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I have about 200' of UFB cable to run to a barn. Does it require conduit, and does it have to be grey electrical conduit or can it be 3/4 black plastic water pipe?

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  • Is there a compelling, red-alert reason not to use aluminum feeder 2 sizes larger? Like "locality does not allow"? Aluminum is a natural for large feeder, since it has proven performance, the panel lugs are aluminum, and the price can't be beat. Cost of 10/3 Cu (30A) is about the cost of 2-2-2-4 Al (90A). It is also made in THHN and XHHW for individual wires. Oct 6, 2021 at 18:34
  • The most compelling reason is that I have already purchased the UF copper and it can't be returned.
    – AKG
    Oct 7, 2021 at 18:33
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    Ouch. Yeah make sure to build the conduit pull-able, i.e. ideally build it empty and pull the wire in after build, backfill and tamp. That way you'll be able to change wires later if the need ever arises. Don't bother trying to keep it watertight, water in conduit is unavoidable. Oct 7, 2021 at 20:28
  • Since you've already purchased the cable, make sure you go big on the conduit if you choose to take our advice ;) The bigger the conduit, the easier it will be to pull that UF-B through it. I've used a bit of it, and it's much stiffer than standard NM-B, it's going to be a bit of a challenge. Make sure you get sweep elbows, too, none of the short, tight radius elbows. I believe that if you stick to proper, UL listed conduit instead of sneaking off to the plumbing aisle, you'll only find sweeps.
    – FreeMan
    Oct 8, 2021 at 11:21

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UF-B is rated for direct burial, though you'd be better served by running through conduit for a couple of reasons.

  1. Conduit doesn't have to be buried as deeply
  2. If When the cable fails, conduit makes it very easy to pull a replacement instead of digging up the old cable and laying new wire.
  3. Conduit will allow you to easily upgrade the wire size (assuming you're still within fill limits), should you ever need more power at the barn.

You can pull UF-B through conduit, though it's more difficult than you'd expect and the conduit has to be larger than you'd think because the conduit fill ratio is based on the largest dimension of the cable (because the cable will end up twisting).

You should return the UF-B (if you've already purchased it) and buy THHN/THHN-2 which will be much easier to pull through conduit and will allow you to use smaller conduit and/or make it easy to upgrade to larger wires in the future should your power needs increase.

You must use grey Schedule 40 or Schedule 80 UL listed conduit (if you're using plastic), and cannot use any other pipe that is not UL listed as conduit. Sch40 is quite suitable for the buried portion. Anything that's above ground (entrance/exit of structures) must be Sch 80 for its added impact resistance.

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    Also, insert typical diy.se mentions that when it comes to burying conduit to outbuildings, go big or go home. Don't screw around with 3/4 conduit, throw in a 2" monster because it's only a little more expensive and it provides a ton of future flexibility. And better yet put two conduits in, one for power, one for data. Oct 6, 2021 at 17:56
  • There are listed conduit products that are similar to "black plastic water pipe" but it's not terribly common in consumer channels, so PVC is usually easier to get.
    – Ecnerwal
    Oct 6, 2021 at 17:59
  • I did specify "not UL listed", @Ecnerwal, so if OP finds something that isn't grey, but has the seal, that should still be covered.
    – FreeMan
    Oct 6, 2021 at 18:14
  • OK. Got it. Maybe not 2", but certainly 1-1/4. I'll make sure its UL. When you mention "data" are you talking about co-ax so I have wi-fi at the barn? Good thought, but I have network extender for that.
    – AKG
    Oct 7, 2021 at 18:45
  • @AKG data can go in the pipe only if it is fiber-optic and non-conductive. The main thing is if you're using it as conduit, the pipe inside diameter must be at least 138% of the width of the cable. This is disadvantageous for UF since it is so flat. Oct 7, 2021 at 20:31
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Aluminum is your friend. Really.

First, I strongly advise aluminum wire such as MH feeder. Copper just doesn't make sense on heavy feeder. Aluminum feeder has always been reliable (unlike what you may have heard about small branch circuits) and the lugs on the panels are aluminum anyway. Why make a dissimilar metal joint?

And if you're thinking "Well this isn't heavy feeder, it's just #10 copper (30A)" - A small feeder is actually the worst case scenario. On paper the voltage drop doesn't look too bad - about 4% in your case. But that assumes balanced 240V loads. Typically on small feeder there is 1 large load, a 120V load - and in that case, voltage drop is much worse.

Whereas if you spent the same money on #2 aluminum, its 90A ampacity is so great that a single large 120V load makes no difference.

Does UF require conduit?

Depends on the burial depth. Pick your depth.

  • Wires in Rigid Metal Conduit or IMC: 6" cover, 12" across vehicle paths.
  • A single 120V <=20A circuit that is GFCI protected at the source: 12" cover
  • All other UL Approved, electrical conduit: 18" of cover
  • Everything else including direct-burial wires: 24" of cover

If you want to enjoy the shallower conduit burial depth, you have to use approved electrical conduits.

UF is allowed for direct burial and so it can be used at the 24" burial depth (or 12" under the GFCI exception). As long as you are burying it that deep, you can sleeve it in anything you want. Ziti noodles. Snake skins.

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