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I'm planning to build a shed next spring, and want to make sure my thoughts on power are correct.

The shed is about 100' from the house, with another 20' inside the house to get to a subpanel. All I plan on having out there is some LED lights and a couple 120v outlets for convenience. The heaviest load I can foresee would be a leaf blower or maybe an electric lawn mower, if I ever get one.

Coming out of the house I'd go underground for about 15' to get under 2 gates, then I'd bring it up and run it on the backside of a fence, to save myself trenching the whole way.

With 8 AWG THHN wire I'm calculating a 2.62% voltage drop (down to 116.86) on a 20-amp load. Is this right, and sufficient? And I would need 3 conductors (black, white, green), correct?

How deep does my conduit need to be for the trenched sections? I'm planning 1" PVC conduit, is this good for both buried and aboveground sections? Any reason to go with a trench the whole way?

Any other tips? Thanks!

  • In the future, please ask one clear, specific question per posts. SE isn't a discussion forum, and questions need to be able to have clear answers provided to benefit the community. – isherwood Dec 13 '16 at 20:10
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One trench the whole way will be easier to pull cable through. You could make it direct, one straight line, or closer to it than otherwise.

Fences can get blown over, run into, replaced for service, or removed for aesthetic reasons, which will become more of a pain to deal with if you run conduit along the fence. Also its less pleasurable to look at conduit than to not.

Your municipality or county should be able to tell you your local minimum depth for buried cable. It depends on your jurisdiction if you're asking about code as I assume.

I would recommend oversizing the conduit significantly to make pulling easier and allow for easy upgrades in the future. 2" can't be that much more than 1". It will be more durable and it could save a lot of work later on.

You can save costs on grounding by using bare wire, or using metal conduit as your grounding conductor. But I would not bury metal conduit to avoid dealing with corrosion. Even a bare ground conductor inside a plastic conduit is going to fare worse than a sheathed over time.

Paint a white line where you plan to excavate and call your utility locating service out to mark obstacles. The white line or shape will keep them from needing to mark unnecessary things in far off parts of your yard.

excavation marking

Rent a trencher and make the work easy on yourself.

trencher

Bury conduit larger than you need, because excavation sucks.

Dig depth is often 2foot minimum for nonmetallic, 6 inch minimum for metallic, but ask your codes department what they will approve.

For wire gaguge, use a calculator like http://wiresizecalculator.net/.

calc

By my estimate you would want 6awg, but maybe the direct patch can cut down distance and let you use smaller.

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Sure, if money is no object

Yes, 6 AWG copper cable will give you a 20A circuit, at 60 cents a foot x 3 conductors x length.

A better choice is 4 AWG Aluminum for 1/3 the price. Yes, aluminum is safe in these large wire sizes.

That's awfully heavy wire for a couple LEDs.

You're not legally required to upsize the cable. As long as the breaker is correct for the wire size (i.e. 20A breaker for 12 AWG wire), it will protect the wiring from overheating, and that is the purpose of breakers.

It's a big mistake on those "voltage drop calculators" to give the amperage of the breaker instead of the amperage of the practical, real-world load. The latter is the one you're concerned with. It's OK to temporarily load the circuit beyond that, so long as the breaker is correct for the wire. In fact, you already do that - how do you get power across the yard? With a long orange extension cord. Look at yours - is it hooty-tooty 6 AWG? Of course not, I bet it's 16 AWG, or 14 at most. And yeah it gets warm - I've melted plenty of snow with mine. The tools got a lot of voltage drop, didn't fry, and we all lived to tell the tale.

So if it's OK to run 16 AWG extension cords, why do you need 6 AWG if it's in conduit? YOU DON'T. You do need to protect the wire though, which is why you need 12AWG wire if the breaker is 20A.

Consider an extreme scenario: a 1000-foot run of 12 AWG to run a couple of post lights, "and a convenience outlet". 2x 15W LED post lights. 30 watts total, at 120V that's 1/4 amp or 0.25A. Now, go and punch that into the voltage drop calculator and allow a 99% drop just to see what happens: enter image description here

Huh what? 1% drop? The calculator goes to minimums - since it's for Code electrical, minimum wire size is 14 AWG. This gives a 1% drop. 14AWG is more than adequate even for a 1000 foot run.

So what happens if our kid is going to school, we put a shelter there, and put a 1500W heater in the convenience outlet? That actually requires some sharp-pencil work with Ohm's Law, but it boils down to the circuit flowing 8.03A, the heater giving 618W of useful heat while 345W is lost in the wiring (0.345 watts per foot, no big deal). And remember that's with 14AWG wire going 1000 feet, which is pretty extreme.

Just for chuckles, go back and plug 20A and 3% voltage drop into the calculator. It'll spec 3/0 copper or 300MCM aluminum. Now price 3000' of that.

I'm only suggesting 12 AWG wire going 120 feet.

So my best answer is, don't worry about it. Breaker it for 20A, or 15A if it really bothers you.

Best of both worlds: reduce voltage drop further with 240V.

OK then, shift gears. Instead of 12AWG, run 14 AWG. what!?

Run 2-wire (hot-hot) 240V power. Fit a NEMA 6 receptacle.

enter image description here

You're skeptical. I get it.

Now add a step-down transformer. This plugs into the 240V source and gives 120V output. Half the voltage, double the amps, or vice versa. If we plug in a huge 16A 120V tool, it's only going to draw 8A at 240V. Go back and plug that into the voltage drop calculator.

enter image description here

Win!

So what do you do about the lights?

  • many LED fixtures and most new fluorescents will accept 240V. (you're not allowed to have E26 Edison bases on 240V lighting, however.)
  • Run a second 120V circuit - 14 AWG is ample, as you saw from the 1000 foot example above. The two circuits can share the ground wire (4 wires + ground)
  • Run hot+hot+neutral (3 wires + ground) put in a sub-panel. Feed the lights with 120V and the transformer with 240V. Yes, on 15A fed with 14AWG. And then you can brag that you did that!

Now you have a wire size, choose conduit

If we were talking about wrestling big old 4 AWG aluminum, I woulda said a 2" conduit if not 3", just to make it possible to pull without hiring an electrician for his truck full of varsity pulling tools. But now that we're talking 14AWG, that's so easy to work with, just get it over there any practical way... bury a little (if you bury, do use large conduit for future expansion)... staple it to the fence, whatever's legal... and if it breaks, splice it.

It really boils down to money. What's the least expensive way that'll do what you want legally, and give you future headroom?

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