Wire Size and Voltage Drop Calculator (AWG)

How to do it

1. 1
   Step 1: Open Wire Size and Voltage Drop Calculator (AWG) and add your input

   Type your numbers or text, add your file, or pick the options you need. Nothing is uploaded: everything stays in your browser.

2. 2
   Step 2: What Wire Size and Voltage Drop Calculator (AWG) does

   Calculate the voltage drop of a copper or aluminium wire and the smallest AWG size that stays under 3%.

3. 3
   Step 3: Read the result and compare

   The result appears at once. Change one value to see how the answer moves, then keep the figure that fits your case.

4. 4
   Step 4: Copy or save it

   Copy the result or download it. For an important decision, confirm it with the official source or an adviser.

About this tool

Choose the AWG size, enter the current in amps, the one-way length in feet and the supply voltage. The calculator gives the resistance, the voltage drop in volts and percent, the voltage at the load and the smallest wire size that keeps the drop under 3%.

Formula

Voltage drop = 2 × current × resistance per foot × one-way length for single-phase or DC; for three-phase it is √3 × current × resistance × length. Resistance is worked out from the wire's diameter and the resistivity of copper (1.724 × 10⁻⁸ Ω·m) or aluminium (2.65 × 10⁻⁸ Ω·m) at 20 °C.

Limits

The NEC recommends a drop of no more than 3% on a branch circuit and 5% on feeders plus branch circuit combined. Temperature, conduit and ampacity limits also matter: the wire must be rated for the current as well.

Not included

The tool works at 20 °C and ignores power factor and reactance, which matter for long runs of large wire in AC circuits.

Worked example

A 100 ft run of 10 AWG copper carrying 20 A at 120 V has a resistance of 0.999 Ω per 1,000 ft, a drop of 4.0 V (3.33%) and 116 V at the load. That is above 3%, so 8 AWG (2.1%) is the smallest size that works.

Tips for best results

Check every box before you rely on a result, because one wrong digit changes the whole answer. Use the exact figures from your payslip, bill, certificate or contract, and keep a note of the date you calculated. Rates, thresholds and rules change from time to time, so for tax, fees and legal matters treat the output as an estimate and confirm with the official source named on this page. You can run as many scenarios as you like; nothing is saved or sent, and refreshing the page clears the boxes.