Voltage Drop Calculator
Calculate the voltage drop in a wire based on current, length and AWG (American Wire Gauge) size. Important for power distribution and electrical safety.
How to use the Voltage Drop Calculator
- Enter your inputs into the Voltage Drop Calculator above.
- Results update instantly as you type — no submit button needed.
- Adjust any value to see how the result changes in real time.
The voltage drop formula
V_drop = 2 × L × R × I (single-phase) · · · R looked up by AWG (e.g., 12 AWG = 1.588 mΩ/ft)
Doubled because current goes out and returns. L is one-way length in feet, R is resistance per foot, I is current in amps. Three-phase systems use √3 factor instead of 2.
Worked example
100 ft of 12 AWG copper carrying 15 A: V_drop = 2 × 100 × 0.001588 × 15 = 4.76 V. On a 120 V circuit: 4.76/120 = 3.97% drop — at the NEC limit of ≤5% for branch circuits.
Frequently asked questions
Why does voltage drop matter?
Excessive drop reduces device performance, heats wires (fire risk) and wastes energy. NEC recommends ≤3% drop for branch circuits and ≤2% for feeders, with a combined limit of ≤5%.
How do I fix excessive voltage drop?
Upsize the wire (lower AWG number = thicker wire), shorten the run, reduce the load, or use a higher voltage (240 V instead of 120 V halves the current for same power).
Aluminum vs. copper?
Aluminum has ~1.6× the resistance of copper at the same gauge. For the same ampacity, aluminum wire is typically 2 AWG sizes larger than copper.