Wire Size / Ampacity Calculator
Enter the load and run length below, then press Calculate. This tool checks both the NEC 75°C ampacity table and the voltage-drop limit you set, then displays the larger of those two planning results. Other installation conditions are not verified by these checks.
This calculator is an estimating aid for planning purposes only. It is not a substitute for a licensed electrician's judgment or for verification against the current NEC and your local electrical code before any work is performed or inspected.
Worked example: inspect both checks
With 20 amps, 240 volts, 100 feet one-way distance, copper, single-phase and a 3% drop limit, the stored model returns 10 AWG. Its simplified voltage-drop arithmetic for that listed size is 2 × 12.9 × 20 × 100 ÷ 10,380 = about 4.97 volts, or 2.07% of 240 volts. This demonstrates the implementation; it is not an installation recommendation.
What these checks leave out
The model uses stored conductor areas, a stored ampacity basis and a simplified drop equation. It does not determine every correction or adjustment, terminal condition, cable limitation, continuous-load treatment, equipment instruction or local requirement. A result labeled for ampacity does not establish that the installation satisfies all applicable conditions.
Check the input basis
The run length is one-way distance; the single-phase equation already accounts for the return path. Use the correct phase and material. Increasing wire size to meet this simplified drop limit does not select a suitable overcurrent device or validate an existing circuit.
How our examples and planning models work · Report an input or result problem