Wire Size & Voltage Drop Calculator (AWG, Copper/Aluminum)
Voltage drop for a single- or three-phase run using NEC Chapter 9 Table 8 conductor sizes.
Formula
VD = 2 × K × I × L ÷ CM (single phase) or √3 × K × I × L ÷ CM (three phase); K = ohm·cmil/ft (approx. 12.9 Cu, 21.2 Al); CM from NEC Ch. 9 Table 8.
Worked example
Inputs: Wire size (AWG) = 12; Conductor = copper; Load current = 16 A; One-way length = 100 ft; Source voltage = 120 V; Phase = 1.
| Voltage drop v | 6.32 |
|---|---|
| Percent drop | 5.27 |
| Volts at load | 113.68 |
FAQ
What drop is acceptable?
The NEC informational notes suggest about 3% for a branch circuit and 5% total feeder + branch for reasonable efficiency; these are recommendations, not ampacity rules.
Does this size wire for ampacity?
No. Ampacity comes from NEC 310 tables and terminal ratings; consult a licensed electrician.
Sources
Embed this calculator
Free to embed on your site; please keep the credit link.
Last reviewed 2026-10-11.