electrical service to tool shed

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've gleaned quite a bit good information from this thread:

https://www.hearth.com/talk/threads...se-10-3-uf-b-or-10-2-uf-b-and-is.76717/page-6

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:

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 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) ?

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.

Thanks for reading !

peakbagger said:

I think the standard recommendation is that the ground rod for the container has to be bonded to the household ground preferably at the primary grounding point on the house with the interconnecting cable routed outside the structure. I believe the container ground would be tied into that ground cable.

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I'm not quite following. Are you saying there needs to be a separate ground cable between house and container, in addition to the ground wire
in the 10-3 UF cable I'm running ?

I envisioned simply a heavy strap, somehow bonded to the container shell and connecting to the 8ft ground rod. Then the ground bar in the subpanel (which is connected to the ground wire in the UF cable and the ground wires of all the branch circuits within the container) is similarly connected to the ground rod. End of story. Is that not enough ?

I expect you don't need to worry about frost but you still need to put a sliding joint in the PVC riser to compensate for the movement in the container.

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The PVC riser just sticks down into the soil almost to the bottom of the 2ft burial trench, and the UF comes out with no fittings or anything. Seems like if the container moves, the PVC just slides up and down in the soil a little and the UF is fine. Am I wrong ?

greg13 said:

For the few extra bucks, put everything in conduit. If you find down the road you need to upgrade you can simply pull new wire through the conduit rather than retrench.

Greg

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It's a thought, but Schedule 80 PVC (which would seem prudent) looks to run $3-5 per foot (for 2-3" sizes). It'd probably cost about as much as renting a ditch-witch for a half-day again (though a lot less trouble).

iamlucky13 said:

Do you need schedule 80 for some reason I'm unaware of? 3/4" Schedule 40 grey PVC is ~$0.25/foot at home improvement stores. I don't think most stores even sell schedule 80 for PVC conduit.

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I think if I'm going to bother laying conduit, it should be a big bigger than 3/4" - maybe 2" or so. And yeah, that's only about a buck a foot in Sch40. I saw elsewhere where a guy said to use Sch80 for burial, but dunno if that's necessary at all.

peakbagger said:

Last thing I knew, Sch 80 is required where the PVC is exposed to UV and at the ground line where the conduit comes through the ground where it might be exposed to abuse (which is also where there needs to be slip joint.). No need for sch 80 in the buried runs.

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Cool, thanks for the clarification. Are there fittings which will allow the transition from horizontal Sch40 in the trench to Sch80 coming up vertically out of the ground ?

Thanks much for that @rwhite - I'll review those pages to make sure I got it right. As far as using THHN (apparently most THHN wire is dual-rated to also be THWN), it seems that four strands of it is only very slightly cheaper than UF of the same gauge (https://www.wireandcableyourway.com/Copper-Building-Wire/).

rwhite said:

Pulling UF is a pain. Won't be a problem if you just pull 10' at a time and piece together the conduit as you go though. I recall a size requirement for the conduit (maybe 1.25") so that you don't get heat build up.

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If I do conduit, I'll probably go 2".

Other issues about possibly using THWN ... The cost (relative to UF) can be further reduced by using bare wire for the EGC. But, I don't see different insulation colors (at least here: https://www.wireandcableyourway.com/8-awg-thhn-building-wire.html) so will require a voltmeter to identify neutral (at the subpanel).

The other thing I would do if you have a larger compressor is put it on its own circuit. A miter saw and a compressor starting at the same time will kick a breaker. Happens more often then I expected and I ended up isolating the compressor.

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Do you mean its own circuit in the subpanel, or another line from the main panel (in the house) ?

rwhite said:

?, there is a color selection box there.

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I looked for it. Not hard enough, apparently. What a maroon.

Just it's own breaker in the sub.

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Sure, I'll probably have four 15 and/or 20 amp single-pole/120vac circuits there.