Underground conduit to shed

[MRCLUNKER]

I am currently having an addition added to my current garage. I have a 12x12 shed with 12awg UFB wire running
thru 1-1/4 schedule 40 conduit to a 20 amp gfci circuit breaker in the garage panel. The conduit is approximately 18 inches deep.
There is a cutoff box in the shed. The shed runs a small compressor, Exhaust fan some lights and a few receptacles. I have never
had any problem with the current wiring.
The question is I need to temporarily remove this cable until construction is complete and then reinstall in the new
garage. This will result in a shorter distance of 20ft, previously 30ft. My question is;

1. Is this type of run to code? I know I can run UFB without conduit but I really would prefer to use conduit.
In the existing setup they simply ran the UFB to the garage and then conduit ends and the UFB goes straight to the
service box.
If I cannot use UFB and use THWN, Do I run 3 wires Black, White & Green and wire them the same as UFB in the garage Panel?
Once inside the garage, do I just continue schedule 40 to the garage panel.
2. Given the load I described is 12Awg sufficient?

Note; I have done basic wiring with NMB/Romex, have never used single wire THWN so give details if this is required.

Appreciate any help from anyone out there.

[HayZee518]

by rights and code, you cannot run romex or uf the continuous length you state, underground. the conduit is used for mechanical protection where the cable enters/exits the ground from/to a panel.
if you wish to use the conduit [full length] then yes replace the UF with THHN/THWN. 12 gauge handles 20 amp. Colors BLACK, WHITE and GREEN. You'll find the single conductor(s) a lot more flexible to pull into the conduit. Use the existing uf cable as sort of a "fish" to pull in the new conductors.

[MRCLUNKER]

Thanks HayZee518

I thought I would need THWN although I have read on some websites where this is a grey area with UFB thru conduit.
At any rate I am having this inspected so I do not want any problems so THWN it is.
Is THWN connections the same as romex with black to breaker, white to the neutral bus bar and green to the ground
bus bar? Also, once inside the garage do I continue with schedule 40 conduit to the service panel? Can I reduce the
schedule 40 diameter? or do I somehow transition to romex. I am supprised at how little info is out there for THWN wiring.
Plenty of stuff for pulling the wire but little on specifics getting to the service panel, Tables for wires thru conduit etc.

[HayZee518]

clearing up stuff

Once inside your garage through an LB fitting to a 4 11/16 junction box you can transition to any wiring method you desire. Romex is ideal if that would work for you. Bore holes in the studs at approx center for long runs. Use the proper two screw connectors or the newer plastic jam down clamp type to secure the romex to the box or panel. The green thhn/thwn - use a ring tongue terminal to ground the box in one of the 10-32 screws at the bottom of the junction box. The romex bare ground also goes here. The panel in your garage is called a sub-panel. If you'll notice there is not a bonding screw between the neutral bus and box ground. If there is, sumbody made a boo-boo. There should be an auxillary grounding bus in intimate contact with the metal box. All green and bare grounds go here. All whites on a 120 volt circuit goes to the neutral bus. If there is a bonding jumper or screw, remove it for the inspector is sure to notice it. THWN and THHN are essentially the same wire. It's the insulation type that's different. T means thermoplastic, H means heat resistant, W means weatherproof N means solvent and gas resistant. The extra H in THHN is a temperature coefficient.
FYI for pulling wire in conduit you are limited to an 80% fill to allow for expansion. Over 4 conductors you have to start de-rating the current draw of the individual conductors by a percentage. The NEC has tables on this.

[MRCLUNKER]

Thanks HayZee518
Your advice is always Top Notch!