Are the red and black two separate circuits and if not, are they both hot? If they are both hot and not the same circuit, then the return must be through the conduit, which is against code. If they are not both hot, then maybe one of them is the neutral, but wrong color. There still needs to be a third, ground wire. If this is really EMT conduit, then pulling the old wires out and pulling 3 or 4 new ones shouldn't be too difficult (4 if you have two circuits, a neutral, and a ground).

'

mtvhike, Thanks for the reply. Both wires are hot and on different circuits. The whole line is EMT conduit. It looks like a setup for 220, but without the neutral. Just trying to avoid pulling wires because the boxes are over 15ft high.

Are you sure they are on different circuits rather than just being connected to the same 240V circuit? There's no conceivable reason why someone would pull hots from different circuits without a neutral.

Ron, I checked again and you're right. It is only one circuit.

Does your heart stop when you touch the metal conduit? :D

What does this line supply power to?

I would guess some goof used the red wire as a neutral and its 120V?

Pics of inside the boxes? Cut power to the circuit and test lines for voltage before going in the boxes.

No, no heart stoppage. I'm in an old factory building and the wiring is mostly way beyond my skills. I don't know how to trace wires other than visually. Some of the boxes are near the top of two story spaces. Some of it is done correctly and some of it is done wrong. A lot of non-electricians (artists) have done a lot of modifications over the years. I've found several places where people have connected 14 gauge wire to 20 amp breakers. I've fixed that.

Pictures of inside of boxes are very uninteresting. The end box has a black and red wire entering, nothing exiting. The next closest box that I can get to is just a pass through box and no connections.

All we can do is guess without more info. Did the circuit ever work properly even? Its either a 240V or a 120V with one of them being a neutral and not white. What did it power?

Sounds like an ordinary 240 V circuit. What voltage do you see across the two hots?

A 240v circuit does not require a neutral. And it would use a red and a black.

But shouldn't it still have a ground, in addition to the conduit?

Thanks everyone. Yes, this is a 240 connection. Now I understand.

358.60 Grounding. EMT shall be permitted as an equipment grounding conductor.

"And it would use a red and a black."

Unfortunately wire colors mean approximately nothing. Green or green/yellow or bare is typically ground, and white is typically neutral, but beyond that you're on your own-- ie you need to check what's going on-- and you should check your neutrals and grounds as well to ensure that's really what they are. I think the rule for hot conductors is only that they should be something other than white or green. When wiring 240 v circuits with two conductor NM cable you typically mark each end of the white wire with black or red to indicate that is is not neutral. Just last night I was working on a motor where the power cable had wires that were green, yellow, blue, and brown; no red or black to be seen. A shielded SO-like power cable I just bought has three numbered blacks and a yellow/green stripe.

While the wire colours don't assure you of anything, a red and a black without any other wires is a really good indication that it's a 240v circuit.

Of course you should still test things out to verify it but the colours of the wires are a good way to indicate what's going on. That's actually the purpose of the colours.

Okay, now that we've established that this is a 240v ciruit, I'd like to convert it to two 110 circuits. Can I add just one white wire, or do I need to add two?

You only need one. It's called a "shared neutral" or "multi-wire branch" circuit.

One white. Adding a wire without pulling the exiting ones and pulling all new can be difficult to impossible unless the conduit is significantly over sized.