Wire Size Calculator (Voltage Drop)

This finds the minimum size for voltage drop only. The wire must ALSO have enough ampacity for the load and the breaker under the NEC — check that separately; the larger of the two governs.

Important: This calculation provides a preliminary estimate. Final conductor and overcurrent protection sizing must account for the NEC as adopted locally, conductor material, insulation temperature rating, ambient temperature, installation method, bundling, terminal ratings, equipment specifications and local amendments. Electrical work should be done or checked by a qualified electrician.

Tapeline — Wire Size Calculator (Voltage Drop)

Printed September 29, 2026

Conductor material	Copper
One-way circuit length	100 ft
Load current	20 A
Voltage	120 V
System	Single-phase (or DC)
Maximum voltage drop	3 %

How it’s calculated

Required cmil = factor × K × I × L ÷ (V × max %)
Pick the smallest standard size with at least that area

Worked example

20 A load, 100 ft one way, 120 V, copper, 3% limit.

Inputs

Conductor material: Copper
One-way circuit length: 100 ft
Load current: 20 A
Voltage: 120 V
System: Single-phase (or DC)
Maximum voltage drop: 3 %

Results

Minimum size for voltage drop: 8 AWG copper
Voltage drop at that size: 2.6%
Voltage drop: 3.13 V

Load this example into the calculator

Assumptions and limits

Voltage drop sizing only — ampacity, temperature correction, bundling and terminal ratings are not evaluated.
Aluminum sizes start at 12 AWG.

Verify the result against the applicable local code, adopted code edition, manufacturer instructions and project requirements. See our methodology.

Questions

What size wire for 20 amps at 100 feet?

At 120 V with a 3% limit, voltage drop alone calls for 8 AWG copper. At 240 V, 10 AWG meets 3%.

Why is this different from an ampacity chart?

Ampacity is about heating the wire safely; voltage drop is about delivering enough voltage over distance. Long runs are usually limited by voltage drop, short runs by ampacity.