NEC 210.4 Multiwire Branch Circuits for the Journeyman Exam: Shared Neutral Current, Handle Ties and Grouping

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Two or three hots, one neutral

Required circuits, GFCI, AFCI and load limits are on the Article 210 branch circuits page. Here we look only at 210.4, the multiwire branch circuit. For the exam, know the disconnect, handle tie and grouping rules, and be ready to work out the current on the shared neutral.

What makes a circuit “multiwire”

Check whether there is voltage between the hot conductors.

System	Multiwire branch circuit	Not a multiwire branch circuit
120/240 V single-phase, 3-wire	Line 1 + Line 2 + neutral	Two hots from the same line + one neutral
208Y/120 V three-phase, 4-wire	Two or three different phases + neutral	Two hots from the same phase + one neutral
480Y/277 V three-phase, 4-wire	Two or three different phases + neutral	Two hots from the same phase + one neutral

Two hots from the same line or phase have no voltage between them, so they do not make a multiwire branch circuit. Their return currents add on the shared neutral instead of canceling, and that can overload it (see the worked example below).

Neutral current: the part the Calculations exam can ask

Single-phase 120/240 V, resistive loads. The neutral carries the difference between the two line currents. With loads at different power factors, simple subtraction no longer gives the exact neutral current:

I neutral = I line 1 minus I line 2 (use the larger minus the smaller)

Three-phase 4-wire wye, resistive loads. The neutral carries:

I neutral = square root of (Ia² + Ib² + Ic² − Ia·Ib − Ib·Ic − Ic·Ia)

Two cases come up often:

• All three phases equally loaded: the neutral current is zero.
• Only two phases used, equally loaded: the neutral carries the same current as each phase. With 16 A on phase A and 16 A on phase B: √(256 + 256 + 0 − 256 − 0 − 0) = √256 = 16 A.

When the neutral counts for ampacity adjustment. On a 3-wire circuit made of two phases and the neutral of a 4-wire wye system, the neutral carries about the same current as the phases, so it counts as a current-carrying conductor. Electronic loads such as LED drivers and computer power supplies add harmonic current that does not cancel in the neutral, so on a 4-wire wye circuit where the major portion of the load is nonlinear, the neutral counts too (NEC 310.15(E)).

What 210.4 requires

• Same origin (210.4(A)). Every conductor of the circuit starts at the equipment that contains the branch-circuit overcurrent protective device, normally one panelboard.
• Disconnect together (210.4(B)). When you open it by hand, every ungrounded conductor of that circuit has to open together. A two-pole breaker does this. Single-pole breakers joined with identified handle ties also do, when the circuit serves only single-phase line-to-neutral loads. A handle tie gives you that manual disconnect; it does not by itself make the breakers trip together on a fault. Handle ties also keep someone from working on a “dead” receptacle while its neutral still carries current from the other half of the circuit.
• Line-to-neutral loads only (210.4(C)). The circuit supplies only loads connected from a hot conductor to the neutral. The exceptions include a circuit that supplies only one piece of utilization equipment, and a circuit where the branch-circuit overcurrent device opens all the ungrounded conductors at the same time.
• Grouping (210.4(D)). In any enclosure where it applies, the neutral of each multiwire circuit is grouped or identified with the hots it serves, by cable ties, wire markers or similar means, under 210.4(D) and 200.4(B). That lets you see which neutral belongs with which hots when the cover is off. The section waives grouping where the circuit runs in a cable or raceway that makes the grouping obvious, such as a single cable used only for that circuit.

Identification and the neutral at receptacles

• Identify the hots. Where a building has branch circuits from more than one nominal voltage system, each ungrounded conductor is identified by phase or line and by system at termination, connection and splice points, and the method is posted at each branch-circuit panelboard or documented in a readily available way (NEC 210.5(C)).
• Splice the neutral. In a multiwire circuit, the neutral’s continuity cannot depend on a receptacle or other device. Article 300 requires the neutrals to be spliced together with a pigtail to the device, so pulling a receptacle out of a box does not open the neutral for the rest of the circuit.

Worked example 1: a kitchen run on 12/3 cable

A 120/240 V, 3-wire, 20 A multiwire branch circuit feeds two groups of 120 V receptacles. Line 1 carries 14 A and Line 2 carries 9 A.

1. Neutral current: 14 − 9 = 5 A.
2. Disconnect: a two-pole 20 A breaker, or two single-pole 20 A breakers with an identified handle tie.
3. Wrong landing: if both hots land on Line 1, the neutral carries 14 + 9 = 23 A. Each hot still has its own 20 A breaker, but no breaker protects the neutral, so the 12 AWG neutral is overloaded and nothing trips.

Worked example 2: an open neutral

On the same circuit, the shared neutral comes loose. A 60 W lamp (120 V) is on Line 1 and a 1,200 W heater (120 V) is on Line 2. Without the neutral, the two loads are in series across 240 V.

1. Resistances: lamp = 120² ÷ 60 = 240 Ω. Heater = 120² ÷ 1,200 = 12 Ω.
2. Series current: 240 V ÷ (240 + 12) Ω = 0.95 A (rounded).
3. Voltage across the lamp: 0.95 A × 240 Ω ≈ 229 V. That number comes from fixed resistances. A real lamp’s resistance rises as it heats, so the actual voltage would differ from the 229 V figure. The point stands: an open neutral can put a 120 V load at close to twice its rating, and it can burn out. That is why the neutral splice matters.

Worked example 3: three phases, uneven load

A 208Y/120 V multiwire branch circuit has resistive loads of 20 A on phase A, 20 A on phase B and 10 A on phase C.

I neutral = √(400 + 400 + 100 − 400 − 200 − 200) = √100 = 10 A.

GFCI and AFCI on a shared neutral

Two separate single-pole GFCI breakers cannot share one neutral. Each one compares its hot current with the neutral current, so the other circuit’s return current looks like a ground fault and the breaker trips. For GFCI protection, use a two-pole GFCI breaker. For AFCI protection, use a two-pole AFCI breaker the manufacturer allows on a shared neutral. If the circuit needs both, the two-pole device has to provide both. Otherwise, run separate neutrals.

Four quick questions

1. Line 1 carries 18 A and Line 2 carries 11 A on a 120/240 V multiwire circuit. Neutral current? 7 A.
2. Two single-pole breakers on a multiwire circuit, no handle tie. Compliant? No. All ungrounded conductors must open together.
3. Phases A and B each carry 15 A on a 208Y/120 V multiwire circuit, phase C unused. Neutral current? 15 A.
4. Which section requires grouping the neutral with its hots in the panelboard? 210.4(D).

FAQ

Sources checked October 2026

• PSI Candidate Information Bulletin, Texas electrician examinations (updated September 15, 2026): https://test-takers.psiexams.com/api/content/bulletin/1682
• NFPA 70, National Electrical Code, 2026 Edition, Articles 100, 210, 240 and 310