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I'm going to be wiring a 50A 240V circuit to a shed through an MF conduit. There will be about 3 feet of conduit connecting to a subpanel and it will remain above ground. I know the neutral wire must be white or grey and the ground _must be green, yellow-green or bare. Can I use 2 black conductors for the hot wires or must they be black and red?
This is for a location in the United States.
The answer is yes you can run your 2 hots the same color and they can be any color except white, gray, or green. If your circuit is only 220. (240 in modern terms) you will also need a ground, the ground can be metallic conduit or if using pvc conduit a green is normally used. If you want to run anything other than 240v you will also need a neutral to set up a breaker panel. If this is a detached shed a ground rod will also be needed. With more info we can help you size the wires & conduit based on the load and distance but more info will be needed to do that but Yes you can use 2 black insulated conductors.
If you are going to a shed and want 120V, you will really, really want a neutral wire. That must be white or gray, with optional colored stripe (e.g. white with red stripe).
You absolutely must run a ground wire even if you have a ground rod at the shed. That must be green, yellow w/ green stripe, or bare.
The hot wires can be any other colors. They can be the same color.
For instance if you are a Prince fan, you can use purple, pink and white :)
One guy left me a system with two 30A 240V circuits: one black-red, and the other black-red. Thanks. I rearranged them so circuit 1 was black-black and circuit 2 was red-red.
No you cannot, THHN is only suited for dry locations. Buried conduit is a damp location.
the wire must be instead (or also) rated THWN or THWN-2 etc.
The other answers cover the matter of neutral and ground conductors which will also be needed.
Assuming this shed is behind your home, then you are working on a single phase system with only one voltage system (240/120v is one system) you can use two blacks as the hot conductors.
Gray, green and white color restrictions also apply as noted by others.
If this shed is behind a building with multiple voltage systems then each phase must be identified "by phase or line and system at all terminations" [NEC 210.5(C)]. Designating wire color and posting it on the panel is the easiest of the allowed methods that follow. (I know this likely doesn't apply to your question, but sometimes people misapply one answer they see to all applications. Like it's true "you can't bury NM or THHN in pvc", but not because you can't put NM or THHN in PVC, but because buried is considered wet location, even in conduit).
