Multiwire Branch Circuit (3-Wire) Voltage Drop
Voltage drop on a 120/240 V multiwire branch circuit, where two hot legs share one neutral.
Example
You enter
- Conductor size (AWG) 12
- Conductor material copper
- One-way circuit length (ft) 100
- Leg A load (A) 16
- Leg B load (A) 4
- Source volts, line to neutral 120
- Conductor temperature (°C) 75
You get
- Neutral current (the difference) 12 A
- Leg A drop 5.40792
- Leg B drop -1.54512
- Balanced vs two-wire comparison 3.09024
- Two wire vd 6.18048
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method.
Details, formula, and sources
ScopeOn a 120/240 3-wire the neutral carries the DIFFERENCE, so VD_A = R(2 I_A - I_B) and VD_B = R(2 I_B - I_A). Balanced, that collapses to half a two-wire circuit's drop because the neutral carries nothing. Badly unbalanced, the lightly loaded leg's drop goes NEGATIVE - it sits above nominal from neutral shift. 16 A against 4 A on 100 ft of 12 AWG: leg A 5.41 V, leg B rises 1.55 V.
Formula or tableNeutral current = |I_A - I_B|; with equal conductors of one-way resistance R, VD_A = R(2 I_A - I_B) and VD_B = R(2 I_B - I_A); balanced this is R x I, half a two-wire circuit's 2 R I.
Source and editionCircuit analysis of a 120/240 V three-wire multiwire branch circuit; NEC 210.4(B) common-disconnect and neutral-continuity requirements named by section, no table reproduced.
Free accessThe circuit analysis is elementary; conductor resistance is computed by this catalog's own resistance model from the size, material, and temperature entered.
What governsEstimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.
API field namesawg, material, one_way_length_ft, load_a_amps, load_b_amps, source_volts, temperature_C, neutral_amps, vd_a_volts, vd_b_volts, balanced_vd, two_wire_vd
- Shared-neutral analysis neutral carries |I_A - I_B|; its drop signs oppositely on the two legsthree-wire circuit analysis
- Equal conductors hots and neutral the same size; the balanced case is half a two-wire dropstated assumption
- DC resistance AC reactance and power factor not modeledstated scope limit