Maximum Circuit Length for a Voltage-Drop Target

Given the wire, how far can I run it? L = VD_target x cmil / (factor x K x I), factor 2 single-phase / sqrt(3) three-phase, K = 12.9 Cu / 21.2 Al, VD_target = target% x source V, cmil the conductor circular mils. A #12 Cu (6,530 cmil) at 20 A on a 120 V single-phase branch reaches ~45 ft before 3% drop; the same wire on 208 V three-phase reaches ~91 ft (sqrt(3) factor + higher voltage). Doubling the current halves the length; a bigger wire or higher voltage lengthens it. DC-resistance drop only (reactance adds on larger conductors); the 3%/5% figures are NEC recommendations. Still pass the 310.16 ampacity check; the AHJ governs.

vd_target_volts = target_vd_pct/100 x source_voltage_v; max_length_ft = vd_target_volts x conductor_cmil / (factor x k_constant x current_a), factor = 2 single-phase / sqrt(3) three-phase, K = 12.9 Cu / 21.2 Al ohm-cmil/ft.

First-principles I x R voltage drop solved for length (public); the NEC 210.19 / 215.2 3% branch / 5% total figures are informational recommendations, not requirements; the AHJ and the adopted NEC edition govern.

The voltage-drop length relation is public Ohm's law; the target percent, current, conductor circular mils, and material constant are the circuit and conductor values.

Estimate. AHJ and licensed professional govern.

Field names used by the API: source_voltage_v, target_vd_pct, current_a, conductor_cmil, k_constant, phases, vd_target_volts, max_length_ft

• I x R solved for length L = VD_target x cmil / (factor x K x I); factor 2 single-phase / sqrt(3) three-phase; K 12.9 Cu / 21.2 Al; VD_target = target% x source Vfirst-principles voltage drop
• Limits DC-resistance only (reactance adds on large conductors, see voltage-drop-reactance); 3%/5% are NEC recommendations; the conductor must still pass the 310.16 ampacity checkNEC 210.19 / 215.2 / 310.16