Q. What does the Code define as a multiwire branch circuit?
A. A multiwire branch circuit consists of two or more ungrounded circuit conductors with a common neutral conductor. There must be a difference of potential (voltage) between the ungrounded conductors and an equal difference of potential (voltage) from each ungrounded conductor to the common neutral conductor [210.4] (see Figure). A multiwire branch circuit can be considered a single circuit or a multiple circuit.
To prevent inductive heating and to reduce conductor impedance for fault currents, all conductors of a multiwire branch circuit must originate from the same panelboard. For more information on the inductive heating of metal parts, see Sections 300.3(B), 300.5(I), and 300.20.
Informational Note 1: Unwanted and potentially hazardous harmonic neutral currents can cause additional heating of the neutral conductor of a 4-wire, 3-phase, 120/208V or 277/480V wye-connected system, which supplies nonlinear loads. To prevent fire or equipment damage from excessive harmonic neutral currents, the designer should consider: (1) increasing the size of the neutral conductor, or (2) installing a separate neutral for each phase.
Informational Note 2: See 300.13(B) for the requirements relating to the continuity of the neutral conductor on multiwire branch circuits.
CAUTION: If the continuity of the neutral conductor of a multiwire circuit is interrupted (opened), the resultant over- or undervoltage can cause a fire and/or destruction of electrical equipment.
Disconnecting means — Each multiwire branch circuit must have a means to simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates.
Informational Note: Individual single-pole circuit breakers with handle ties identified for the purpose can be used for this application [240.15(B)(1)].
CAUTION: This rule is intended to prevent people from working on energized circuits they thought were disconnected.
Line-to-neutral loads — Multiwire branch circuits must supply only line-to-neutral loads.
Exception No. 1: A multiwire branch circuit is permitted to supply an individual piece of line-to-line utilization equipment, such as a range or dryer.
Exception No. 2: A multiwire branch circuit is permitted to supply both line-to-line and line-to-neutral loads if the circuit is protected by a device such as a multipole circuit breaker with a common internal trip that opens all ungrounded conductors of the multiwire branch circuit simultaneously under a fault condition.
Grouping — The ungrounded and neutral conductors of a multiwire branch circuit must be grouped together by cable ties or similar means at the point of origination.
Exception: Grouping isn’t required where the circuit conductors are contained in a single raceway or cable unique to that circuit that makes the grouping obvious, or if the conductors have circuit number tags on them.
Grouping all associated conductors of a multiwire branch circuit together by cable ties or other means within the point of origination makes it easier to visually identify the conductors of the multiwire branch circuit. The grouping will assist in making sure that the correct neutral is used at junction points and in connecting multiwire branch-circuit conductors to circuit breakers correctly, particularly where twin breakers are used. If proper diligence isn’t exercised when making these connections, two circuit conductors can be accidentally connected to the same phase or line.
CAUTION: If the ungrounded conductors of a multiwire circuit aren’t terminated to different phases or lines, then the currents on the neutral conductor won’t cancel, which can cause an overload on the neutral conductor.
These materials are provided to us by Mike Holt Enterprises in Leesburg, Fla. To view Code training materials offered by this company, visit www.mikeholt.com/code
