Wire Gauge (AWG) Calculator
Picking the right wire size isn't just about amps — distance matters too. This free wire gauge calculator sizes copper wire by current, one-way run length, system voltage and your maximum acceptable voltage drop, then cross-checks the result against NEC ampacity ratings so the wire is safe as well as efficient.
Enter your numbers
The NEC recommends 3% max on branch circuits (5% total). Range 1–10%.
Your results
How to use this calculator
Enter the current your circuit will carry in amps (check the appliance nameplate or the breaker size you're planning), and the one-way length in feet from the panel or power source to the outlet or device — not the round trip. The calculator doubles it for you.
Pick your system voltage: 120 V for ordinary household circuits, 240 V for large appliances like dryers and EV chargers, or 12/24 V for DC runs like RV, solar and automotive wiring, where voltage drop is especially unforgiving. Set your maximum voltage drop — 3% is the NEC's recommended limit for branch circuits. Everything recalculates instantly as you type.
The material is fixed to copper, the most common residential wiring. If your numbers need a wire bigger than 4/0 AWG, the tool will tell you to get professional help instead of guessing.
How it works
The tool sizes wire two ways and takes the larger of the two. First, the voltage-drop calculation: the maximum allowed drop in volts is your system voltage times your drop percentage (for example, 3% of 120 V = 3.6 V). The required wire cross-section in circular mils is:
CM = (2 × K × I × L) / VD — where K is copper's resistivity constant (12.9), I is current in amps, L is one-way length in feet (the factor of 2 accounts for the round trip), and VD is the allowed drop in volts. The smallest AWG size whose circular mils meet or exceed that number is the voltage-drop pick.
Second, the ampacity check: every AWG size has a maximum safe current (NEC 60 °C ratings for copper, e.g. 14 AWG = 15 A, 12 AWG = 20 A, 10 AWG = 30 A). If the voltage-drop pick's ampacity is below your load, the tool sizes up to the next wire and flags it as "sized up for ampacity". The results also show the actual drop of the recommended wire in volts and percent, its diameter, and whether capacity covers your load.
Assumptions: copper conductors at 60 °C ampacity ratings, single circuit, and the standard 12.9 resistivity constant. This is general guidance only — consult a licensed electrician and your local electrical code (NEC) before wiring anything.
Frequently asked questions
What size wire do I need for 20 amps?
For a short run, 12 AWG copper is the standard choice for a 20-amp circuit, since its NEC ampacity is exactly 20 amps. For longer runs, voltage drop can push you to 10 AWG or larger — for example, 20 amps over a 50-foot one-way run at 120 volts with a 3% drop limit sizes at 10 AWG. Enter your actual distance in the calculator above to be sure.
What is voltage drop?
Voltage drop is the voltage lost as electricity travels through wire — some energy is converted to heat along the way. If the drop is too large, appliances get less than their rated voltage, so motors run hot, lights dim and electronics misbehave. The NEC recommends keeping drop to 3% on branch circuits (5% total from source to outlet), which is the default limit in this calculator.
Does wire length matter for AWG?
Yes — doubling the wire length roughly doubles the voltage drop, so long runs need thicker wire to stay within the same drop limit. Enter the one-way distance in the calculator; the math automatically doubles it to account for the round trip out and back along the circuit.
Can I use aluminum wire instead of copper?
Aluminum is cheaper but has higher resistance, so it needs a larger AWG size than copper for the same current and distance. This calculator is sized for copper wire only (K = 12.9). If you plan to use aluminum, consult a licensed electrician rather than sizing it yourself.