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**proposed feed**

I don't want unprotected UF (wife is a busy planter-gardener). The plan is #8 THWN stranded in 3/4 or 1/2 SCH 40 PVC conduit. 125' total length of run @ 12" depth.

The home has 100A service in the main panel and this will land on a 30A breaker. I drew a 1-pole breaker.

At the outbuilding, the subpanel will have a ground rod, and the planned loads are

  • 1 - 20A microwave/space heater
  • 2 or 3 - 15A lighting/outlets/ceiling fan

It'll be the wife's get away "nest". Never going to have a need for 240v. Used 2-3 times a week (less in winter).


A few more questions based on ThreePhaseEel's answer:

  1. "wayward naturally-induced surges" - AKA lightning, yes?
  2. Why the two hots? (I will never have a need for 240V).
  3. Do I need the 30A breaker in the home load center, or can i direct connect to the panel? (Or Two hots-each on a 30A?).
  4. Is it mandatory to allow for "pulling"? I had planned on installing conduit over run as I buried. Is 3/4" allowable with conduit fill equation?

Location: Michigan/USA.
Any feedback would be helpful.
Thank you all very much.

1 Answer 1

1

Close, but not quite a banana

You are close to a correct setup here, but there is an issue: your specified conduit burial depth is too shallow for PVC under normal conditions. Code requires the top of the conduit to be 18" underground for a run through a residential yard, so you'll be trenching to about 20" deep.

Also, I would upsize to a 1" or 1.5" conduit here to ease pulling, and make it Schedule 80 instead of 40 as the former is more rugged against mechanical damage than the latter. You will also need a pair of prefabricated wide-sweep elbows to bring the run aboveground and expansion fittings aboveground at the transitions to compensate for earth movement.

As to the wires -- 8AWG THWN is a suitable wire size (a bit more than strictly necessary here, but fine nonetheless), and you'll need to run four wires in the conduit: two hots, a neutral, and a grounding conductor -- the ground here is critically important as it serves a very different, and complementary function, to the ground rod at the outbuilding. (Namely, the ground wire returns wayward utility power back the way it came, while the ground rod is for returning wayward naturally (aka lightning)-induced surges back to terra firma.)

As to why 240V? This lets you balance the loads across the two legs of the feeder, reducing voltage drop, as well as providing 240V for the next bloke who owns the place.

The part about laying the entire conduit run and then pulling the wire through it is essential to avoid damaging wire insulation and jacketing excessively. You can use a shopvac to suck a foam "mouse" (or even a plastic grocery bag in a pinch) tied onto a length of twine through the conduit to serve as a pulling string, and then tie the string off to the wire and use it to pull it back through, though.

9
  • "wayward naturally-induced surges" - AKA lightning? Entertain my newb questions: Why the two hots? Do i need the 30A in the home load center, or can i direct connect to the panel? (Or Two hots-each on a 30A?). I get the wide sweeps and expansion fittings - freeze thaw MI. Is it mandatory to allow for "pulling", planned on installing conduit over run as I buried. Is 3/4" allowable with conduit fill equation? Thanks for your time 3PhaseEel
    – MI Brian
    Jul 29, 2018 at 16:46
  • @MIBrian -- yes, aka stuff induced by lightning bolts. The two hots let you provide 240VAC to the panel, which is more efficient than 120V, and yes, you need a 2-pole 30A breaker in the house panel. It is absolutely mandatory to allow for pulling the wire through the conduit (pulling conduit over wire mangles the wire insulation too much), and 3/4" is allowable, but going up to 1" makes it less likely you'll need to beg an electrician for help rescuing your pull. Jul 29, 2018 at 17:24
  • I thought you had to below frost line for burials to prevent heaving? You need 2 hots to give 240 bolts do you can run future 240 stuff...kiln, compressor, heaters etc Jul 29, 2018 at 17:25
  • @LimoDRIVER -- the expansion fittings take up any motion due to frost heaves etc. Burial below the frost line is only a concern for water lines to prevent water from freezing in them. Jul 29, 2018 at 17:28
  • Thanks, ThreePhaseEel. I am in South Florida, don't worry too much about freezing down here. Jul 29, 2018 at 17:32

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