Multiwire Branch Circuit (3-Wire) Voltage Drop
The shared neutral the generic voltage-drop tile has no input for. On 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 and source
Neutral 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.
Circuit 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.
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