Multiwire Branch Circuits NEC 210.4
By: David Herres | Mar 08, 2013
A guest post by:
David Herres, Master Electrician living in Clarksville, NH.
A common application of multiwire branch circuits as described in section 210.4 is for a 120/240-volt, single-phase system where three wires do the work of four, the two ungrounded conductors sharing a single neutral.In a 20 amp multiwire branch circuit if the two ungrounded conductors each carry 15 amps, some people think the neutral carries 30 amps, or the combined total of the current on each ungrounded conductor. The load on the grounded neutral conductor will not be the sum total of the load on each ungrounded conductor if the two ungrounded conductors are connected to different phases at the panelboard. With a multiwire branch circuit connected to different phases in the panelboard the current in the neutral is equal to the difference in current flow in the two ungrounded conductors; the larger current in one phase minus the smaller current in the other phase, in a single-phase panelboard. If the phases are equally loaded, the current in the neutral will be zero because the opposing phases cancel out.
All of this depends upon the two ungrounded conductors being connected correctly at the panel. If both hot wires are hooked to the same phase by mistake, the current in the neutral will be the sum of the currents in each phase conductor, rather than the difference, which could be more than double its current carrying capacity. This can generate enough heat to ignite nearby combustible material.
Multiwire branch circuits, properly configured, offer a number of advantages. Less copper is required (one 12-3 rather than two 12-2s), so there is less material and labor initially. In commercial or industrial work, less raceway fill means additional savings. In residential work, the round three-wire Type NM (Romex) is easier to run and staple than the flat two-wire cable. Less knockouts in the service panel is another plus. Additionally, for long runs there is less voltage drop. Since a multiwire branch circuit is actually two branch circuits, a single run will suffice to meet the Code requirement for two small-appliance branch circuits supplying countertop receptacles in the kitchen of a dwelling. Some electricians run multiwire branch circuits throughout every installation.
The National Electrical Code permits multiwire branch circuits, but adds requirements to make them safer. Section 210.4(B) states that in the panelboard where the branch circuit originates, all ungrounded conductors must be provided with a means to disconnect them simultaneously. This is usually a double-pole breaker, but two single-pole breakers may be used if they have an identified handle tie.
Another requirement in Section 210.4(D) is that the ungrounded and grounded circuit conductors are to be grouped by cable ties or similar means in at least one location within the panelboard. This requirement is designed to ensure that the wires will be hooked up correctly, with the ungrounded conductors connected to opposite phases. An exception states that the requirement for grouping does not apply if the circuit enters from a cable or raceway unique to the circuit that makes the grouping obvious.
Posted by David Herres, Master Electrician living in Clarksville, NH
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Circuit breakers on the left side of a single phase panelboard are numbered 1, 3, 5, 7, etc. The busbars on the panelboard are arranged so that circuits 1 and 3 are connected to different phases. So a 2-pole circuit breaker, or two single pole circuit breakers with a handle tie, are connected to phase A and B.
Most breakers that are directly across from each other are on the same finger of a buss bar (the same phase). Panels are typically arranged so that a different phase in to each side. That way you can insert a 2 pole breaker and have 240v, or share a neutral.
In order to share a neutral you need to have a 2 pole breaker for both circuits to tie to, or be able to insert a handle tie for 2 single breakers.
Also a twin or 1/2 size breaker is almost always on the same phase or buss bar.
You have it backwards. A breaker directly above or below another is on a different phase. And any two breakers directly next to each other (across the panel) are on the same phase. All breakers at the left are the odd numbered locations. All right are equal numbered. So, breakers 1 and 3 are on different phases, but breakers 1 and 2 are on the same. This differs with mini breakers…
Our graphic shows two multi-wire branch circuits. One connected to circuits 1 and 3, and the other one connected to 2 and 4.
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This is not accurate if the panel has 3-phase power. In that case, complex math (e^j*theta) or a long string of trig functions is needed (assuming unbalanced loads).
This blog post was written about a common application of a single phase multiwire branch circuit where there was 240 volts between 2 phase conductors. We did not get into an example of a 3-phase system.
If the system is three phase, the current in the neutral of a multiwire circuit will not exceed the current in the most heavily loaded hot wire. If the current in both hot legs is the same and the hot legs are feeding two 120-volt loads, the current in the neutral is the same as in the hot legs. If the two hot legs are feeding a 208-volt load connected phase to phase, the neutral current is zero. Bottom line; Three-conductor cable can be used for two multiwire circuits on a three-phase system. Actually, 4-conductor cable (black, red, blue and white) could be used for three multiwire circuits in a three-phase system. The current in the neutral will not exceed the most heavily loaded phase. The three circuits must be fed by three different phases.
