12-2 UF with a 'big' dimension of 0.463 results in a calculated area (pretending it is now circular with a diameter of 0.463 inch) 0.463^2 x pi/4 = 0.1684 in^2
A single chunk of 12-2 UF would then meet the Chapter 9 Table 4 fill values in Rigid Schedule 40 PVC at the 2-conductor 31% fill-ratio with 1" (nominal) conduit.
But "cable" is supposed to be counted as 'one conductor' for fill area calculations. So that puts the single chunk of 12-2 UF in the 1-wire/conductor 53% column, so 3/4 inch rigid PVC Sch 40 or Sch 80 meets that requirement. Not that I would really like to pull 50+ ft of UF 12-2 through some 3/4 inch plastic conduit.
But running two chunks of 12-2 UF in the rigid PVC conduit then triggers an ampacity 'adjustment' down to 80% (2 current-carry conductors per cable x 2 = 4 current-carrying conductors), as well as increasing the calculated cable cross-sectional area up to 2x the previous number or 0.3367 in^2, which then pushes up the required conduit size up to (now using the two wire column) 1-1/4 Sch 80 or Sch 40 rigid PVC.
Of course, all this won't matter if the AHJ doesn't allow UF inside plastic conduit, or doesn't like the burial depth you are putting any of this in, or whatever else they don't like.
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Whoops I did forget to calculate the area It did seem odd, 2", but it was late and I just threw that out and went to bed. Moonrise is dead on.
In reality we never have to calculate wire dimensions, we just use the annex C tables.
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Thanks for all the replies. I've been down at the shore working all week and haven't had internet access despite the fact Comcast says there's an internet hot spot on the block, so I couldn't reply until now. Talked to the electrician briefly this evening. He's going to pull the permits next week for the job. I just have to give him the exact address etc for the paperwork. We'll see what they want.
I'm going back tomorrow with the customer to do layout for lights, ceiling fans and so on, so I'll probably know by the end of the week if she wants the lights switched or not. Original wiring to the shed was UF I believe, but I haven't found the house end of the loop. My guess is it's under the house and like everything else got flooded and may be questionable.
I like the idea of the switches you don't have to run wire for. I'll have to ask the electrician about them. They may simplify things. As far as THHN in conduit, not sure how well that would hold up to salt water immersion. Almost every piece of conduit I've ever had to dig us was full of water it seemed. Add in the very real potential of flooding ( yard floods pretty regularly in winter storms when the bay backs up) and I'm not sure I want to take that risk. FEMA seemed to be concerned with corrosion issues if standard Romex was immersed even for a short while. Not sure how standard Romex coating on the wires varies from THHN, but I know UF holds up to most things. When in doubt, go overkill...
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THHN that is also THWN is just fine when wet. Pretty much all underground conduit is wet BTW.
Romex's only issue is that there is paper inside, where UF has none (and has a jacket that is not affected by sunlight).
I'd say go with THHN. Oh, and if the ends of the wire get wet, you're better off with solid wire than stranded. Stranded wire can wick water between the strands.