Shared neutral

A shared neutral is a connection in which a plurality of circuits use the same neutral connection.

Split phase shared neutral

In split phase house wiring, for example, a duplex receptacle in a kitchen is typically connected with a cable that has three conductors, in addition to ground. The three conductors are usually colored red, black, and white. The white serves as a common neutral, while the red and black each feed, separately, the top and bottom hot sides of the receptacle. Typically such receptacles are supplied from a ganged breaker, i.e. a breaker in which the handles are tied together for a common trip, so that if one kitchen appliance malfunctions and pops the breaker, the other side of the duplex receptacle will be shut off as well.

Three phase shared neutral

In a three-phase circuit, a neutral is often shared between two or three phases. In the case of two phases sharing one neutral, the worst-case current draw is one side has zero load and the other has full load, or when both sides have full load. The latter case results in 1 + 1@120deg = 1@60deg, i.e. the magnitude of the current in the neutral equals that of the other two wires.

In a three-phase circuit with three loads, the shared neutral has zero current in it when the loads are equal, and at most one unit of current in it, where the maximum load on any one branch is one unit.

Double and triple breakers for shared neutral

In order to avoid the danger of having breakers later re-arranged onto the same leg (which would create a potential overcurrent hazard), shared neutral circuits should be connected to multigang breakers rather than to separate single gang breakers.

Common trip on shared neutral circuits

Common trip is always required if there are loads that use more than one phase simultaneously.

In situations where there are only single-ended loads, common trip is still generally advisable for the following reasons:

An inexperienced user might only check one of the sockets and think the circuit is off when turning off only one breaker, i.e. a homeowner might go into the kitchen, plug in a lamp, and turn off breakers until the lamp goes off, not realizing that the two duplexes of the duplex receptacle are fed from different breakers.  Subsequent electrocution could result;

Loop currents could be present, when someone is working on the equipment, or otherwise.