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.

  • Thanks for your comprehensive response. A few questions/comments foillow. – RustyShackleford Feb 27 '17 at 19:21
  • Re #1, not a bad idea, but might be cheaper to just run 8-3 UF (much smaller cost than Sch80 PVC, which is $3-5/foot. – RustyShackleford Feb 27 '17 at 19:23
  • Re. #2, I have a good Fluke multimeter, so can't I do this myself ? I assume I attach one lead to the ground rod, and the other to the ground somehow. But how ? Another temporary ground rod ? Should the ground be wet or not - I imagine that'd make a huge difference. – RustyShackleford Feb 27 '17 at 19:24
  • Re. #3, do you think it's worth the cost of bringing a welder in, versus just bolting a connector on carefully ? Why does the weld not work if I plan to move the thing ? – RustyShackleford Feb 27 '17 at 19:26
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    @RustyShackleford: re: #1 -- consider the cost of conduit now an investment in not having to spend money digging later. #2 is not something you can do with a DMM (you need a special test set for the measurement). #3 is something most electricians can do as it's all wrapped up in a nice package (it'd be...adventurous as a DIY job, but not impossible), but a bolted lug should work provided you can get a good connection to bare metal. On #4, you're fine without I suspect given the situation, and you're correct on how #5 works. – ThreePhaseEel Feb 28 '17 at 0:03
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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.

  • Hey @Harper, it's you from my LED tube question - thanks for chiming in. Funny, I just saw an old EE coworker who also urged me to do EMT. I'm sold. Run exactly the conductors you need, using THHN, and kinda fun he said; plus no need for an EGC wire, the conduit does the job (I guess you just tie the ground terminal on fixtures to the box). I'll probably still try to have an explicit ground connection the the structure (doable without penetrating the envelope). And a ground rod (into the earth) sounds like a good idea. All connected with bare 6gauge. – RustyShackleford Mar 22 '17 at 1:39
  • Yes, the folks who sold the container urged me not to penetrate the shell. They pointed out the square tube that runs along the top corners (where walls and ceiling meet) can be drilled from the inside and not compromise the envelope. I have another thread about doing that - short answer: use #14 TypeF screws. I ran 2x4s along those tubes (an on-edge piece bolted to steel, and a flat piece nailed to the bottom of that). The latter supports rafters, and I can run studs down to wood floor. Not fully populating the studs, but can add them as need for shelves, back edge of a bench, etc. – RustyShackleford Mar 22 '17 at 1:52
  • Just realized, it's essential for me to have separate ground connection to frame, since my EMT is going to be attached to wood, not steel (since I'm not planning to do any welding, and I doubt I'll bother tapping any of those hollow frame tubes for EMT mounts, when there's all that wood available). – RustyShackleford Mar 22 '17 at 4:27

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