Are multiwire branch circuits becoming antiquated for most new installations, or are they still being used in most circuit applications? Changes have been made in the last few National Electrical Code (NEC) editions to ensure circuits supplying critical loads—such as those provided for operating room and critical-care equipment in hospitals, exhaust fans for uninterruptible power supply system battery rooms, and emergency and other life safety equipment—will have individual nonmultiwire branch circuits. Disconnecting the power at one circuit breaker supplying a bank of lights at a patient bed location should not interrupt power to lights for other patient bed locations. Critical equipment and lighting cannot be indiscriminately disconnected from power if that shut down will affect other similar circuits.
Over the past decade, the electrical industry has been moving toward the use of individual branch circuits with dedicated neutrals for each ungrounded circuit conductor to ensure that each circuit can be independently disconnected and a failure or faulty operation in one circuit will not adversely affect another. By requiring each multi-wire branch circuit to be provided with a means to simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates, such as at the panelboard or load center, a change to 210.4(B) in the 2008 NEC prompted the industry to move away from multiwire branch circuits. Remember, a multiwire branch circuit “consists of two or more ungrounded conductors that have a voltage between them, and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral or grounded conductor of the system.”
This change prompted further adjustment for healthcare facilities in Article 517. In general-care patient bed locations covered by 517.18(A) and critical-care bed locations covered by 517.19(A), a new sentence states, “the branch circuit serving patient bed locations shall not be part of a multi-wire branch circuit.” Restricting the branch circuits from being multiwire branch circuits ensures that disconnecting the power to one circuit will not affect the operation of any other. In other words, each branch circuit supplying general- or critical-care patient bed locations, as well as other critical circuits, must be individual branch circuits with a separate neutral for each ungrounded conductor and cannot be a multiwire branch circuit using multiple circuit breakers tied together either in the field or by the manufacturer.
Wherever a multiwire branch circuit occurs, various sections of the NEC, such as 225.33(B), 230.71(B) and 240.15(B), permit two or three single-pole switches or breakers on the multiwire branch circuits that are capable of individual operation—one pole for each ungrounded conductor and one multipole disconnect—provided these devices are equipped with identified handle ties or a master handle to disconnect all ungrounded conductors. Disconnecting all of the ungrounded conductors simultaneously provides disconnection of the circuit’s neutral current and affords additional safety while working on the multiwire branch circuit by disconnecting both ungrounded and grounded currents.
Another problem that can occur with multiwire branch circuits—or with any circuit requiring a neutral—is the loss of a neutral conductor in the circuit. With an individual branch circuit that consists of an ungrounded conductor and a neutral (grounded) conductor, the loss of the neutral means the circuit is no longer operational. With a multiwire branch circuit, if the ungrounded conductors have the same load on each ungrounded conductor, there will be balanced loads on all of the ungrounded circuits, so the loss of a neutral may not be as noticeable since there will be very little, if any, imbalance.
However, as loads are decreased on a particular ungrounded conductor in the multiwire branch circuit, the loss of a neutral will be very noticeable. Rather than having all of the loads from the ungrounded conductors balanced and in parallel with each other with the neutral carrying the unbalanced current, the ungrounded conductors will be in series, with current in the series ungrounded circuit being the same and voltage drop being different, thus, having more wattage on the series circuit with the most impedance. Some lights will be dim, some appliances will run much hotter than others, and the imbalance will not have a path back on the neutral.
Ensure that any load is supplied by the appropriate circuit, either circuits with an ungrounded conductor and a grounded (neutral) conductor where the branch circuit is essential, or by multiwire branch circuits where the load is not as essential.