I don't have formal training as an electrician but I have done a lot of wiring over the years. I have to run a lighting and outlet circuit to a new area in my basement. A friend suggested I run one strand of 12/3 to service the 2 circuits. I was not aware this could be done?? I thought each hot lead required it's own neutral.

Are there any drawbacks to running 2 circuits on one 12/3 wire?

 

The answer to your question is yes, you can do this.

This is called a multiwire circuit. To make this work with a single neutral, the two hot wires must come from different legs of the circuit. The current on the neutral will be the difference between the current on each leg. If one leg draws one amp and the other legs doesn't draw any current then the one amp will be on the neutral if one leg draws 3 amps then other leg draws 3 amps then there is no current on the neutral.

The advantage of a multiwire circuit is that you only need to run one cable from the panel to the location where the circuit is used. This is often done to the first location, and then separate cables are run from the first junction box.

The danger with a multiwire circuit is that working on one half of the circuit requires both halves to be shut off. For this reason you need to provide a common trip for the breakers.

Multiwire circuits are frowned upon (by some) for residential installations, because homeowners do not understand them very well or are unfamiliar with them, but are common in commercial applications. The distances in residential applications usually do not justify the use of a multiwire circuit.

 

I am running aprox. 70 feet before the curcuits hit the first junction box and it is a hard run to fish wire. That is why I thought 12/3 was wonderful.

As I understand it, as long as the breakers are stacked on top of one another, one will pull from one bar and one from the other.

 

As long as they are stacked on on top of the other, and as long as they are not those skinny tandem breakers, they will get power from separate legs.

I generally try to discourage multiwire circuits. However, they do have their uses, and you situation may be one of them. But you may find that you can simply fish two 12/2 cables at the same time about as easily as one 12/3.

If you do decide to use the multiwire circuit, I have three suggestions:[list=1][*]Use a voltmeter to ensure that you have 240 volts between the black and red.[*]If you need GFCI or AFCI protection on this circuit, then split the 12/3 to two 12/2s in the first junction box you come to. If you don't understand why I said this, then maybe you don't understand multiwire circuits well enough yet to install one.[*]Use a double-pole breaker, even in situations where it is not technically required.[/list=1]

 

Another consideration, one of the circuits is for a home theatre system. Could I get interferrence if it is one leg of the 12/3 wire?

 

A few questions/comments.

"The danger with a multiwire circuit is that working on one half of the circuit requires both halves to be shut off. For this reason you need to provide a common trip for the breakers."

This is not code but a suggestion. Code says only if both circuits terminate on the same device yoke.


"The distances in residential applications usually do not justify the use of a multiwire circuit."

You obviously haven't wired many houses. Multi wires are a good way to lighten the wiring fill in a panel. Also it greatly cuts down the number of homeruns. Try it next time you have a new house with a full 40 circuit panel.


"and as long as they are not those skinny tandem breakers"

The most important thing to remember!


"If you need GFCI or AFCI protection on this circuit, then split the 12/3 to two 12/2s in the first junction box you come to. If you don't understand why I said this, then maybe you don't understand multiwire circuits well enough yet to install one."

I'm a licensed electrician and I still don't get what you mean. The only way to use GFCI/AFCI with a multi-wire is a two pole GFCI/AFCI breaker.

 

Yes, very true of AFCI, since AFCI receptacles haven't been marketed yet. But GFCI protection can certainly be provided with ordinary GFCI receptacles, providing you do as I said.

 

OH, OK, I see what you meant. The AFCI threw me off.

 

My comment regarding multiwire circuits not being justified in houses had to do with adding circuits, not with new construction. I was also thinking more along the lines of do-it-yourselfers, not electricians.

Even so, if I were building a house or having my house completel,y rewired, I would make it clear that I wanted no multiwire circuits.

 

Funny how things are foreign to different people. Multi-wire branch circuits are the norm where I come from. We only use seperate neutrals if it is spec'd or if it is for an isolated circuit (clean power).

 

I would highly recommend running a separate 12/2 for this purpose.

 

Dare I ask why?
--
Tom H

 

If it were me, I would run a 14/3 to a 2 gang box with 2 duplex receptacles each on a circuit. Then a power strip off each outlet. With today's home theaters there is no way you'll be able to plug everything into the wall. You'd need 4 or 5 duplexes. I must have 8 or 9 things to plug in at mine.

I also don't understand people's obsession with 12 wire. Any residential circuit (barring kitchen, bath, basement, garage, laundry, any utilitarian circuits) can be run in 14 wire.
If you use this stuff you know what I'm talking about. Pig tails and splices are much easier and neater in 14 and device hook up is also much easier. You just cannot put as many devices or lights on a circuit.
With the advent of AFCI's we put each BR on it's own circuit anyway. Having 4 or 5 receptacles and a light on a circuit is far from full.
I have been told running 12 is safer which is total BS.

Running spares to an attic I can see using 12/3, just in case, but for general use 14 is fine.

 

Interference on the neutral from the other circuit.


For home theater you want everything to be on the same phase, no questions. Besides the obvious of ground loop potential, this way if you have an appliance that is causing interference you could change the phase of that item (fridge, dimmer, etc.) Also if you're going through the hassle of running a dedicated line for a system, why not use 12? Do you realize that there are amps that require a 20A circuit. Also, if you're running just a single line, you need the capacity of a 20A circuit.

 

I don't understand ANY reason not to. It's definitely not a cost issue. Besides, what do you think teens have in their bedrooms anyway...a 13" TV and a table lamp? Teens these days are running full surround sound setups with big screen TVs and powered subs while running their computers all at the same time. Plus lets turn on that ceiling fan and recessed lighting and now tell me that 14 is enough for anything while maintaing that 20% overhead.

 

It is. In 15 years I've NEVER had a complaint that breakers are tripping due to too much load on a general use circuit.

 

True most circuits don't call for any more than a 15 amp max. load these days. But then again, when my first house was built in the 1950s, four 15 amp fuses ran the whole house. Let's see you run a whole house now on four 15 amp circuits. I'm running 20 amp outlet circuits on the chance maybe someday my wife will want to plug in a space heater on a cold day. Or who knows what new electronics they will come up with in 20 years. As long as I'm running the wire I might as well run #12 and hopefully avoid having to fish additional circuits through finished walls in the future.

 

I have wired a number of radio installations including the studio of a major news radio station. We made every effort to keep all of the equipment in a single audio pathway on a single ground. One of the ways we did that is by the judicious use of multiwire branch circuits because those circuits can carry two or three times the current and only have one grounding conductor in the raceway or cable. IMuHO using multiple cables in place of a multiwire branch circuit is more rather than less likely to cause a ground loop.
--
Tom

 

Actually, I meant to say interference on the neutral from the other circuit. I was thinking about the other reply at the same time. I edited the above reply also.

Also, how would you avoid ground loop with a multiwire circuit? The circuits can't be on the same phase unless they are exactly balanced.

Can you explain this.

 

Exactly my point