On a 3-phase system if two phase conductors are used with a grounded conductor in a multiwire branch circuit, the common grounded conductor will carry approximately the same current as the line-to-neutral load currents. See 310.15(B)(5). According to the definition of multiwire branch circuit in Article 100, a 4-wire multiwire branch circuit is considered a single multiwire branch circuit not three multiwire branch circuits. On a 3-phase system, both 3-wire and 4-wire multiwire branch circuits are permitted. Cable or conduit are both OK.
Multiwire branch circuits are permitted with handle ties. Handle ties allow for a manual disconnect of each individual circuit in a multiwire branch circuit. But on a multiwire branch circuit with handle ties, if there is a fault on one circuit the other circuit will not reliably trip. A 2-pole or 3-pole circuit breaker with a common trip will de-energize all of the circuits on a multiwire branch circuit.
Dave, Thanks. I’m aware of handle-tied breakers versus 2 pole or 3 pole breakers. Sometimes it is not desired to clear all phases for a fault or overload on just one of the multi-wire phases, especially when the loads on the different phases are unrelated. Is a handle tie or multi-pole breaker required for multi-wire circuits, or can the different phases (or legs of a 120/240-volt single-phase multi-wire circuit) just be protected with independent, single-pole breakers. I believe independent, single-pole breakers are OK. I’ve certainly seen a lot of multi-wire circuits protected that way. My code book is old, or I’d look it up myself…….Don
I was hired to determine why a house burned down in an electrical fire. The original contractor used 12/3 romex in a branch circuit to the kitchen because it was on a long run from the electrical panel. This is best because it reduces cost and resistance associated with long runs by sharing a neutral with two circuits that are 180 degrees out of phase from each other. Later the owner decided to put in a swimming pool. To make room for the pool pumps 220VAC breaker the pool contractor rearranged the box and inadvertently put both branches on the same phase. When this is done, instead of the opposing phases subtracting current from the neutral wire it instead returned 40 amps which caused the house to burn down. NEC 210.4 requires that the branch circuits be on a double pole breaker with the black and red legs tied together so it is obvious to others working in the box that these legs need to be on opposite phases. Unfortunately many electrical contractors cut corners on boxes that are getting full and don’t take the extra time to make sure it is clear to the next contractor working in the box that these two circuits must remain together as a pair.
Section 210.4 of the 2017 NEC requires grouping the conductors of a multiwire branch circuit together within a panelboard or other enclosure in accordance with 200.4. This should allow a qualified person to identify multiwire circuits and ensure that proper phase connections are maintained in the event circuit breakers are rearranged to accept new loads. However, grouping conductors is not required where the branch circuit conductors enter the panelboard enclosure from a cable unique to the circuit that makes the grouping obvious as would be the case for a 12/3 NM cable supplying 2 kitchen small appliance branch circuits. If both ungrounded conductors of the 20-amp multiwire circuit are connected to the same bus in a single phase panelboard, the grounded conductor will carry the load present on both ungrounded conductors. Although each ungrounded conductor may not be carrying a full 20 amps of load at all times, the 12 AWG grounded conductor will be overloaded anytime the total current on both ungrounded conductors exceeds 20 amps. In many older homes, especially where wiring has been added over the years, it is not always obvious that multiwire branch circuits are present as conductors within the panelboard may be tangled and cable entries not clearly visible. Although 240.15(B) permits the use of double pole breakers or single pole breakers with identified handle ties for multiwire branch circuits that serve only line to neutral loads, in many older homes double pole breakers or single pole breakers with handle ties were not used. Before relocating circuit breakers in an existing panelboard, it is the responsibility of the qualified person to clearly identify the circuits that are being relocated to avoid overloading a grounded circuit conductor.
It depends on when that house was built. Breaker ties were not always code for multi wire branch circuits. Why wouldn’t you put the blame on the mouth breathing pool installer who should be in an electrical panel in the first place. Just sayin!
Looking for some clarification on a “Branch Circuit, Individual”
Is it permissible to use two, single pole breakers (no tie bar or handle ties), on a 220 volt branch circuit, suppling a single piece of equipment (laundry clothes dryer) or (stove/oven)??
Thank you in advance!