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I am planning to run power to a very small shed by extending an existing 15a circuit that currently feed only a gate operator (4.5amp). This is located in southeast Michigan. I only need power at the shed for a light and a single receptacle for the occasional small power tool. The gate is fed by (3) #14 wire in PVC conduit about 100'. I plan to tap into the existing junction box at the gate and run THWN #14 wire underground (18" depth) in a 3/4" conduit about 75' to the shed. I will use a small disconnect and a GFCI at the first receptacle. My questions:

  1. Will there be a problem with voltage drop if running #14 wire that far? I assume it does no good to run heavier wire unless I replace the existing run of #14 with the same.
  2. Is schedule 40 PVC conduit sufficient for running a 120v, 15amp circuit 18" down in a residential yard?
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    Assuming that this is in the US, I believe you're in the clear. NOTE: this is based on things I've read here, not in-depth knowledge of the NEC. Please edit your post, though, to include WHERE you are located (country or US state). Imperial measurements and 120v lean heavily toward the US, but code varies by state. – FreeMan Aug 17 at 14:29
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    18" TO THE TOP OF THE CONDUIT, NOT TRENCH DEPTH. – JACK Aug 17 at 15:02
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#14 is not adequate for a 175 foot, 50% circuit capacity load. Voltage drop is a function of resistance of the wire in series with the load. Extending a #14 circuit with lower resistance #12 is completely acceptable and does accomplish reducing the accumulated resistance that causes voltage loss on the extension.

5% is the total drop generally considered the maximum acceptable. Figure a distance to the gate of 100', add your 75' extension, you have 175', with an 7.5 amp load (a worm drive saw) you have a 3.5% loss just getting to the gate, then an additional 3% getting to your new receptacle. But if you reduce the resistance on the extension by using #12 you will get only about 1.5% loss on your extension, bringing you back to 5%. (Numbers get dramatically worse if you exceed the 50% circuit capacity that I used above.)

The 18" cover requirement is adequate for most conditions, be aware it is 18" of "cover", meaning the conduit is to be below 18" of material. Do an internet search for table 300.5 that could identify conditions that could change depth requirements.

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  • Thanks! I don't expect to exceed 50% of the circuit capacity. This is mostly to get lights out there. What would be the result of exceed 50% circuit capacity if used #12 for the 75' run? Would using #10 instead help much? I imagine it gets exceedingly difficult to pull and work with #10 wire. – Inquisitor Aug 17 at 16:15
  • Using #12 or the extension would result in right around the limit of 5%. You probably don't need #10, but pulling 3 #10's into a 75' of conduit is not noticeably more difficult than #12's. – NoSparksPlease Aug 17 at 16:41
  • @nosparksplease There is no actual limit there are 3 & 5% recommendations in the NEC but these are fine print notes not code enforceable. – Ed Beal Aug 18 at 18:16
  • @EdBeal I didn't say whether Informational Notes in the code were or weren't enforcable in the US or Canada, or pick apart the difference between 3 and 5% total, I merely said " generally considered". – NoSparksPlease Aug 18 at 19:04
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Voltage drop only matters for your actual load

You have to compute voltage drop on each segment individually, however, you only have to worry about the gate motor when it is actually running.

Voltage drop is a function of the resistance of the wire X the current you're drawing Right Now.

So if you only have a 15W actual light burning, voltage drop will be negligible. If you fire up a 12A circular saw, now you get pummeled.

You can use a voltage drop calculator to compute your practical voltage drop based on your actual tool loads. Most voltage drop calcs are run by wire salesmen, who are trying to upsell you into bigger wire, so you have to override their default "3%". Raise it to 30% if you're just playing "what-if"...

Let's be very clear on this: Achieving 3% (or even 5%) voltage drop at breaker trip (15A) is a complete waste of time. You'll never run the circuit at breaker trip, so you'll never have that voltage drop.

That said, 175 feet is awfully long for 120V.

My rule of thumb is don't even bother crunching the numbers til it's over 90 feet @ 120V. You're well over that. So yeah, I'd crunch the numbers.

You might get by without a bump if your loads are under 7 amps or so. Otherwise a bump to #12 is probably warranted.

One option is pull the #14 from house to gate, using it to pull in #12 THHN. Then reuse the old #14 for the gate-shed run.

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  • I will definitely use #12. On a side note, do you think it's a good idea to add a GFCI breaker to reduce required burial depth, or is it not worth the risk of extra tripping (it's not easy to get to the sub-panel from the shed? – Inquisitor Aug 17 at 20:13
  • @Inquisitor Yeah, a GFCI breaker is a good idea. If anything would trip the GFCI it'd be the gate motor, not your tools. My GFCI never trips, and it's coming to a "receptacle in a junction box" type extension cord that lives outside and is directly rained into. Yes, the GFCI tests ok. You know you can reduce burial depth to 6" of cover if you use Rigid conduit: no GFCI required. However Rigid is hella expensive and requires a trip or two to the hardware store to thread pipe to fit... – Harper - Reinstate Monica Aug 17 at 20:42
  • GFCI a gate circuit? Not unless all members of the household are capable of mechanically opening the gate if a failure occurs. Which if it does it will be when most inconvenient. – NoSparksPlease Aug 17 at 23:09
  • I was thinking a GFCI breaker at the subpanel for the entire circuit, so if it tripped, the shed and gate would both lose power. It requires accessing the subpanel on the side of the building which requires unlocking a separate gate. Not terrible, but would like to avoid if it's potentially a more common occurrence. I don't think rigid conduit is warranted in this case. If I can use PVC at 12" depth that would be good. I just want to make sure I don't regret it if I have a circuit that trips all the time -- NOt sure how a gate motor might work on a GFCI circuit. – Inquisitor Aug 18 at 0:42
  • A GFCI is required at the out building no matter what the burial depth is. I usually recommend a battery back up all of the gates I have installed the actuator has been 12 or 24vdc only 1 was 24v. – Ed Beal Aug 18 at 18:12
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I completed the project and everything is functioning properly. Interestingly I took voltage readings at the gate motor prior to the circuit extension and found 122v. I took a reading at the new shed disconnect installed at the end of the additional 80' run of #12 wire and got a reading of 123v. The lower value at the motor is likely due to some impedance within the gate motor circuitry, but I'm still surprised to see a reading of 123v at the shed. Makes me wonder why I was so concerned about voltage drop.

Anyhow, Thank you all for the advice! I truly appreciate it.

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