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14/3 cable for 2 circuits?

frederic | Posted in Construction Techniques on August 31, 2004 08:51am

Could anyone shed some light on this type of installation using a 14/3 cable to supply two seperate 15amp circuits?

I was mapping my electrical circuit box when I discovered that two individual circuits  supplying the second floor are fed from two seperate circuit breakers but carried by one 14/3 cable.  I haven’t been in our house very long (dated early 1900’s) but this cable appears to be fairly new.  Is this legit wiring practice and if so can both circuits be loaded to their full (80%?) capacity?  What is the impact/purpose of the neutral wire in this case and in general?  Does the neutral carry a load?

Thank you in advance for any information on this.

Freddy

 

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  1. User avater
    BillHartmann | Aug 31, 2004 09:08pm | #1

    It is called a multi-wired circuit. It is OK if done CORRECTLY, but special care is needed and can be confusing to people working on it later. That is why a lot of people don't recommend them.

    The breakers need to be on separate legs of the 240. In most cases that means in adjacent slots in the pannel. By doing this neutral only carries the DIFFERENCE between what is in the two hots. So worst case, when on side is fully loaded, it will only carry 15 amps.

    If it is connected to any device that uses both hots (either a 240 device or a duplex receptacle which has been "split") then the breaker needs to be a 2 pole so that both sides disconnect at the say side.

    The other requirement is that the neutral be pigtailed to connect to a load and not use a receptacle as the feedthrough.

    1. CPopejoy | Aug 31, 2004 09:33pm | #2

      Bill,

      Nice explanation. Didn't we have a similar discussion a few weeks ago?

      Anyway, I've seen many Edison circuits (another name for a multi-wire circuit) where someone has put both supply (hot) conductors (usually the red and the black) on the same pole or leg of the service. This often happens when someone who doesn't understand the concept starts moving breakers around in the panel.

      In that instance, the neutral (white) carries the SUM of the current on the two hots. The neutral isn't protected by a breaker, of course, so you get overheating and the insulation melting off of the neutral. Sort of Dali-like! And in extreme cases, the neutral burns off in the panel where it's been nicked where the insulation was stripped off. That's if you're lucky--if not, it might burn open anywhere along the circuit. If burns partially open and it starts arcing, you'd better hope it's in an electrical box. And one that doesn't have a lot of the romex paper filler in it...

      When I'm working on old houses, sometimes they've been partially re-wired and several multi-wire circuits installed. That's a lot of fun because these circuits are rarely marked, and sometimes the wires do not enter the panel as romex (which would identify the miltiwire circuits), but as free conductors run in conduit from a j-box in the attic or basement/crawl space. For instance, the 14/3 cable runs to a j-box and is spliced to stranded wires that run to the panel. The color codes of the romex are rarely maintained. And the point at which the multiwire diverges into two circuit legs is anyone's guess.

      I wish there was an easier way to identify multi-wire circuits than to physically trace the wiring. A long test lead and a continuity tester is good to confirm the circuit, but you have to find the point where the multiwire branches to do that efficiently.

      Cliff

      1. User avater
        BillHartmann | Aug 31, 2004 10:42pm | #3

        "Anyway, I've seen many Edison circuits (another name for a multi-wire circuit) where someone has put both supply (hot) conductors (usually the red and the black) on the same pole or leg of the service. This often happens when someone who doesn't understand the concept starts moving breakers around in the panel."

        Found one in my own home. I have 2 furnaces side by side and 14-3 was run for them.

        But then some one connected both hots to together and to a single switche for a disconnect. Then moved one lead supply to the next breaker.

        And of course none of the breakers where labeled. I found it while trying to ID the breakers.

        "When I'm working on old houses, sometimes they've been partially re-wired and several multi-wire circuits installed."

        There are 2 application that I can see where multi-wire is a real advantage. One is rewiring where conduit is used. You can get 6 circuits at 80% adjusted rating which in most cases you won't have to upsize the wire. If you use separate circuits then you 12 current carrying wires and 50% rating and upsize.

        The other is workshops where you will line the wall with receptacles and what to be able to easily reconfigure the loads. But I wout use a 2 pole breaker for that so I can also through in a 240 if needed.

    2. frederic | Sep 14, 2004 05:50pm | #4

      I forgot to say thank you for the information.  It is greatly appreciated.

      Freddy

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