Multiwire Branch Electrical Circuits and Split-Wired Receptacles - Electrical Wiring Safety Requirements
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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.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
Introduction to Split Wired Electrical Receptacles
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 plain English, a "multiwire branch circuit" or "split-wired receptacles" means that two hot wires are sharing a neutral wire.
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.
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.
- 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.]
- 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
- Double pole breakers can be required by National Electrical Code for split receptacles, as per paragraph
210-4-b.
- 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.)
- 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
- 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.
- NEC Paragraph 210-4 addresses multiwire branch circuits.
- 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.
-
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.
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Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.
- 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
- 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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