Shared neutral wires, split-wired receptacles, multi-wired branch circuit wiring:
This article provides an explanation of electrical wiring and safety defects regarding split-wired (multi-wired or shared neutral) electrical receptacles. We include electrical code citations for mutliwire circuits.
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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 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.
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.
This topic has moved to ELECTRICAL WALL PLUG ADAPTERS - using a wall plug adapter, power strip, surge protector, or electrical spike protection device
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(July 28, 2015) George said:
I see in this article Copyright 2015 InspectApedia.com, but only in one place NEC 2008 is referred.
Could you please help to understand if shared neutral is still permitted for the same definition of multiwire branch circuit (as two or more hots and one? neutral)by the NEC 2011 and NEC 2014 where in Article 200.4 is written like as follows:
Neutral conductors shall not be used for more than one branch circuit, for more than one multiwire branch circuit, or for more than one set of ungrounded feeder conductors unless specifically permitted in this Code. I would be sure that is still permitted one neutral for one multiwire branch circuits, but it is written in plural - Neutral conductorSSS.
But if to understand in this way - feeders' hots (circuits) should be with dedicated neutrals too. Could you please explain if one shared neutral is still permitted for one multiwire branch circuit and in what cases? I would so much appreciate your help. Thanks, George.
Yes George I share your interpreation - one neutral can be shared on one multiwire circuit by the NEC. The issue is that some devices such as AFCIs and GFCI's do not perform well on shared neutral circuits
(Oct 7, 2015) Jimmy Ray said:
In our trailer, my son moved in two strippers from the local club. I was against this but he did it anyway.
They try to plug in all kinds of stuff in the two outlets in the bedroom. This is blowing the fuses and fuses ain't cheap. Can i run a double 12/2 to a outlet so they can plug all that in without popping the fuse?
Running 12/2 wires to a dedicated electrical outlet is a reasonable solution to the overload probelm you describe PROVIDED that the circuit is properly installed and protected, that is code-compliant. It will need its own 20-Amp breaker or fuse, and you'll want to use an electrical receptacle ("outlet" or "wall plug") rated for 20-Amps.
See SIZE of WIRE REQUIRED https://InspectAPedia.com/electric/Electrical_Outlet_Wire_Size.php
See ELECTRICAL RECEPTACLE TYPES https://InspectAPedia.com/electric/Electrical_Outlet_Types.php
properly wired, routed, grounded, etc.
(Jan 25, 2016) DaveH said:
I'm wiring a small woodworking shop and providing multiple metal surface mount outlets (fed by EMT).
Is it allowable to wire a multiwire branch circuit that provides both a single 50A 240 outlet and two 120 outlets (one on each phase) as long as I have a 2 pole tied breaker feeding the circuit.
Furthermore is it allowable to have two surface mount outlet boxes touching each other side-by-side so the wiring from the 240 outlet can be split and feed the 120 outlets in the adjacent box. Or do I have to do the single 240, exit the box, go through a short length of EMT, enter another box and then be split out to the two 120 outlets.
I've spent hours going through my code book and I'm pretty sure the answer to the first question is "yes" but I can't find if side-by-side outlet boxes are allowable.
I'm afraid, I'm very afraid. The placement of a 120V receptacle (normally a 15A or 20A circuit) on a 50-Amp electrical circuit is asking for a building fire, or for users, shock, or injury or worse. A short circuit in wiring or a defective appliance plugged into one of those 120V receptacles is being fed 50Amps rather than being limited to the proper 15 or 20 Amps of current.
Don't do it.
I'm not sure what code says about the mixed-circuit you describe but it's a hybrid that sounds totally unsafe and unacceptable to me. Certainly if the 2-pole breaker has a common internal trip mechanism then an overcurrent on either leg would trip the whole breaker off. But you're using a 50A breaker to power not just a 240V receptacle but also two 120VAC receptacles.
Those "normal" 120VAC receptacles typically are on 15A or 20A electrical circuits. Not a 50A circuit. You may be under-protecting them by potentially allowing too much current to flow to devices plugged into them.
(Apr 12, 2016) Ron Swoverland said:
doing solar you can't use multi branch circuits
Can you give me some details of what you mean, where the problem occurs, what sort of circuits you refer-to?
BTW I agree that multiwire circuits can be trouble. In particular, both GFCIs and AFCIs will not work properly on a shared neutral circuit. But to be clear, shared neutral wires, properly connected, are certainly permitted by the electrical code.
2017/02/01 Dave H said:
My understanding was the panel breaker is intended to protect the wall wiring and not necessarily the appliance - thus you could over-protect a outlet but not under-protect an outlet.
Having said that, I can see what you mean by allowing too much current to go through a 20A outlet.
You could melt the outlet before the breaker would pop, the wall wiring (being sized for it) could handle the current but the outlet couldnt.
Looks like I'll need to run separate 240 and 120 runs to each outlet. love - it would have saved me a lot of wire runs.
If you mean that it's ok to put a 20A rated receptacle on a 15A circuit because you've over-protected the outlet by limiting its current to just 15 Amps, that sounds reasonable to me, but I'd not like to see a 20A receptacle (See ELECTRICAL RECEPTACLE TYPES) on that 15A circuit for a different reason: the receptacle's physical appearance (that extra horizontal slot) tells some users that it's on a 20A circuit - risking repeated nuisance tripping of the 15A circuit.
I certainly agree that putting a 15A receptacle on a 20A circuit could be inviting trouble for the opposite reason: you may overheat receptacle components.
The worries in an earlier question about spinning off 15A or 20A 120V circuits off of a 50A circuit is a still more serious fire hazard.
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