running two circuits on a single piece of cable

[Doug Miller]

Safety, practicality - you're fine. Code approval - maybe, maybe not. The National Electrical Code permits that. But not all jurisdictions have adopted the NEC for their local code. Some have adopted the NEC with restrictions. It varies all over the place. The only way to know what is acceptable where you live is to ask the code inspectors where you live. Knowing that you're in NJ isn't necessarily much help, as it could vary considerably from one place to the next within the same state.

-- 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?

[Pop]

...

===> True; that's one way to do it. Each of the two hots have 120Vac on them, just 180° out of phase. Just be certain you take the red/black to opposite phased connections in the breaker panel. Usually they alternate per breaker; use a meter to be certain. 240V = two phases. 0V (or nearly 0) indicates same phase. You want both phases, one on red, other on black.

BUT, if you're talking about ONE Duplex outlet, one with TWO receptacles, and having each receptacle in the duplex outlet on a different breaker, you also need to be sure you isolate the two receptacles in the duplex outlet by breaking the bar that connects them together.

===> Yes. Because of the phase difference between the two lines, one does not linearly add to the other; they only add by phase relationship and thus will never put excessive current into the neutral or earth.

===> Well, they're at the same "potential", but not at the same time. If you've wired it right, one is at 0 when the other is at 120, and vice versa, so at any instant in time current is 20A in your case. _-_-_-_- If you put the wires on the SAME phase, then each could carry 20A and you would have double the current flowing in the neutral and that wouldn't meet code since it would be possible to put 40A in the neutral wire.

It's really much better to run one cable for each outlet, from two breakers. Easier to identify, and easier to keep straight when you're doing the wiring, too. Then you don't have to worry about getting the phases right. This is oversimplified, but: red to black is intended to be 240Vac difference between them. Black\white is intended to have 120Vac between them.

Pop

[Takoma Park Volunteer Fire Dep]

You can still run a single cable. there is cable available that has two separate neutrals as well as two ungrounded conductors and one ground in the same cable jacket. The only advantage is the effort saved in not running two cables. One such cable had a black, red, white with black tracer, white with red tracer, and a bare ground. The cable was developed to save labor on runs serving bedrooms that are required to have AFCI protection which prevents using Edison circuits because AFCIs are not yet available for those circuits.

-- Tom H

[Calvin Henry-Cotnam]

Interesting. I can't say I have seen this prohibition in our code, at least for residential dwellings, but I have done it on occasion without problems from inspectors.

We have a cold storage room and I wanted to put in a small (300W) baseboard heater with a thermostat just above the freezing mark just to prevent freezing if we were hit with a real cold spell. The room needed lighting and I wanted an outlet, so I ran a 120/240 volt circuit that powers the light on one leg, the outlet on the other, and the heater using both. The inspector approved it.

Also, at my parents house they have a pool pump that is configured for

240 volt operation and one leg of the circuit provides power to an outlet and some lights in the yard. To add to it, a few years back when we were renovating the kitchen and needed to add some new circuits, I asked the inspector about using the other leg of that circuit to power the garbage disposal and he had no problems with us doing that.

As long as you are within the 80% loading rule, there's no problem here.

[Calvin Henry-Cotnam]

Same here. ;-)

Come to think of it, it is more foolishly considering the existence of mini-breakers (or twin, or half-size) that squeeze two breakers into one position -- in the case of those, two side-by-side breakers are not necessarily on opposite legs!

[jwalters]

The real problem is that this device was not installed by a professional electrician. If you knew what you were doing, it would work. Obviously you are incapable of doing this yourself, and the average homeowner should never touch any electrical wiring. Leave it to the professionals to insure your safety. Besides that, tampering with electrical things will void your homeowners insurance.

A general rule of thumb is this: A homeowner should only flip light switches, and plug UL Approved electrical things into outlets. That's ALL you should do, except to turn off a circuit breaker in an emergency. You should hire a Union Electrician for all wiring. Hire an electrician to change light bulbs, and hire an electrician to turn ON or OFF all circuit breakers in non-emergency situations. Also, if you have the old fuses instead of breakers, DO NOT touch them even during an emergency. They are extremely dangerous and can explode if handled by an inexperienced home owner. Even light bulbs are capable of exploding and killing people. Thousands of persons die every day as a result of electricution and fire caused by non-certified people tampering with electricity. Just last week an entire family consisting of three adults and seven children died as a result of an inexperienced homeowner attempting to change a light bulb, which exploded and released toxic gasses, killing the entire family, and causing one of the worst fires in U.S. history

Learn not to burn.

