Consider a three-conductor cable (red, black, and white) that originates at two circuit breakers which are connected to opposite poles, and the neutral buss bar. One can connect a 120-volt load between the red and white wires, placing that load on one of the circuit breakers, and one can connect another 120-volt load to the black and white wires to place it on the other circuit breaker.
The breakers, of course, are sized appropriately to protect the wiring. But what about the current in the white wire? The white wire is the return for both loads. Won’t it be carrying more than its rated current?
The answer is that the neutral does not carry the sum of the two currents; it carries the difference. If both loads draw the same amount of current, the neutral will carry no current. The voltage on one “hot” wire is always of opposite polarity to the voltage on the other “hot” wire. Therefore, the current returning by way of the white wire for each circuit flows in the opposite direction to the current for the other circuit, so that the two currents subtract, and the white wire can never carry more current than one of the two “hot” wires. This technique is called a multiwire circuit.
Beware, however, of connecting to the wrong breaker. If the two breakers supplying the two “hot” wires are connected to the same pole, the voltage on both hot wires will be of the same polarity, and the current for each of the two circuits will return in the same direction through the white wire, and thus add together. The white wire may carry as much as twice the rating of the circuit so look for those browned white wires in those panels fella’s. This is a dangerous condition.
To check that a multiwire circuit is supplied correctly, measure the voltage between the two “hot” wires. If the two “hot” wires are connected to opposite poles, as they should be, there will be a difference of 240 volts between them. If they are incorrectly connected to the same pole, there will be zero (or nearly) volts between them.
There are some restrictions on the use of multiwire branch circuits, including these (2002 NEC 210.4):
- “All conductors must originate from the same panelboard.”
- If a multiwire circuit in a “dwelling unit” supplies “split-wired” receptacles, a means must be provided to disconnect both ungrounded conductors simultaneously. This can be done with a two-pole switch, but usually it is done by tying the handles of the two circuit breakers together.
*]The same simultaneous disconnection requirement also exists if the circuit supplies a mixture of 120v and 240v loads (whether or not in a “dwelling unit”), but the disconnection means must be the "branch circuit overcurrent