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This document provides an explanation of electrical wiring and safety defects regarding split-wired (multi-wired or shared neutral) electrical receptacles.

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A Multiwire Branch Circuit (in the electrical code) is defined as a branch circuit that consists of two or more ungrounded conductors (two or more "hot" wires) that have a voltage between them (they are not on the same electrical phase and so are connected to different buses in the electrical panel), and a grounded conductor (the neutral wire) that has equal voltage between it and each ungrounded conductor (hot wire) of the circuit and that is connected to the neutral or grounded conductor of the system. (Paraphrasing NEC Article 100).

In English, this means that two hot wires are sharing a neutral wire.

A split-wired receptacle [electrical outlet] is a duplex [two openings for plugs] electrical receptacle that has been converted functionally into two single, receptacles that are individually partly or completely electrically independent. The photograph shows a red and black wire pair powering a shared neutral circuit. They are improperly connected in this panel. This article explains why that is the case and what to do about it.

Each receptacle opening of the pair is individually supplied with electricity by its own electrical circuit and fuse or circuit- breaker. Thus there is one electrical circuit for each individual plug-receptacle opening in the individual duplex electrical outlet.

Introduction to Split Wired Electrical Receptacles

A split-wired receptacle [electrical outlet] is a duplex [two openings for plugs] electrical receptacle that has been converted functionally into two single, receptacles that are individually partly or completely electrically independent. Each receptacle opening of the pair is individually supplied with electricity by its own electrical circuit and fuse or circuit- breaker. Thus there is one electrical circuit for each individual plug-receptacle opening in the individual duplex electrical outlet.

By providing two power sources at one duplex electrical receptacle, split-wired receptacles permit the user to plug-in two power-hungry electrical devices at the same location without overloading and thus tripping a circuit breaker or blowing a fuse as might happen if the same two power-hungry devices were operated simultaneously on a single circuit. [Imagine trying to simultaneously operate both a large electric toaster and a microwave on the same kitchen circuit.]

In completely electrically-independent split-wired receptacles, each receptacle also has its own independent neutral wire and possibly ground wire back to the electric panel. In a multi-wired or shared-neutral receptacle, a single neutral wire is shared by both of the independently-powered receptacles.

How to Fuse Multiwire (shared-neutral) Electrical Branch Circuits

Use of linked double-pole or two-pole circuit breakers is recommended: Pending further research and development of authoritative citations, the following is the opinion of the author:

Multiwire branch circuits should be protected by a double-pole common-internal trip circuit breaker, including the physical "trip tie" which bonds the two circuit breaker switches together. This is a safety measure which protects people working on the building wiring and which helps assure that the circuit is wired properly at the panel. Even if local building inspectors do not require this measure we recommend it as a safety item and as good construction practice.

Background: the author has observed two electrical wiring hazards associated with failure to observe the recommendation above.

1. Case-1: both breakers placed on same panel bus: Performing work on an electrical panel in which a multi-wire branch circuit was not wired using a double-pole common-trip circuit breaker, an electrician relocated one of the individual breakers such that both of the multi-wire feeds were on the same electrical bus. This placed both breakers on the same electrical phase in the electric panel. When both circuits were in maximum use the result was a neutral wire current which exceeded the permitted rating.
   
   If a double-pole internal trip breaker had been used it would have forced the two feed circuits to be on opposite electrical phases in the panel, thus reducing rather than increasing the observed current on the neutral wire. [Two electrical currents in opposing electrical phase, introduced on the same wire, are self-canceling. I am speaking approximately, since in common residential electrical wiring, the two panel buses are not in exactly-opposite electrical phases.]
   
   
2. Case-2: neutral wire energized at a location where power was "off": Performing repairs on an individual electrical circuit which used a shared neutral wire, the worker turned off the circuit breaker feeding an electrical receptacle in order to install a new outlet at that location.
   
   After confirming (by simple neon tester) that power was "off" at the receptacle, the worker began re-wiring the new device in place. Meanwhile the worker's assistant plugged-in and began operating a power-hungry electrical tool at another electrical circuit in the building - a circuit which shared the same neutral wire as the one being worked-on. The worker, on touching both a ground path (BX cable) and the neutral wire, was shocked.[Personal experience of author.]

