Voltage Drop Calculator
What a cable run steals from your voltage, by wire size and length.
The calculation
Copper resistance is 0.01724 Ω·mm²/m; the round trip doubles the length, and V = I × R does the rest. 50 ft of 12 AWG carrying 15 A on 120 V drops about 2.4 V (2%), losing 36 W as heat in the wire.
How much drop is acceptable
The usual guideline is 3% for branch circuits, 5% total. Low-voltage systems suffer far sooner: the same 2.4 V that is a rounding error at 230 V is a fifth of a 12 V automotive or solar circuit, which is why 12 V wiring gets thick fast. Long runs: go up a gauge size (or a mm² step) before you need to.