Wire Gauge
This free wire gauge calculator estimates voltage drop and percent drop for a given wire gauge, conductor material, circuit length, current, and source voltage, then rates whether the result is acceptable. Nothing you enter is sent anywhere or stored.
How to Use It
- Select the wire gauge and conductor material.
- Enter the one-way circuit length, the load current, and the source voltage.
- The voltage drop, percent drop, and a plain-language rating update instantly, no submit button.
How It Works
This calculator looks up the selected wire gauge and material's resistance per 1,000 feet from a standard published conductor resistance table, then multiplies by the actual current path length (twice the one-way distance you enter, since current travels out and back) and the load current to find the voltage lost along the way. Dividing that voltage drop by the source voltage gives the percent drop, which gets a plain-language rating.
Formulas
voltage drop = 2 × length(ft) × resistance(ohm/1000 ft) ÷ 1000 × current(A)
percent drop = voltage drop ÷ source voltage × 100
A worked example: a 15 amp, 120 volt circuit run 50 feet with 12 AWG copper wire (1.98 ohms per 1,000 feet) drops about 2.97 volts, 2.5%, rated acceptable. The same circuit run 100 feet with thinner 14 AWG copper wire (3.14 ohms per 1,000 feet) drops about 9.42 volts, 7.9%, past the point where a larger wire gauge is genuinely needed.
Everything runs client-side in your browser. Nothing you enter is sent anywhere or stored.
Frequently Asked Questions
What is voltage drop, and why does it matter?
Every wire has some resistance, so some voltage is lost as current travels along it, and the load at the far end receives slightly less voltage than the source is providing. A small drop rarely matters, but a large one on a long run can dim lights, make motors run hotter and less efficiently, and cause sensitive equipment to underperform or shut off.
Why does this calculator double the length I enter?
Current has to travel from the source to the load and then back again to complete the circuit, so the actual length of wire carrying current is twice the one-way distance you measure. Entering the one-way length and letting the calculator double it internally is less error-prone than asking you to measure and enter the round-trip distance yourself.
Why does aluminum wire need a larger gauge than copper for the same job?
Aluminum has higher electrical resistance than copper, so the same AWG size in aluminum drops more voltage over the same length carrying the same current. This is why aluminum branch circuit wiring is typically sized up from what a copper circuit would use for the same load, not just swapped in at the same gauge.
Is a 3% or 5% voltage drop a hard limit from the electrical code?
No. Keeping branch circuit drop to 3% and combined feeder-plus-branch drop to 5% is a widely followed engineering guideline for reasonable equipment performance, not a strict National Electrical Code requirement for most residential circuits. Some jurisdictions and specific circuit types do enforce stricter limits, so always check your local code.
Can you give a real-world example?
A 15 amp, 120 volt circuit run 50 feet with 12 AWG copper wire drops about 2.97 volts, only 2.5%, well within the acceptable range. The same 15 amp, 120 volt circuit run 100 feet with thinner 14 AWG copper wire drops about 9.42 volts, 7.9%, well past the point where a larger wire gauge is genuinely needed.