Maximum Circuit Length for a Voltage-Drop Target
The longest one-way run that still meets a voltage-drop target -- the other inverse of the VD tile (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 -- see voltage-drop-reactance); the 3%/5% figures are NEC recommendations. Still pass the 310.16 ampacity check; the AHJ governs.
Formula and source
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.
Audience
This tile is built for electricians and the adjacent professions in the Electrical group. The interactive calculator runs entirely in your browser. No account, no fee, no advertising, no tracking.
Related tools
Posture
Rough Logic answers the math question the working professional asks on the job. The site is a calm, fast, ad-free, account-free, ever-free reference. It does not interpret code. It does not replace the licensed professional. It does not store your inputs. The Authority Having Jurisdiction governs all installations and inspections.