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I'm running a circuit out to our shed/chicken coop. We live in New Hampshire. The electrical demand won't be high, so one 15 amp circuit will be sufficient. We currently have 3/4" schedule 40 buried 12" deep. Long story short, we tried to get it all 18" deep but the work wasn't done properly and about half ended up 12". I'd prefer not to have to redig it, but that is an option if we absolutely had to.

I know that at 12" deep I have this limitation:

"It has GFCI protection before it enters the ground, is limited to 120
volts, and is protected by no more than a
20-amp fuse or breaker."

Which our circuit totally fits, but . . . I did some calculations for the voltage drop, and the recommendation is that I use 8 gauge wire. I am planning on having electronics at the coop (camera, smart light bulb), so i do want "good" electricity.

Trouble is, not only does that add a lot of cost, I'm not entirely sure how voltage drop "works". Do I only have to have 8 gauge for the longest part of the run? Only at the beginning? Fine to run 14 gauge after the connection to the shed?

Plus, the complication of a 12" deep trench makes it hard for me to understand how to combine the requirement for GFCI and 8 gauge wire.

I've made a diagram of what I've got to work with. I could move the GFCI at the exit of the house onto this circuit and use that to my advantage.

My specific questions are on the diagram and I'd love to be directed/corrected on how voltage drop works and what the larger gauge is doing to prevent it.

Thanks!

Update: The most I'll possibly need at the shed/coop is 650 watts. However, I use my shop vac or circular saw at the existing GFCI location.

Voltage drop is additive or you might say cumulative. If you use a heavy wire for the long portion, and small wire for the short sections, you'd prevent most of the voltage drop.

Your concern about your electronics may be unfounded; most of your electronics will run off a power supply that handles a wide range of voltages, and voltage drop will be no problem.

As mentioned in @EdBeal 's answer and the comments, the load is the key.

It seems to me that up to 5 amps / 600 watts you don't have any issues at all. With a total run of 180' and load of 5 amps, you can limit your voltage drop to less than the NEC recommended 3% with 12 AWG wire, which is the minimum for a 20A circuit, which is the maximum for your 12" buried conduit.

The electronics and some LED lights might not add up to much load, so if you don't have much else out there, you might very well stay under the 5 amps and never have any voltage drop issues.

Beyond that it's hard to say just what might happen. The NEC 3% voltage drop is conservative, most things tolerate significantly more voltage drop than that before you have any serious issues. (It depends on the utility voltage too - your actual voltage may be a bit over or under the nominal 120 volts.)

If you occasionally run a circular saw or shop vac at the end of the line, the load will be significantly higher, maybe 10 amps. At 10 amps, you'd want #10 wire to limit voltage drop to 3%. Now if you run both the shop vac AND the circular saw, you could be up over 15 amps and you'd need that #8 to limit voltage drop to 3%.

But again, 3% is very conservative, you might have no major issues with #12.

If I wanted to be safe, I'd run #8 underground to the disconnects, and #12 within the buildings. It's not a big bump in cost to run #8 THWN in the underground, it's a pain to terminate / splice compared to #12, but you'd only have to deal with it in the disconnects.

"convert to GFCI" is a fine way to resolve the depth problem. So is covering the route with more dirt. You need 18" of cover over the top of the pipe.

You can always use larger wire. If Code requires #12, you can use #8, #2 or 2000kcmil if that's what you happen to have lying around.

If that wire will not attach to a receptacle, easy peasy - Pigtail it. Splice it to a smaller wire that can, but again you cannot go below the minimum size. Say on a 20A curcuit you cannot use #14 just because it fits in the backstabs.

There is no requirement to upsize wires for voltage drop. That is not in the Code anywhere, except implied in the "have basic competence" rule. There is nothing magical or required about 3% or 5%. You have to evaluate your loads and see. Letting a load sag the voltage 20% is fine if this is not deleterious to any of the loads.

Look again at your electronics loads. I bet they have "wall warts" that have switching power supplies and they auto-range from 90-264V. That's Japan's 100v -10%, and UK's 240V +10%. So more than 30V of sag (25%) is unacceptable.

Your incandescent lamp and heater loads behave like resistors. If power drops 10%, their output drops 10% squared, or 19%. That's the only thing that happens.

Go through each load turn by turn, and figure out what voltage drop is workable. Then back through the voltage drop calc.

The #1 mistake people make using the voltage drop calc (besides 3% for allowable drop) is when they get to the amperage entry, they type in the number on the breaker handle. That's wrong. Put in the amp number you actually expect to run.

Now if the circuit has receptacles, do keep in mind the temptation to use the circuit in lieu of an extension cord, for temporary loads which are much larger, e.g. A circular saw. These will wreak havoc on your calculations. As a CYA, many will say "spec for the breaker size": ok fine, but design load is only 80% of that. "They might load it to 100%?” Sure, they might load it to 200% and get away with it (breaker trip curves) - where does it end? We have to settle on a number to design to. That depends on the installed loads' tolerance.

The amps drawn will max out at the rated watts divided by 120.

You only have 15 amps available a camera and a heat lamp would be fine on that line what we really need to know is the actual load not the size of breaker. I have 1 outside light almost 2x that distance and it works fine on 120v line but the led dawn to dusk fixture draws about an amp maybe slightly more.