electrical service to tool shed

RustyShackleford Posted By RustyShackleford, Feb 26, 2017 at 1:30 AM

  1. RustyShackleford

    RustyShackleford
    Minister of Fire 2.
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    Jan 6, 2009
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    Loc:
    NC
    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/new-30-amp-shed-sub-panel-underground-electrical-feed-should-i-use-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 !
     
  2. peakbagger

    peakbagger
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    Jul 11, 2008
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    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. I think the concept is if the conex is hit, that the all the grounds are at equal potential. so there is no reason for ground current through the household wiring. By interconnecting the grounds outside the structures, if there is difference in potential, the high current flow runs outside the structure.

    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.
     
  3. RustyShackleford

    RustyShackleford
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    Jan 6, 2009
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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 ?


    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 ?
     
  4. peakbagger

    peakbagger
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    What was explained to me is the secondary panel ground should not be routed through the house. The problem is that with two ground points there is inevitably going to be a difference in ground potential so a strike is going to potentially run through the house wiring to the "best" ground that usually is the house ground point. By running a separate buried ground wire outside of the house between the house ground and the new container ground rod if a strike hits the container, the charge will travel through the ground outside the house and stay outside of the house and main panel and go direct to the house ground. The container also gets tied to the ground rod. The neutral is still isolated from the ground in the secondary panel. Note I had one electrical engineer and one long term master electrician argue on both sides of this so the code must not be specific on this.

    I missed that you are going direct burial with the wire from the house. I don't think there is need for an expansion sleeve on the riser like there is if you went conduit.
     
  5. greg13

    greg13
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    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
     
  6. RustyShackleford

    RustyShackleford
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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).
     
  7. iamlucky13

    iamlucky13
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    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.
     
  8. RustyShackleford

    RustyShackleford
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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.
     
  9. peakbagger

    peakbagger
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    Jul 11, 2008
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    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.
     
  10. RustyShackleford

    RustyShackleford
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    Jan 6, 2009
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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 ?
     
  11. festerw

    festerw
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    Nov 16, 2009
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    Unlike water pvc and cpvc, conduit has the same OD so the fittings are interchangeable.
     
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  12. peakbagger

    peakbagger
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    Give the man a cigar, I generally buy a stick of 80 and use up the leftover the trench. When I worked in a mill we usually used sch 80 underground in areas prone to abuse as its pretty rugged compared to sch 40. I have never bought the expansion coupling they use to allow movement between the ground and the service panel as my garage entrance is too old. I have underground conduit from the utility to my house and that also doesn't have one but its code now.
     
  13. rwhite

    rwhite
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    Ipurchased this book several years ago and it has been in valuable for me

    https://www.amazon.com/dp/162710674X/?tag=hearthamazon-20
    But here are the pages that may help. As I recall when I did mine I ran 3 separate 6 guage thwn wires + ground since it was in conduit anyway. Using direct burial seems like a waste since it will be in conduit. At a 75ft run I'd consider a sub panel breaker so your not having to run back and forth to the main every time you need to kill power

    20170303_050622.jpg 20170303_050653.jpg 20170303_050737.jpg 20170303_050925.jpg
     
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  14. RustyShackleford

    RustyShackleford
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  15. rwhite

    rwhite
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    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. 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.
     
  16. RustyShackleford

    RustyShackleford
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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).

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

    rwhite
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    ?, there is a color selection box there. I got mine at HD. They had multiple colors.


    Just it's own breaker in the sub.
     
  18. RustyShackleford

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

    Sure, I'll probably have four 15 and/or 20 amp single-pole/120vac circuits there.
     
  19. Don2222

    Don2222
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    Feb 1, 2010
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    Hello
    I put a 30 amp panel in my shed and 10 outlets.
    Using 90 foot run of orange color 10-3 direct burial cable it was easy.
    See all the pics and details here.
    https://www.hearth.com/talk/threads/new-30-amp-shed-sub-panel-underground-electrical-feed-should-i-use-10-3-uf-b-or-10-2-uf-b-and-is.76717/page-6#post-1852593

    We put R15 fiberglass batts and reflective foiil in a the 2x4 walls under the paneling
    Then I made a raised tile Hearth and installed a pellet stove for some heat in the winter.

    The only need for black 6 gauge wire is for a 50 amp welder outlet. I put one in my new 12x20 workshop under the deck on the side of the house.

    See welder outlet install tips and pics
    https://www.hearth.com/talk/threads/welder-hookup-question-any-tips.155105/#post-2088439

    Don't forget to post pics when it is done. :)
     

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  20. stee6043

    stee6043
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    For whatever it's worth I recently installed a spa panel using THWN, 50' run +/-, and where I'm at (including the cost of the conduit both inside the house and out) the total cost of the run was less than half of what direct-burial cable of the same size was going to cost me. I used four insulated conductors (black, green, red, white). 50AMP circuit. THWN is cheap at Lowe's. As-is PVC...

    Pulling THWN couldn't be easier if you oversize the conduit. Dealing with that direct burial cable can be quite a challenge.
     
  21. burnham

    burnham
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    It's a much better way of doing it as well. I'm not 100% certain, but I don't think it's legal to feed a sub-panel with type UF wire anyway.
     
  22. jeanw

    jeanw
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    Sep 23, 2008
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    Hubby did the gray conduit but only 3/4 inch.. Unfortunately couldnt find a rental ditch availble so it wasnt buried very deep.. Now unfortunately with all the erosion its starting to be visible.Paid a guy to just spade it in. DUH>>>>
    He told me he had an estimate for the 125 foot trench but he balked...back 2 yrs ago...duh
    the couple who did our buried new phone line.. said they might do the trenching. I called many times with the phone number they gav us. But they never returned our calls in this small town....
    Now he had them back out here this morning to give a new estimate to ditch witch right next to the old(2yr) placement...
    Unfortunately we are seniors and he has health problems more now.
    we prob should just put in more dirt since We wanna get out of this place...too much yard work...
    so I like saying "do it right the first time"
    just chiming in....
     
  23. jeanw

    jeanw
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    Sep 23, 2008
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    Correction 1 inch conduit. the creepos wanted over $400 just to ditch witch it. not putting the dirt back or helping in anyway..
    He drove the 30 miles and rented the DW.....Unfortuntely neither one of us thought about putting in THE drainpipe
    to help with further erosion on this hilly hell place... Duh. he even ditched down 24 inches when possible. He rented the whole day... the 4 inch pipe was laying right close by two at least several 10 ft sections.....DUH
    It s hello getting old and not hinking things out...LOL
     

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