What’s a Shared Neutral?

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A shared neutral wire is used in electrical circuits for neutral lines with low current. It is also called a common neutral wire and is used in the Edison circuit. Copper wire is needed to carry current during load imbalances. Single-phase wiring is more expensive than split-phase wiring, which uses a shared neutral. Three-phase motors require five wires, including a shared neutral between two motors. Circuit breakers prevent the neutral current from increasing beyond a few milliamperes.

A shared neutral, in electrical circuits, is a common connection for neutral lines that usually carry near zero mains current. In the case of a three-phase circuit, the neutral line current for a balanced three-phase load is zero or very little. For two-phase electrical power, when there are balanced loads on each of the two phases, the net current across the common neutral is almost zero. The shared neutral wire is also called the common neutral wire, and this arrangement is known as the Edison circuit.

While the current in a shared neutral can be nearly zero when loads are balanced, it is important to use copper wire that can carry currents during a load imbalance. A similar situation occurs for balanced three-phase loads when a power failure occurs from one of the phases. This condition can occur when an unexpected open circuit occurs on one of the three-phase loads.

In terms of distribution cost, it is more expensive to run single-phase wiring. In single-phase wiring, the hot and neutral will need to provide equal current carrying capacity. For example, given a 1,000 watt (W) load, a 100 volt (V) power line should run approximately 10 amps (A). If a split-phase power line were to provide twice the power, the amount of wires needed only increases by 50% and not 100% because the split-phase power wiring will use a shared neutral, which will share the same neutral wire with two hot wires supplying a phase-shifted voltage between them. The return currents on the neutral cancel out when each hot phase supplies the same current to the load.

A typical three-phase motor requires five wires. These are the earth connection, the neutral connection and the first, second and third phase connections, all of which are “hot” or live. The ground connection can have peak currents only during normal transient conditions or during thunderstorms. It is possible that surges can be produced across 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 three-phase connected breakers, so any overcurrent of at least one phase will trip the combined breaker and disconnect all phases from the motor. This results in a mechanism to keep the neutral current from increasing beyond a few milliamperes.

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