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210-4 Multiwire Branch Circuits
Branch circuits recognized in Article 210 can be installed as multiwire circuits. Last month’s In Focus covered different types of branch circuits, including multiwire ones. A multiwire branch circuit consists of two or more ungrounded (hot) conductors and one grounded (neutral) conductor. They can be installed in various applications.
Generally, these circuits feed receptacles, lights, equipment, or any combination thereof. Certain multiwire branch-circuit combinations are not permitted for temporary wiring installations. Receptacles must not be connected to the same ungrounded (hot) conductor of multiwire circuits that supply temporary lighting. [305-4(d)] Receptacles can be installed in a variety of ways on multiwire branch circuits, which can consist of two (or more) single receptacles, one (or more) duplex receptacles, one (or more) multiple receptacles, or combinations of these.
Although a multiwire branch circuit may seem like one circuit because it has only one neutral, it can be considered as multiple circuits. Therefore, in most installations, individual overcurrent protection is permitted. Except where limited by Section 210-4(b), individual single-pole circuit breakers, with or without approved handle ties, can serve as the protection for each ungrounded conductor of multiwire branch circuits that serve only single-phase, line-to-neutral loads. [240-20(b)(1)]
For example, three circuits of fluorescent lights are being fed from a three-phase remote panelboard (subpanel) in a commercial occupancy. These three circuits share a common neutral. The overcurrent protection can be three single-pole breakers without tie handles. Each circuit must be connected to a different phase. Section 210-4(a) also states that all multiwire branch-circuit conductors must originate from the same panelboard.
The fine print note (FPN) to Section 210-4(a) explains that where multiwire circuits feed nonlinear loads on a three-phase, four-wire, wye-connected system, that power system may need a design that allows for the possibility of high harmonic neutral currents. Sometimes, under these conditions, the size of the neutral conductor is increased. A nonlinear load is a load where the wave shape of the steady-state condition does not follow the wave shape of the applied voltage. [Article 100] Nonlinear loads may be electronic equipment, electronic/electric-discharge lighting, adjustable-speed drive systems, and similar equipment.
Unlike the general rule in 210-4(a), specific multiwire branch circuit installations in dwelling units cannot be considered as separate circuits. In dwelling units, if a multiwire branch circuit supplies more than one device (or equipment) on the same yoke, a means must be provided to disconnect all ungrounded (hot) conductors simultaneously. The simultaneous disconnecting means must be located in the panelboard where the branch circuits originate.
For example, a dwelling unit contains a multiwire branch feeding a duplex receptacle. The tab has been removed on the ungrounded side of the receptacle. Since there is no tab, the top receptacle is connected to one circuit and the bottom to another. Under this installation, the breakers must disconnect both circuits simultaneously. This can be accomplished by using either one double-pole circuit breaker, or two single-pole breakers with approved tie handles.
Where both circuits are not connected to the same device on the same yoke, the two circuits can be considered as separate circuits. Two single-pole breakers can be installed as the disconnecting means. For example, a dwelling unit contains a multiwire branch circuit feeding two duplex receptacles. Only one circuit supplies each receptacle. Each receptacle is supplied by only one circuit. Under this installation, the breaker(s) can disconnect the circuits separately or individually. Therefore, two single-pole breakers can be installed.
Switch-controlled receptacles are sometimes installed to meet Section 210-70(a)(1), Exception No. 1 requirements. In dwelling units, every habitable room (except for kitchens and bathrooms) is permitted one or more wall switch-controlled receptacle in lieu of a lighting outlet. These switch-controlled receptacles are sometimes split-wired. The tab is removed on the ungrounded side so one receptacle outlet can be controlled from the switch, and the other receptacle can remain live regardless of the switch’s position. The circuit is not a multiwire branch circuit if the split-wire receptacle receives power from a single-voltage source.
In multiwire branch circuits, the continuity of a grounded conductor must not depend on device connections such as receptacles, lampholders, etc., where the removal of such devices would interrupt the continuity. [300-13(b)] This simply means that the continuity of the grounded (neutral) conductor must not be dependent upon the device.
If removing the receptacle interrupts (or breaks) the grounded conductors’ continuity, the grounded conductors must not be joined by way of the receptacle. Simply splice the grounded conductors, and install a jumper wire to the receptacle. If the receptacle is removed, the continuity of the neutral is not interrupted. This helps ensure that there will be a proper return path to ground.
Section 210-4(c) stipulates that multiwire branch circuits shall supply only line-to-neutral loads unless an exception is met. Exception No. 1 states that a multiwire branch circuit can supply other than line-to-neutral loads if it supplies only one piece of utilization equipment. Exception No. 2 permits other than line-to-neutral loads where all of the ungrounded (hot) conductors are opened simultaneously by the branch-circuit overcurrent device.
Where more than one nominal voltage system exists in a building, each ungrounded conductor of a multiwire branch circuit, where accessible, must be identified by phase and system. The identification means can be separate color coding, marking tape, tagging, or other approved means. [210-4(d)] Generally, identification is accomplished by using different-colored conductors.
For example, a multiwire circuit may consist of ungrounded (hot) conductors with an outer insulation of black, red, and blue. Section 210-4(d) also stipulates that if more than one nominal voltage system exists in a building, and it contains multiwire branch circuit(s), the identification means must be permanently posted at each branch-circuit panelboard.
For example, a commercial building contains two voltage systems, 480/277 and 208/120 volts, three phase. The building’s panelboards will contain multiwire branch circuits. Because of these multiwire branch circuits and the two nominal voltage systems, each branch-circuit panelboard must be permanently labeled with the means of identification.
Next month’s In Focus, beginning with Section 210-5, will continue discussion of general branch-circuit provisions covered in Article 210.
MILLER, owner of Lighthouse Educational Services, teaches custom-tailored classes and conducts seminars covering various aspects of the electrical industry. He is the author of Illustrated Guide to the National Electrical Code. For more information, visit his Web site at www.charlesRmiller.com. He can be reached by telephone at (615) 333-3336, or via e-mail at [email protected]