Voltage Drop Calculator
Uses standard resistivity constants (12.9 for copper, 21.2 for aluminum, ohm·circular-mil/ft at 75°C) and the geometric AWG-to-circular-mil formula — not a lookup table with gaps. 5% is the commonly cited maximum acceptable drop under full load.
Free voltage drop calculator for a wire run — enter the gauge, length, current, and system type, and see the voltage drop, percent drop, and voltage at the load, with a warning if it exceeds the commonly recommended 5% maximum.
How it works
- Choose single-phase/DC or three-phase, and copper or aluminum wire
- Enter the AWG wire gauge, one-way length, load current, and supply voltage
- See the voltage drop in volts and as a percentage, with a warning if it's too high
Frequently asked questions
Where do the wire size numbers come from?
AWG wire cross-sectional area (in circular mils) is calculated from the standard geometric AWG formula, not a lookup table — so every gauge from 14 AWG to 4/0 comes out exactly matching published reference values (e.g. 12 AWG = 6,529.9 circular mils).
Why 5%?
It's the commonly cited maximum acceptable voltage drop under full load for a branch circuit — beyond that, equipment can underperform or run hotter than it should. Some codes and applications call for tighter limits, so treat this as a general guideline, not a code requirement for your specific situation.
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Update history
- 2026-09-02 Launched: voltage drop calculator using the geometric AWG-to-circular-mil formula and standard copper/aluminum resistivity constants, with a 5% threshold warning. Entirely client-side.