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 October 9, 2026

Conductor materialCopper
One-way circuit length100 ft
Load current20 A
Voltage120 V
SystemSingle-phase (or DC)
Maximum voltage drop3 %

Results update as you type.

Distance from the panel to the load, not the round trip.

A
%
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Watch the step-by-step video

Watch it explained

A step-by-step video using your numbers: what to enter, what the result means, and a “what if” example. On-screen instructions follow each step as it happens.

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What the animation shows

The animation and the step-by-step video above use your numbers. Here is the same walkthrough written out for the default example.

Finished frame of the wire size calculator (voltage drop) animation for the default example
The last frame of the animation for the default example.

What you enter

  • Conductor material: Copper.
  • One-way circuit length: 100 ft. Distance from the panel to the load, not the round trip.
  • Load current: 20 A.
  • Voltage: 120 V.
  • System: Single-phase (or DC).
  • Maximum voltage drop: 3 %.

What happens, step by step

  1. Each bar is the voltage drop for 20 A over 100 ft one way.
  2. 10 AWG loses 4.14%, over the 3% limit. Try the next size.
  3. 8 AWG is thicker: 2.6% drop, within the 3% limit.
  4. 8 AWG copper is the minimum for voltage drop; also check its ampacity.

The answer: minimum size for voltage drop is 8 AWG copper.

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.