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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).
Our photo (above-left) illustrates how a shared neutral circuit can be easily fouled-up and made dangerous.
The two circuit breakers I am pointing to in this electrical panel have been inserted into the panel in a position so that they are on the same phase or power circuit. As a result the shared neutral wire will be carrying double its intended load rather than a fraction of it. Simply moving one of these breakers to a different panel position could correct the problem but unless the two breakers are placed side by side and connected with a common trip tie the wiring would still be unsafe.
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.
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.
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.
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.
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.
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