OK to double up 2 #10 wires instead of 1 x #6?

[Nick]

Hey,

I'm thinking of installing an electric on demand water heater in the garage and already have a bunch of #10 MC and as the run is 20' or so I was thinking of using that if it was OK. It would run across an unfinished ceiling (ie under the floor above) so I think MC would be needed?

Thanks

Nick..

[RBM]

MC is used in places of public assembly, not that you can't use it in a garage. The smallest cable allowed to be run across the edges of beams is

6/2 or 8/3. The smallest size wire you can parallel (double up) is 1/0 if I'm not mistaken, but certainly not #10

HTH

[Dan]

Short answer, according to the NEC, no. Get the proper size cable. Drill through the joists, or use protector strips.

Dan

[Michelle P]

[Ralph Mowery]

I don't know the code, but doubling the wires go down 3 sizes so you get the equal of a # 7 wire. The lengths of the wire (within reason ) will not make any differance in the voltage. One wire will carry more current ( probably less than 1/4 of an amp if the length is off by a foot out of the

20 foot run. At work we parallel 4/0 wires all the time on 480 3 phase circuits and never worry about the exact lengths.

[trader4]

two. if the wires are not the exact same length the phase in the too will be a little out that can give a lower or higher than expected voltage. Michelle (EE major in college)

Michelle, you should have stuck with the first answer. Any phase difference due to wire length issues of two conductors run side by side in a garage for AC are totally negligible.

EE - graduate

[Andy]

Good answer, Michelle. But with a wavelength of 5 million meters, I don't think a foot or two will make much phase difference. The cross section and purity of the copper is usually fairly well controlled, so the one-tenth of a volt difference or so won't do much either.......

So, while you are absolutely correct, I suggest that the difference won't amount to squat, in my opinion...

The code requirement is something else entirely.... If wiring wasn't required to be done in a standard way, we will have as many different collections of wires as we have electricians...... It's nice to open up a box and see that it was done in a manner that let's you know that the last guy/gal there did it the same way as the two previous guys/gals there......

Just an opinion from an old, retired, electrical engineer :>))) ( whose work is mentioned in the bibliography of some of your textbooks (grin) )

Andy

[Doug Miller]

Short answer: no.

Long answer: Same subject was discussed to death just a few weeks ago in this newsgroup. Check Google for what was said then.

-- Regards, Doug Miller (alphageek at milmac dot com)

Nobody ever left footprints in the sands of time by sitting on his butt. And who wants to leave buttprints in the sands of time?

[Jeff Wisnia]

My entering freshman class at MIT (1953) had zero female EE majors in it.

So, we could get away with saying, "If G-d wanted women to become electrical engineers he would have given them brains instead of boobs."

That expression isn't PC these days, so I quit using it - about a year ago.

Jeff (With apologies to Michelle.)

[Pop]

Uhh, no, that's not correct at 60 Hz. I'll bet you just got into Field Theory and/or sufrace-conductivity, or em's, didn't you? The phases would be identical at each end of both wires unless one was incredibly longer than the other, and coiled to boot. Not an unusual mistake; keep on studying - you'll be glad you did.

Pop

[RMUH]

I understand the desire to put that cable to good use (waste not...) but for 20 feet, it just isn't worth making up a kluge. By the time you follow all the rules for properly paralleling the two sets of conductors, it won't be worth the trouble.

[RMUH]

At 60 HZ, the phase change over a few feet is not an issue. The issue is the resistance of the wire. Any difference in length will cause a reciprocal difference in resistance, causing the current to divide unevenly between the two conductors.

[Nick]

Thanks All.

Guess I'll go out and pick up the right wire.

Nick..

[Doug Miller]

At one-point-two ohms per thousand feet, the change in resistance over a few feet is not an issue either.

-- Regards, Doug Miller (alphageek at milmac dot com)

Nobody ever left footprints in the sands of time by sitting on his butt. And who wants to leave buttprints in the sands of time?

[Ralph Mowery]

The differance in resistance of a few feet is not an issue either at the current levels being used.