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I've seen discussions of using an old 240V wire (two hots, a neutral, and a ground) and reconfiguring it to have two 120V circuits.

As an example, if it is a 20A cable, you could have two 20A circuits, each one using one of the two phases and both sharing the neutral and ground.

If each circuit is powering a device using the full 20A, would this result in 40A of current going onto the neutral, causing overheating?

Additionally, will sharing the neutral cause interference between the circuits?

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    Are the 2 hots wired to the same or different phases? If of the same phase there may be an issue.
    – chux
    Commented Nov 11, 2020 at 18:33

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Actually, it would result in a net 0 A on the neutral. So this actually can work quite well and is called a Multiwire Branch Circuit or MWBC.

The one catch is that the breakers powering the circuit need to be set up for common shutoff - i.e., if you turn off one, you always turn off the other. If the breakers are actually set up for common trip, which is normally the case for a 240V circuit like a stove or dryer, then that provides common shutoff as well.

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  • Dah, the phases cancel out! Very cool, thanks. What are the trade-offs between a common shutoff and a common trip. Any disadvantages to a common trip in a case like this?
    – brentonstrine
    Commented Nov 11, 2020 at 1:40
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    it would only result in a net 0A on the neutral if both loads were balanced which is rare in real life.
    – Ted Mittelstaedt
    Commented Nov 11, 2020 at 2:00
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    Common disconnect is for the sake of maintainers, and must be on ALL circuits of this type. Common trip is to protect bad side effects from failed/tripped circuits, and must be on a 120V MWBC only if it also serves 240V loads (which you can do by the way).
    – Harper - Reinstate Monica
    Commented Nov 11, 2020 at 3:55
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    @manassehkatz-Moving2Codidact: In some cases, having separate trip may make it easier to identify, in case of overload, which outlets are served by the half that tripped, and which outlets could be used instead when trying to better partition loads. That wouldn't be as useful if outlets were labeled by phase, but they almost never are.
    – supercat
    Commented Nov 11, 2020 at 15:23
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    Does this mean that the neutral wire will see the difference in current between the two phases? So, at worst, 20A?
    – Bryan B
    Commented Nov 11, 2020 at 19:03
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Just to expand the explanation in the answer from @manassehkatz:

On a USA-type 240 volt circuit, you actually have two 120 volt hots, as you know, but the AC signal is 180 degrees out of phase. So when one hot is at +170 volts (peak voltage for 120 volts RMS), the other hot is at -170 volts and when one hot is at 0 volts, the other hot is also at zero.

The result is that on a 20 amp circuit, the most current that can travel over the neutral is 20 amps and this occurs when the load is completely unbalanced: 20 amps on one hot and 0 amps on the other. In this case, all of the current from the load returns over the neutral.

On the other extreme, if both hots have a 20 amp load, then all of the current from one leg returns over the other (effectively, the two loads are in series) and no current has to travel over the neutral.

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  • That really works out perfectly! Why isn't this strategy used more commonly? E.g. if a room needs two circuits you can run only one cable and have 2 less wires.
    – brentonstrine
    Commented Nov 11, 2020 at 1:44
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    @brentonstrine They do this. As manassehkatz noted, this is called a Multi-Wire Branch Circuit (MWBC). The major disadvantage is that the breakers must trip together so either circuit tripping will also trip the other.
    – DoxyLover
    Commented Nov 11, 2020 at 3:32
  • @brentonstrine in addition to what DoxyLover noted about the breakers, it also requires a 4 wire cable instead of 3; increasing costs in areas where you don't anticipate enough power draw to need more than one circuit.
    – Dan Is Fiddling By Firelight
    Commented Nov 11, 2020 at 17:50
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    12/3 cable doesn't work with the currently available common configuration of required AFCI circuits, and 12/2/2 NM is typically more than twice the price of 12/2. Your question was only theory, but in practice this modification may produce NEC compliance issues depending on location of the circuits. The NEC requires modifications to a circuit meet current code, you would need to check with your Authority Having Jurisdiction for which addition of the code they last adopted, and local enforcement exceptions, particularly to the AFCI requirements.
    – NoSparksPlease
    Commented Nov 11, 2020 at 18:06

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