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Calculation Machine

Wire Gauge Calculator

Cable size for current, length and voltage drop.

Reviewed by James Whitfield · How we check our maths

Your values

ⓘ Live results

Results
Recommended gauge
10 AWG
Actual voltage drop
3.28 V (2.73%)
Maximum allowed drop
3.6 V
Power lost in the cable
65.6 W
Note
Always confirm against local electrical code ampacity tables.

Quick answer

Choose a wire gauge whose resistance keeps voltage drop under 3% for the current and one-way run length.

The formula

Vdrop = 2 × I × L × R_per_metre

Worked example

Worked example

Using the values the calculator loads with.

Inputs

  • Current20 A
  • One-way run length25 m
  • System voltage120 V
  • Allowed voltage drop3 %

Result

  • Recommended gauge10 AWG
  • Actual voltage drop3.28 V (2.73%)
  • Maximum allowed drop3.6 V
  • Power lost in the cable65.6 W
  • NoteAlways confirm against local electrical code ampacity tables.

How to use the Wire Gauge Calculator

  1. 1Enter the load current in amps.
  2. 2Enter the one-way cable run length.
  3. 3Set the acceptable voltage drop, usually 3%.

Frequently asked questions

Why 3% voltage drop?

It's the widely used design limit for branch circuits; more than that wastes energy and dims or overheats loads.

Does length really matter?

Yes — drop is proportional to run length, so long runs often need a gauge or two thicker than ampacity alone suggests.

About this calculator