Calculate voltage drop over a wire run and check it against code limits.
Voltage drop in a wire run is proportional to current, wire length and wire resistance. For single-phase, the factor of 2 accounts for the current flowing through both the hot conductor and the return (neutral) conductor. For three-phase, the factor is √3 because the three phases are offset and the net drop across a balanced load is smaller. The NEC (210.19) recommends limiting branch-circuit drop to 3% and total drop (feeder + branch) to 5%.
Single-phase voltage drop
Vdrop = 2 × length(ft) × resistance(Ω/ft) × current(A)
Three-phase voltage drop
Vdrop = √3 × length(ft) × resistance(Ω/ft) × current(A)
Drop percentage
Drop % = (voltage drop ÷ source voltage) × 100
NEC 210.19(A)(1) recommends (but does not mandate) that branch-circuit conductors be sized so voltage drop does not exceed 3%. The combined feeder and branch-circuit drop should not exceed 5%. High voltage drop wastes energy and can damage equipment.
Single-phase uses 2× because current travels through both the hot and neutral conductors. Three-phase uses √3 ≈ 1.732 because the three phases are 120° apart; the geometric relationship reduces the effective round-trip distance.
Select the next larger wire gauge (lower AWG number). Each gauge step up reduces resistance by about 20%. Alternatively, shorten the run by relocating the panel or adding a sub-panel closer to the load.
The calculation is valid for DC and resistive AC loads. For AC circuits with significant reactance (long runs of large conductors), the AC impedance from NEC Table 9 should be used instead of the DC resistance.