A shared neutral, in electrical circuits, is a common connection for the neutral lines that usually carry a near zero net current. In the case of a three-phase circuit, the neutral line current for a balanced three-phase load is zero or very minimal. For split-phase electric power, when there are balanced loads on each of the two phases, there is almost zero net current on the common neutral wire. The shared neutral wire is also called a common neutral wire, and this arrangement is known as an Edison circuit.
Although the current in a shared neutral may be almost zero when the loads are balanced, it is important to use copper cables that can carry currents during a load imbalance. A similar situation happens for balanced three-phase loads when there is an interruption in current from one of the phases. This condition can be experienced when there is an unexpected open circuit on one of the three-phase loads.
In terms of distribution cost, it is more expensive to run single-phase wirings. In single-phase wirings, the hot and neutral will be required to provide the same current-carrying capacity. For instance, given a 1,000-watt (W) load, a 100-volt (V) supply line needs to run about 10 amperes (A). If a split-phase power line were to provide twice the power, the amount of cables needed increases by only 50% and not 100% because the split-phase power cabling will use a shared neutral, which will share the same neutral wire to two hot wires that provide a voltage that is out of phase with each other. The return currents on the neutral will cancel when each hot phase is feeding the same current to the load.
A typical three-phase motor will require five wires. These are the earth connection, the neutral connection, and the first-, second-, and third-phase connections, which are all "hot," or live. The earth connection may only have surge currents during normal transient conditions or during times of lightning storms. It is possible that surge currents can be produced through the three-phase lines caused by electromagnetic pulses from lightning.
If two three-phase motors are connected to the three-phase line, the neutral line can be a shared neutral between the two motors. Under normal conditions, there will be no current in the neutral wire. Circuit breakers are ganged three-phase breakers, thus any over-current at least one phase will trip the ganged breaker and disconnect all phases from the motor. This results in a mechanism for preventing the neutral current from increasing beyond a few milliamperes.
