How voltage drop is calculated
Voltage drop is the current times the resistance of the conductors carrying it there and back. Resistance depends on the conductor's material, cross-section, length and temperature: copper's resistance rises about 0.4% for every degree Celsius.
- Resistance:
R = ρ·(1 + α·(T − 20))·L / A - DC and single-phase:
V_drop = 2·I·R - Three-phase (line to line):
V_drop = √3·I·R
Worked example: 12 V solar run
Running 15 A at 12 V over 8 m (one way) of 6 AWG copper at 50 °C drops about 0.35 V, or 2.9%. The same run in 10 AWG drops about 7.3%, enough to trip low-voltage cut-offs on many inverters and charge controllers.
Good to know
- A common guideline is 3% for a branch circuit and 5% for feeder plus branch.
- This checks voltage drop only. Conductors must also be sized for ampacity under your electrical code.
- Pro finds the smallest conductor that meets your target and the longest run your chosen size can handle.