What kind of conduit should I use with UF-B to reduce depth of burial?

Question

I currently have a 50' length of 14/2 UF-B. I only need to dig a 40' trench, but I know it should be buried 24" deep. That seems like it will be a lot of work to bury it that deep. It seems like it would be a good idea to run it through some form of conduit so that I don't have to dig so deep. There's also the added benefit of being able to run a different size cable in the future. I'm not sure if there are different requirements with running UF-B through conduit, so I'd like some advice on which conduit I should use, or how I should dig this trench more easily.

I assume that the 14/2 UF-B is still the cheapest cable option. I looked into renting a trencher, but the cheapest I could find was $150 for 4 hours. since it's a 40' trench, it didn't seem worth the price if I could just use a shovel. PVC conduit is pretty cheap, but as far as I can tell, it still needs to be buried to a depth of 18". As far as I know, the only other option is IMC, which only requires a depth of 6". Looks like I can buy 10' lengths of IMC for $16 (so $64 for 40'). I don't know if there are any other options. I found 50' length of flexible PVC conduit for $35, which is cheaper, but I'm not sure how deep it needs to be buried.

Answer 1

I'd use conduit with THHNs in it and save the UF for wiring the office-shed

One thing most people don't realize about UF is that it can be used as a substitute for NM with the same makeup; the NEC permits this in NEC 340.10 point 4, and while it's usually uneconomical, it's a good way to use up spare lengths of UF. This is because while it's legal to use UF in conduit, it's a waste of conduit fill compared to using individual wires, and also much harder to pull through the conduit compared to individual THHN conductors.

I'd use a 1.5" and a 1" PVC conduit at 18" to top of conduit (20-24" deep trench) in your situation; if you don't need anything else, you can use the 1" conduit for 3 (1 circuit) or 4 (1 MWBC = 2 circuits) 12AWG THHNs to a 60A non-fusible AC disconnect box on the outside of the office-shed. You can then run your branch circuit(s) from that box, and just provision a 15A feeder breaker at the panel you're running this from, with the 1.5" conduit left as a spare for future provisions of more power, or fiber Ethernet to the shed for that matter.

If trenching that is out of reach, you could use rigid or intermediate metallic conduit in the same sizes in an 8-9" deep trench, but keep in mind that that conduit is going to pose an obstacle to landscapers and others going digging in the yard in a way a 24" deep PVC conduit won't. You'll also need to use metal (RMC) expansion fittings in the riser at each end instead of the much cheaper PVC versions, in addition to the conduit being considerably costlier than PVC, and occasionally requiring either compression (threadless) couplings or a threading machine.

So, it's either more work trenching, or more work hauling, cutting, and threading conduit, with the latter creating a hidden annoyance for landscapers as well. Pick your poison.

Answer 2

If your goal is to minimize trenching, Rigid conduit is the best choice... it only needs 6" of cover, you can trench it with a garden trowel. Note this is 12" under a vehicle pathway (driveway etc.) Use largest size you're willing to pay for but 1/2" is enough for small stuff. Note that Rigid conduit and the metal boxes it is bolted to qualifies as the grounding path, so 1 less wire.

Now to be clear about conduit -- you are required to assemble, complete, and bury the conduit before you start pulling any wires. You cannot assemble the conduit around the wires. Then you pull all the wires in.

See what ThreePhaseEel says about ... well, anything lol... but in particular, wire types. I would use stranded THWN myself. If you bring over 2 hot wires, they can be the same color. Neutral must be white or gray.

THWN cannot go "loose in walls" so at the end of the conduit run, install a junction box and transition to normal NM cable. The disconnect box will suffice on the shed end.