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I need to run a 20amp outlet in my shed 80 feet away. I have lots of moles/voles in my yard. Can I put "UF-B 12-2" in buried PVC conduit ? Will this overheat ? What size & type of PVC should be used ? Thank you for the assistance.

Will Schmahl
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  • I would think that an 80' straight pull of UF-B wouldn't be horrible (not fun, but not horrible). The ugly part is that your run is likely not straight (you've got some elbows to get the line down from where it exits the source, turns to go horizontally underground, then back up into the shed). The individual THWN wires in NoSparksPlease's answer is definitely the way to go (and is probably cheaper, too)! – FreeMan Aug 17 '21 at 15:10
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    On the bright side, directly burying the cable will help eliminate the hungry critters! :) The down side is constantly digging it up to replace the cable, so that's probably a no go. – FreeMan Aug 17 '21 at 15:11
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    @FreeMan ..and ground fault requirements might defeat that method! I'll stick with the .223 method for the ground squirrels I have in my alfalfa. – NoSparksPlease Aug 17 '21 at 15:24
  • I struggle to get UF-B through a hole in a 4-inch joist. I don't see how you'll get it through 80 feet of anything even if it's dead straight, and it can't be unless your house and shed both have basements with parallel walls. Go with the suggestion in the answer. – jay613 Aug 17 '21 at 19:48

2 Answers2

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Assuming you're dealing with 0.450" wide UF-B, a single cable in conduit must be 138% of that (to be 53% fill), so you need 0.621" inside diameter conduit. That means 3/4" conduit is the code legal minimum.

I'm not sure why voles would find PVC conduit any less tasty than PVC wire insulation, but the 18" of cover that is required for PVC conduit may help. If the line was GFCI protected at the source (i.e. so a ground fault in the wires would trip the GFCI), 12" of cover would be permitted.

If Rigid Metal Conduit or Intermediate Metal Conduit were used, it could have 6" of cover. Voles won't eat that. With that, 1/2" conduit would suffice since you can run a black and white THHN wire. The conduit is the ground.

Overheating is decided by NEC 310.15(B)(3)(a) and says that you cannot have more than four 20A circuits in the same conduit before having to derate. I think you'll be OK. You have a long distance that will cause some voltage drop, but that doesn't cause overheating because you have a long distance to spread it over.

Harper - Reinstate Monica
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You can. If you size the conduit right it won't overheat. It won't be easy, the pvc doesn't slide well. I wouldn't do it, I would pull THWN conductors into the pipe.

The conduit has to be sized as a single round conductor of the greatest dimension of the cable. Dimensions vary a little, I think 3/4" would be required, but the exact brand would be needed for 100% answer.

NoSparksPlease
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  • Then 1" should be safe. Thank you !! – Will Schmahl Aug 17 '21 at 15:14
  • Belden cable fill calculator returns conflicting answers, it says 0.450 (Cerrowire) with 1/2" will not exceed 53% of sch40, but then says 3/4" is minimum size. I" would certainly be overkill. – NoSparksPlease Aug 17 '21 at 15:17
  • https://www.southwire.com/calculator-conduit says 0.450 would fill 1/2" to 56%, so 3/4" would be minimum since 53% is max for single conductor calculation. – NoSparksPlease Aug 17 '21 at 15:30
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    Overkill will never be unappreciated in the future, @WillSchmahl, when updates/upgrades become necessary. Much cheaper & easier to do it now than dig it up again later – FreeMan Aug 17 '21 at 15:36
  • thanks for all the advice.. Better safe than sorry.. As they say, never enough time to do it right, but always enough time to do it over. – Will Schmahl Sep 08 '21 at 23:08