Cable Sizing Calculator
Ampacity tells you the minimum safe wire size; voltage drop tells you the actually useful size for long runs. This calculator computes the real voltage drop for your circuit — accounting for the round-trip conductor path — checks the 60°C ampacity limit, and suggests the gauge that keeps the drop under 3%, the NEC-recommended target for branch circuits.
Circuit Parameters
The Voltage Drop Formula
Drop is proportional to resistance and distance, and doubled for the return path:
Vdrop = 2 × one-way length × Rper 1000 ft × I ÷ 1000
At 120 V, 20 A, over 50 ft of 12 AWG (R = 1.93 Ω/1000 ft): drop = 2 × 50 × 1.93 × 20 ÷ 1000 = 3.9 V (3.2%). That is already above the 3% guidance — most "12 AWG for 20 amps" advice quietly assumes a short run. Stretch the same circuit to 100 ft and the drop doubles to 6.4%, which is why long garage and outbuilding runs always need a size or two larger.
Ampacity and Common Wire Sizes
| AWG | Ampacity (60°C) | Drop / 100 ft @20 A | Typical Use |
|---|
Ampacity values are for copper at 60°C termination. Drop figures assume 100 ft one-way, 120 V, 20 A. Always follow local code and equipment terminal ratings.
How to Use This Calculator (3 Steps)
Case Study
Frequently Asked Questions
How do I size a cable for voltage drop?
Drop = 2 × length × resistance × current ÷ 1000. Pick the gauge under 3% for the run.
What is the 3% voltage drop rule?
NEC guidance: feeders + branch circuits should stay within 3% (5% total). Exceeding it dims lights and overheats motors.
What wire size for 20 amps?
12 AWG copper, standard for 20 A branch circuits. Long runs may need 10 AWG for voltage drop.
Why does drop matter on long runs?
Resistance grows with length — 200 ft has 4× the drop of 50 ft at the same current.
One-way or two-way?
Two-way. Current returns on the neutral, so the formula uses 2 × one-way length.