Multiwire Branch Circuits NEC 210.4
Published on March 8, 2013 by David Herres
A guest post by David Herres, Master Electrician living in Clarksville, NH.
A common application of multiwire branch circuits as described in section 210.4 is for a 120/240-volt, single-phase system where three wires do the work of four, the two ungrounded conductors sharing a single neutral.In a 20 amp multiwire branch circuit if the two ungrounded conductors each carry 15 amps, some people think the neutral carries 30 amps, or the combined total of the current on each ungrounded conductor. The load on the grounded neutral conductor will not be the sum total of the load on each ungrounded conductor if the two ungrounded conductors are connected to different phases at the panelboard. With a multiwire branch circuit connected to different phases in the panelboard the current in the neutral is equal to the difference in current flow in the two ungrounded conductors; the larger current in one phase minus the smaller current in the other phase, in a single-phase panelboard. If the phases are equally loaded, the current in the neutral will be zero because the opposing phases cancel out.
All of this depends upon the two ungrounded conductors being connected correctly at the panel. If both hot wires are hooked to the same phase by mistake, the current in the neutral will be the sum of the currents in each phase conductor, rather than the difference, which could be more than double its current carrying capacity. This can generate enough heat to ignite nearby combustible material.
Multiwire branch circuits, properly configured, offer a number of advantages. Less copper is required (one 12-3 rather than two 12-2s), so there is less material and labor initially. In commercial or industrial work, less raceway fill means additional savings. In residential work, the round three-wire Type NM (Romex) is easier to run and staple than the flat two-wire cable. Less knockouts in the service panel is another plus. Additionally, for long runs there is less voltage drop. Since a multiwire branch circuit is actually two branch circuits, a single run will suffice to meet the Code requirement for two small-appliance branch circuits supplying countertop receptacles in the kitchen of a dwelling. Some electricians run multiwire branch circuits throughout every installation.
The National Electrical Code permits multiwire branch circuits, but adds requirements to make them safer. Section 210.4(B) states that in the panelboard where the branch circuit originates, all ungrounded conductors must be provided with a means to disconnect them simultaneously. This is usually a double-pole breaker, but two single-pole breakers may be used if they have an identified handle tie.
Another requirement in Section 210.4(D) is that the ungrounded and grounded circuit conductors are to be grouped by cable ties or similar means in at least one location within the panelboard. This requirement is designed to ensure that the wires will be hooked up correctly, with the ungrounded conductors connected to opposite phases. An exception states that the requirement for grouping does not apply if the circuit enters from a cable or raceway unique to the circuit that makes the grouping obvious.
Posted by David Herres, Master Electrician living in Clarksville, NH
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