Find the right copper wire gauge for a circuit based on the load, the voltage and the length of the run. The calculator checks both ampacity and voltage drop, so you can see when a long run needs a bigger wire.
Last updated: October 2, 2026
Copper conductors. Results are estimates for planning, not a substitute for your local code or a licensed electrician.
How this calculator works
Wire size is chosen in two steps. First, the wire must safely carry the current without overheating. This is its ampacity, based on the National Electrical Code (NEC) ampacity table for copper conductors and the small-conductor limits that cap 14, 12 and 10 AWG at 15, 20 and 30 amps. Second, on long runs, the wire may need to be bigger so the voltage drop stays acceptable.
voltage drop (V) = 2 × K × I × L ÷ CM
K = 12.9 (copper), I = amps, L = one-way length in feet, CM = wire area in circular milsCommon residential circuits
| Breaker | Typical copper wire | Common uses |
|---|---|---|
| 15 A | 14 AWG | Lighting, general outlets |
| 20 A | 12 AWG | Kitchen, bathroom and laundry outlets |
| 30 A | 10 AWG | Electric dryer, water heater |
| 40 A | 8 AWG | Electric range (some models) |
| 50 A | 6 AWG | Range, EV charger, subpanel |
Always check the appliance’s installation instructions. They may require a specific wire size or breaker.
Frequently asked questions
Why does a long run need bigger wire?
Every foot of wire adds resistance. On long runs, too much voltage is lost along the way, which can make motors run hot and lights dim. A thicker wire has less resistance and keeps the drop small.
Can I use aluminum wire?
This calculator is for copper only. Aluminum needs a larger size for the same current and special connectors. Ask a licensed electrician before using aluminum conductors.
What is a continuous load?
A load expected to run at full current for three hours or more, such as an EV charger or a water heater. The conductors for these loads are sized at 125% of the load current.
Safety note: Electrical work can cause fire, injury or death. Many jurisdictions require permits and licensed electricians. Use this tool for planning and always follow your local code and the authority having jurisdiction.