John Walters Professional Electrical Consultant and Union Certified Electrician

1030 Market St. Los Angeles, California [e-mail]

[kzinNOSPAM99]

I'll get right on that. I think I'll hire a plumber to flush the toilet for me as well. Can never be too careful.

[Chris Lewis]

According to :

It's legal in Canada. In fact, until recently, it was the _only_ way to wire kitchen counter outlets.

_Not_ in the same outlet box.

12/3 with split receptacles is the way to go.

I wired my shop that way.

The Canadian inspectors were quite happy.

[Goedjn]

Well, that makes sense, if you're allowed to have two un-tied breakers feeding different legs. All *KINDS* of fun things can happen if you pull one breaker of a mixed circut like that. Starting with but not limited to melting any motor-driven equipment anywhere on the loop, and ending with presenting you with line current in a box you just pulled the breaker on.

[Chris Lewis]

According to Calvin Henry-Cotnam :

Egad, and this was one place where I thought the CEC was smarter than the NEC.

Sheesh.

[zxcvbob]

Just curious, but could you put a 240V convenience outlet on that multiwire branch circuit without pissing off the inspectors? That could be quite handy in a shop.

Best regards, Bob

[Chris Lewis]

The CEC does prohibit it. It's just that with a multi-wire branch with only one receptacle, you'd not be "mixing voltages" if you swapped it to 240V.

The inspector likely approved it because the circuit wasn't "general purpose".

Now _that_ is somewhat strange.

[Chris Lewis]

According to Chip C :

There are _some_ panels where a dual breaker does not guarantee opposite legs. Ie: certain Federal Pioneer/Pacific panels, where it's possible to install a regular dual breaker and both breakers are on the same leg.

You should examine the backplane carefully (or use a voltmeter) to be sure.

[Chris Lewis]

You couldn't have both 240V and 120V outlets on the same circuit.

[Henry thinks differently, but unless our code has changed in this regard since my copy, CEC still prohibits mixed voltages. I believe the exceptions he encountered was an inspector giving him a break on a very specific situation.]

[Chip C]

Oh yeah; somehow I had it in my head that the ends are on separate breakers.

(I guess in the UK the breakers have two wiring connections for this purpose?)

So now I'm pretty darn sure that's a big I Haven't A Clue.

Chip C

[Chris Lewis]

According to Chip C :

I'd be willing to assume that there hasn't been any devices _in_ Canada with those plugs pre-installed... ;-) Because, until recently, those T-slot receptacles were illegal, and what _few_ 120V/20A circuits there were, were almost always hard-wired, and didn't even have non-T'd

20A receptacles.

There must be some equipment out there with aftermarket 20A plugs on them, because you _can_ buy them ;-)

[zxcvbob]

Window air conditioners, about 15000 to 18000 BTU/hr.

Best regards, Bob

[Doug Miller]

Right - and I need to amend my statement above, which should have read "... prohibits mixing 240V and 120V loads on a multiwire circuit unless there's a single-point disconnect."

-- 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?

[Calvin Henry-Cotnam]

Chip's wording should be: "two pole" breaker, not "dual".

A two pole breaker will guarantee opposite legs, and will have a common trip (tied handles).

"Dual" simply means there are two in one housing, and given the presence of mini-breakers (where two fit in the space of one, and are therefore fed from the same leg!), one must check to be sure.

[Chris Lewis]

According to Calvin Henry-Cotnam :

I _meant_ "a two pole breaker does NOT necessarily guarantee opposite legs".

A certain not-ancient generation/model of Canadian Federal Pacific panel (Federal Pioneer in the US) had (has?) the breaker backplane having the legs sequenced as "AABBAABB".

Which meant that a full size standard two pole breaker with integral tiebar could (can?) be installed such that both breakers are on the same leg.

During a renovation for a friend (new panels), I switched their stove over to the new panel on a FP two pole breaker. I had noticed the odd backplane arrangement, and had to be careful about slot allocation because of it.

Tested stove, worked fine.

The next day, my friend called me and told me that only the stove clock and stove accessory receptacle were working, nothing else was - no heat.

It was a long trip to their house, and we weren't planning on going there for a while, but they needed the stove.... Just before sighing, hanging up, and travelling down there, he casually mentioned that he had pulled the breaker for some reason then reinstalled it.

Bing!

He slid it over a slot, and the stove started working again.

When you first start doing 240V work on a panel, you should double check the backplane arrangement to be _sure_ it's not dumb like that FP panel was.