For a detailed article about how multi-wire electrical circuits are wired, see the ASHI Technical Journal, Vol 2 No 1 Winter 1992 p. 27-30 In addition to the author, Neal Macneale III, Douglas Hansen and Daniel Friedman edited and illustrated that material.

Explanation of Shared Neutral Electrical Circuit Wiring Requirements

1. Double pole breakers can be required by National Electrical Code for split receptacles, as per paragraph 210-4-b.
   
   
2. Using a double-pole breaker assures that the two legs of the circuit will be forced (in most panels) to opposite legs of the 240-volt panel - a requirement when circuits use a shared neutral, such as in multiwire branch circuits. If individual 120V breakers are used, it's possible for a future modification or rearrangement of breakers in the panel to in advertently move one or both individual breakers so that they both end up on the same 120V leg of the panel - which is improper when a shared neutral is involved. (Improper because the shared unbalanced load could exceed the rating of the wire.)
   
   
3. Using a double-pole assures that if one leg of the circuit is being turned off for electrical repair/modification work in the building, the other leg is forced off as well. Otherwise it is possible for the mechanic to be shocked while working on the circuit, since the neutral wire of the supposedly "dead" circuit could be carrying current from the sister "live" circuit.

National Electrical Code Citations for Multi Wire and Split Wire Devices

1. The 2008 National Electrical Code, 210.4(B) Multiwire Branch Circuits, now requires that effective 1 January 2008,information about conductors of multiwire branch circuits originating from the same panelboard or distribution equipment has been relocated to 210.4(A). Now 210.4(B) addresses disconnecting means for simultaneously disconnecting all ungrounded conductors of all multiwire branch circuits. Simultaneously disconnecting all conductors of multiwire branch circuits is now expanded to all multiwire branch circuits, not just those that supply more than one device mounted on the same yoke or mounting strap. -- Minnesota Electrical Association.
2. NEC Paragraph 210-4 addresses multiwire branch circuits.
   
   
3. NEC 210-4-b makes clear that split receptacles must be protected by a simultaneous disconnect to all ungrounded (hot) conductors (i.e. use a double-pole breaker with a common trip tie installed). NEC Paragraph 210-4 addresses multiwire branch circuits.
   
   
4. Split-receptacle means each half of a duplex receptacle is wired to a different "polarity" or phase and the single grounded conductor (neutral) is used). NEC Paragraph 210-4 addresses multiwire branch circuits.

Electrical Code Notes on Shared Neutral and Split Wired Electrical Devices

ASHI Member Frank Luciano spoke with Al Weiss, New York State building code authority (Building code support office at World Trade Center, New York City) regarding the requirement for linking or common-trip ties for these circuits.

Mr. Weiss' opinion was that if he sees individual breakers in the panel on a multiwire circuit he will not call it out as an issue for failure to link the breakers together. The discussion did not review possible relocation of one of the breakers to the same phase or "leg" of the panel as the other.

Mr. Weiss' interpretation of the National Electric Code (NEC) is that if, on a multiwire circuit, the two phases are wired to the same electrical receptacle (upper portion to one phase, lower to another phase, by breaking the tie on the receptacle sides) then a common-trip breaker should be used on that circuit. He also opined that if breakers were wired in parallel, rather than in series, as is done in some states, then common trip ties are not required.

Questions & Answers about Multiwire Electrical Circuits & Their Electrical Safety

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• 01/17/00 note: thanks to David Shapiro for additional editing comments
• The Minnesota Electrical Association has posted an article of the most important electrical code changes for 2008 at electricalassociation.com/catalog/2008NECTop10.aspx - by Michael J. Johnston, IAEI

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• Multi-wire electrical branch circuits For a detailed article about how these circuits are wired, see the ASHI Technical Journal, Vol 2 No 1 Winter 1992 p. 27-30 In addition to the author, Neal Macneale III, Douglas Hansen and Daniel Friedman edited and illustrated this material. Reprints of the ASHI Technical Journal are available from ASHI - www.ashi.com or copies of individual articles may be available from the authors.
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