The problems with multiwire circuits
On March 20, 1883, a U.S. Patent was issued to Thomas A. Edison for a three-wire distribution and branch circuit electrical system consisting of a neutral conductor having a voltage of 120 between it and each of two “hot” conductors with a voltage between them of 240. In Article 100 of the National Electrical Code, there is this definition: “Branch Circuit, Multiwire. A branch circuit that 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.”
While there are multiwire branch circuits supplied by other systems, such as a three-phase four-wire system, we are speaking of a single-phase system of 120/240V. For the three-wire system compared to two, two-wire 120V circuits serving the same load, there is a saving of copper, three wires doing the work of four, with a weight of 75 pounds of copper for the three-wire compared to 100 pounds for the two two-wire circuits. Also, voltage drop is lowered by 50 percent where the loads are balanced on both sides of the neutral.
It is important that the outside legs of the multiwire circuit be connected to different busses in the panelboard. If both are inadvertently connected to the same phase leg, the current in the neutral (which has no overcurrent protection) will be the sum of the loads on both outside wires and will no doubt overheat, a hazardous condition. The loads on each side of the neutral should be balanced to the extent possible.
Despite these advantages, there are risks associated with multiwire circuits, and they are subject to two special Code restrictions for safety reasons.
First, receptacles are furnished with screw terminals or push-in terminals, two for each circuit conductor, so that two conductors can supply the receptacle and two other conductors can be “daisy-chained” to other receptacles downstream, using the receptacle as a splicing means. These receptacles are listed by UL, but their use as splicing means is frowned upon by many inspectors and many old-timers. While replacing one of these receptacles, all of the downstream receptacles are without power. If the neutral is opened while the circuit is energized, loads downstream will be subject to variations of voltage depending on the watts called for by the loads. The high voltage can destroy lamps, while a motor can suffer from both high and low voltage.
This possible effect is taken care of to some extent by NEC: “300.13 Mechanical and Electrical Continuity—Conductors. (B) Device Removal. In multiwire branch circuits, the continuity of a grounded conductor shall not depend on device connections such as lampholders, receptacles, and so forth, where the removal of such devices would interrupt the continuity.” This means that the neutral conductor in a multi-wire branch circuit must be spliced with pigtails serving the devices, so that the removal of one device will not interrupt the continuity of the neutral conductor. Good workmanship requires that all of the conductors be pigtailed so that the removal of a device will not interrupt the circuit to downstream devices, but this is not a Code requirement. It may be noted that the replacement of a receptacle should be done with the circuit off, but this rule indicates that the NEC recognizes that this safety procedure is often ignored.
The second Code restriction on three-wire circuits is: “210.4 Multiwire Branch Circuits. (B) Dwelling Units. In dwelling units, a multiwire branch circuit supplying more than one device or equipment on the same yoke shall be provided with a means to disconnect simultaneously all ungrounded conductors at the panelboard where the branch circuit originated.” This means that whenever the two hot legs of the multiwire circuit appear at devices on a single yoke, a means to simultaneously disconnect them at the panelboard shall be provided. This could be a two-pole circuit breaker, two adjacent single-pole breakers with the handles tied for simultaneous manual operation, or in the case of fused branch circuits, a two-pole switch adjacent to the panelboard and clearly marked as to the load it controls.
In the 2002 NEC, a new 210.7(C) was added, similar to 210.4(B) except that it affects only receptacles, not switches or pilot lights, is not limited to a multiwire branch circuit but applies also to two separate branch circuits sharing a yoke, and applies to all occupancies, not only dwellings.
Duplex receptacles are provided with a break-out connection between the terminal screws so that the two receptacles, one on each end of the yoke, can be supplied by separate hots of a multiwire circuit. Unless both hot conductors are simultaneously disconnected at the panelboard, a person replacing such a receptacle might leave one alive. This might not be discovered until too late, after the person has received a shock. This is the hazard that 210.4(B) and 210.7(C) are intended to eliminate.
CORRECTION: In the March 2004 issue of this magazine, “permanently installed fire alarm” in 210.8(A)(5) Exception No. 3 was misread to include permanently installed smoke detectors. However, many inspectors feel that since smoke detectors are not GFCI protected, they do not need to be AFCI protected. Thanks to David Kravitz, of Power Wiring & Emergency Response, Inc., for noticing this. EC
SCHWAN is an electrical Code consultant in Hayward, Calif. He can be reached at firstname.lastname@example.org.