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Questions: Will 8/3 UF Cable or 6/2 UF Cable fit inside 3/4" Rigid Conduit, and will it be up to national code?

Background: I am planning on wiring a shed for power. Approximately 50 amps, and about 111 feet in total from breaker panel on side of house (97' + (2 * 7')). I spoke to an an inspector who told me I had three options to bury the cable:

  1. 6" deep inside "Rigid" Conduit.
  2. 18" Deep in "conduit".
  3. 24" deep "direct burial".

It is difficult to find contractors out here to dig the trench, so I am leaning toward Rigid Conduit because I am not willing to dig a trench that deep in the hard ground here.

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    6" is to the TOP of the conduit. But if you pour concrete, you can be even shallower.
    – Ecnerwal
    Oct 31, 2021 at 22:08
  • Rent a trencher and do it yourself, that would be easily doable in less than a day. Nov 1, 2021 at 3:11
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    Also, dont use 3/4 conduit. Go big or go home. Future proof yourself and put a massive size in there, it will never be any cheaper to do so than now. Nov 1, 2021 at 6:07
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    I own a post-hole auger with an 8" bit. I used it for trenching by drilling a series of holes as reasonably close to each other as I could get. Making a trench out of the perforations wasn't much more difficult than drilling the holes in the first place. You, of course, could rent a post hole digger. You don't indicate if the ground is hard because it's hot & dry, or frozen solid, but if it's hot & dry, hosing it down a bit to soften it (don't turn it into a mud pit) would making the digging easier. Of course, once you've got a trench that wide, it's easy to put big conduit in it!
    – FreeMan
    Nov 1, 2021 at 16:28

3 Answers 3

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You can't use 6/2 if you want to use it with a normal subpanel. If you are balking at the price of 6/3, we have an answer for that.

You can't use 8/3 because in NM and UF, it's only good for 40A.

I don't know why you want to use UF cable inside conduit,

"It's the devil I know"

... understandable. But the 6/3 UF you would actually need would require 2” conduit (really).

If the run is entirely inside conduit, or for the segments which are inside conduit, then meet a new devil: THWN or XHHW or USE. It is sold as single individual wires. Buy what you need.

If you like saving money, also, it comes in aluminum, which is fine for these large sizes, especially going to a subpanel, where the lugs are aluminum!

As a bonus, THWN and XHHW are allowed higher amps on the same size wire!

For 50A using THHN, XHHW or USE, you can use #8 copper or #6 aluminum for hots and neutral. Use #10Cu or #8Al for ground. You can mix and match.

Rigid Metal Conduit

RMC is a specific type of conduit that looks like heavy steel pipe for compressed air. Because a shovel strike won't even faze it, it is allowed to be buried with only 6" of cover.

It is threaded, and you should thread most of it, but they also make threadless couplers so you don't have to get into pipefitting for that last few feet that need to be precise.

However, it is hella expensive. As much as $3.00 a foot! depending on size.

If the conduit route is complete, you can use the shell for ground. But I wouldn't. #1 that only lasts until the pipe rusts out. #2 that means you can't switch to PVC conduit for the parts that are not underground. (You could switch to EMT, however; that can carry ground.)

The conduit must be assembled absolutely complete, backfilled, and tamped before ANY wires can be pulled into it. That means you need to build it properly so it can be pulled. No plumbing elbows! Access points must be built-in where they need to be (e.g. LB conduit bodies at the ends of the stub-ups).

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Use THWN in the conduit. It's easier to pull because there's no bulky outer jacket around the conductors. The W is for wet locations (like underground conduits, which are likely to get wet over time.)

UF is for direct burial, not conduit applications.

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"Rigid" means metal conduit (or EMT). And it's probably your best bet here because

  1. It's allowed to be shallow
  2. It counts as ground

Switch to USE-2 6 gauge aluminum tho. It's better for this application since it will be in conduit, and it should be cheaper

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    Rigid is not EMT. Rigid will take all sorts of abuse that EMT will not, and thus is gets the low cover requirement. The wall thickness is easily 3-4 times thicker, without going and looking it up. Rigid, IMC (intermediate Metallic conduit) and EMT (Electrical Metallic Tubing) are 3 distinctly different products, though the first two are more similar than the third.
    – Ecnerwal
    Oct 31, 2021 at 21:07
  • I did not know the metal pipe could save me from running a ground wire (can someone else confirm?) Question/clarification: Two 6-gauge aluminum will meet national code for ~50 amps at 110 feet?
    – Roger
    Oct 31, 2021 at 21:18
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    @Roger -- provided you're landing on 75degC terminations, it should, yeah Oct 31, 2021 at 22:00
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    You still need a neutral wire for this. So you're looking at 3 wires (not 2) plus ground-via-conduit for any normal USA/Canada 50A service.
    – Ecnerwal
    Oct 31, 2021 at 22:07

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