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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.

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  • If it's in the US and it's not a time machine post from ~60 years back, it's 240V. – Ecnerwal Apr 12 '20 at 12:12
  • I'm familiar with calling it 120/240, but most products I'm seeing are still being marked 110/220 so I went with 220 in the question. – psaxton Apr 12 '20 at 17:55
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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.

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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.

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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.

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    Virtually all THHN is ALSO THWN (marked as both, but typically "sold as" THHN. – Ecnerwal Apr 12 '20 at 12:09
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    I have contacted mfg’s when THHN was not stamped with both thhn/thwn-2 and told them I would be returning it unless they sent the appropriate documentation. Inspectors don’t even check and haven’t for years, possibly some super cheap stuff from China is not dual rated+ – Ed Beal Apr 12 '20 at 16:19
  • non waterproof insulation will turn to chalk (in a few years) if it gets damp – Jasen Apr 12 '20 at 20:00
  • Thank you for your answer, it is useful information. I neglected to say in my question that the conduit will not be buried. It is about 3 feet of MF connecting a 1' span from an existing 50 amp termination intended for a hot tub to a shed built on a cement pad originally intended for a hot tub. – psaxton Apr 13 '20 at 14:49
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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).

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  • 240v is split phase and identified as L1 and L2 marking is not required as they are 180 degrees out of phase with each other so it dose not matter . Marking on 3 phase matters especially with corner grounded delta as this sets up a wild leg that is required to be marked B leg and identified with orange phase tape the phases are 120 degrees out of phase so it matters how they are connected. It is very uncommon to have 3 phase in residential in the US but other parts of the world it is common. – Ed Beal Apr 12 '20 at 16:10
  • @EdBeal you got your deltas mixed up -- it's the center-tap-grounded (high leg) system that has the high/wild/power/stinger leg. Corner grounded (grounded B phase) just has an annoying situation where the grounded leg is a pseudo-neutral of sorts, but not really a neutral because it's not connected to a neutral point – ThreePhaseEel Apr 13 '20 at 0:58
  • I don’t think so but L1 and L2 on US systems are always 180 out and do not require marking! All deltas with the wild leg identified as L2 or the orange require marking , code has required marking 1,2,3 front to back. Top to bottom. Or left to Right since I got my first license . Look at it a corner grounded has One of the 3 legs grounded a center grounded is only available as A Wye or star are you just a bit? This is ELECTRICIAN 101. – Ed Beal Apr 13 '20 at 1:26
  • I was aware of high leg marking, but figured if the OP couldn't identify 208 or 240 vs. 220v then when and where high leg deltas needed to be marked was way out in left field. – NoSparksPlease Apr 13 '20 at 2:45
  • NEC 110.15 requiring marking defines the High-leg configuration. "on a four wire, delta connected system where the mid point of one phase is grounded, only the conductor or busbar having the higher phase voltage to ground shall be durably and permanently marked by an outer finish that is orange in color or by other effective means. Such identification shall be placed at each point...if the grounded conductor is present." – NoSparksPlease Apr 13 '20 at 3:13

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