Cable Size Calculator

Work out the current your load draws, then find the smallest standard conductor whose corrected ampacity covers that current with margin. Separately check the voltage drop over your cable run. On long runs, voltage drop often forces a larger size than ampacity alone would. Use whichever result comes out bigger.

It gives a solid first estimate using standard published ampacity and resistivity references, but it cannot account for every local code rule, unusual installation method, or inspector requirement. Treat the result as a starting point and confirm against your local electrical code, ideally with a licensed electrician for anything beyond a small DIY job.

Both, always. Ampacity tells you the minimum size the conductor can safely carry without overheating. Voltage drop tells you the minimum size needed to keep the run efficient over distance. Short, high-current runs are usually ampacity-limited; long, lower-current runs are usually voltage-drop-limited.

A commonly used guideline is a maximum of 3% for a branch circuit and 5% total from source to load, which is why 3% is the default here. Sensitive electronics, long solar or battery runs, and motor-starting circuits are often designed to a tighter limit.

Ampacity works the same way regardless of phase count, since it's about how much current one conductor can carry. Voltage drop is where the difference shows up: three-phase uses a √3 factor instead of 2, because the return path is shared across three conductors rather than doubling back through one. This calculator applies that automatically when you select AC (three phase).