Electrical question, can I double up 12awg t90 to make 10awg?
 

I'd like this to be answered by a real electrician ;)

My idiot ex-buddy (who is suppose to be an electrician, ex-buddy mostly because he became a miserable drunk) wired up my 5hp compressor 230v with 12 gauge 20amp breaker. It ran fine for years but has started to trip the breaker.

The motor is 5hp, 1740rpm, single phase, 208/230v and 24.5 amp. Fresh rebuilt, new bearings, run cap and switch. My research says it should be 30amp using 10awg wire. It's about a 50' total run
The exact same motor is in the vid below, showing the spec plate at 1:54
https://youtu.be/4RRB7tvm-eU

Now, I have all kinds of 12awg T-90 wire and conduit running almost all the way to the compressor and then I have a long enough piece of 10awg BX to get me the rest of the way. I know how to twist a proper joint.

Is it OK/legal to double up the 12awg T-90 to make 10awg?

No, not legal.

Minimum size for parallel wire runs is 1/0.

If you have 1/2 inch emt for most of the run, why not stop at an electrical contractor and have them pull off the amount you need for the run?
Use the old wire to pull in the new. Red, blue or black. You may have to pull a ground too (green). Some areas the conduit is no longer sufficient as a ground.
I would worry about one of your wire connections failing and the remaining conductor having to carry the load.

Noooo. Pull new wires.

I'd have to look but it's either 3/4" or 1" EMT and it has a ground wire run through it and only has two #12 t-90 (THHW is similar) running though it. So I think I have plenty of room. ;) The old wire is in the wall, but I know how to pull cable, that's not the issue.

From what I'm reading #12 T-90 is legal for 30amps.

Sounds like you've made up your mind.
I'd run 10 ga.

Small conductor rule says 20 amps for #12. You need #10 wire.

Ontario Electrical Code says #10 I think...pull a new continuous wire and be done with it.

If you have to join them Marr connectors in a box are legal. As is a CSA labeled terminal strip.

http://uk.rs-online.com/web/p/terminal-strips/2508940237/

His forum the Mod who is a mike holt instructor is one that says #12 THHW is good for 30amp

Ampacity of 12 THHN conductor

I'm not going to debate code with you. You've made up your mind it sounds. Do what you will, pull a permit and call for inspection and see how that goes... If you PM me an Email address I can send you a copy of the 2005 NEC in PDF format (it's the only one I have in electronic format.)

He needs to look at the Ontario Electrical code, it's the one that applies.

I will try to post a picture later today.

Just found a 55' chunk of 10/3 BX in my stash. might be a bit short to do a sanitary install, but long enough to get me by till I can run the EMT (then cable w/strain relief) right to the compressor.
This is how I wired my 2 post lift, so 2/3 of the way the EMT is already done.

I agree

The reason you shouldn't run parallel wires is that if one becomes disconnected you wouldn't know it and it would no longer be protected by a breaker. A single 12 awg would conduct 30 Amps just fine until it got real hot.

From what I've read #12 T-90, which is a stranded wire, is totally legal to 25amps and OK 30amps if it's just a spike on start up, but even licenced electricians are all over the map on this question.
I assume the maths may say they can, but it's easier to just go by the rule of thumb.
I'll look at the code when I have time and just run the #10 BX I have for now.

http://members.rennlist.org/warren/elec-hbk.pdf

60 deg C rating is 20 Amps for #12, 30 Amps for #10.

If you use 110C you can go to 35 and 45.

That is if it is by itself in free air

Did 5500 people really watch that video? Do you get like a penny apiece?

What is a design L motor? Is that some Canadian thing? If I figured this right, the startup current (Code G) should be about 130 Amps.

At the motors rated load it should draw 23.5 Amps so if it was working on a 20 Amp breaker it must not have been running at its full rated torque.

If you are curious about how the operating point is determined look at this chart Torques in Electrical Induction Motors and overlay a torque vs RPM curve for a reciprocating compressor (guessing a straight line, 0 at 0 RPM and climbing to the right). Where this intersects the motor torque curve, the curly line, is where the motor will operate. Drop straight down to the current curve to see the relative current.

To draw 20 Amps the motor must have been running faster than nameplate 1740 RPM (less torque). Motor torque would go up and down with voltage and the compressor load line slope would go up if more torque was required. The torque curve is pretty steep in that region.

Yes, I meant to use Table 2...corrected.

Well I ran the #10 BX and it ran perfect to 160lbs without popping the breaker, woo-hoo!
The run is a hack, up over the lights, but good enough for getting the job at hand done. Putting on a steel roof and I bought a framing nailer for the strapping.
In the winter I'll square it away right.

I don't think it was running faster on 20amp, I'd say runs the same, just it would pop the breaker as the pressure builds. For the life of me I don't know why my buddy, the union electrician, would run it on #12 with piss poor grounding. At the time I just bought what he told me, I wasn't cheaping out on the materials. It also worked that way for 15 years, but I bought a new compressor head with a bigger draw/faster recovery time.