Voltage Drop Calculator
Long runs and small wires starve motors, heaters and chargers. Enter the run and the load to see the drop in volts and percent before you pull cable.
Your results
The 3 percent branch-circuit and 5 percent overall figures are common design targets referenced by electrical codes, not a compliance check. The Canadian Electrical Code and your inspector govern your installation.
Worked example
30 metres of 12 AWG copper feeding 15 A at 120 V
drop = 2 × 0.030 km × 5.211 ohm/km × 15 A = 4.69 V, which is 4.69 ÷ 120 = 3.91% of supply, leaving 115.31 V at the load. The same run in imperial (98 ft) gives the same answer; a 50 ft run drops 2.38 V (1.99%).
How this calculator works
For a single-phase circuit the current travels out and back, so voltage drop = 2 × length (km) × resistance (ohm per km) × current. For a balanced three-phase circuit the factor is the square root of 3 (1.732). percent drop = drop ÷ supply voltage × 100.
Resistance values used (copper, ohm per km): 14 AWG 8.286, 12 AWG 5.211, 10 AWG 3.277, 8 AWG 2.061, 6 AWG 1.296, 4 AWG 0.815; metric sizes 1.5 mm2 12.1, 2.5 mm2 7.41, 4 mm2 4.61, 6 mm2 3.08, 10 mm2 1.83. Conductor resistance rises with temperature; these are standard operating-temperature values.
Common questions
How much voltage drop is acceptable?
Common design practice targets no more than 3 percent on a branch circuit and 5 percent total from supply to load. Motors, compressors and EV chargers are the loads that suffer first; heaters simply deliver less heat (output falls with the square of voltage).
My run is 30 m round trip. What length do I enter?
Enter the one-way distance (15 m). The factor of 2 in the formula already accounts for the return conductor. Entering round-trip distance double-counts and overstates the drop.
Does aluminum wire change the result?
Yes. Aluminum has roughly 1.6 times the resistance of copper for the same gauge, so multiply the drop by about 1.6, and note aluminum terminations have their own code requirements. This calculator's table is copper only.
Estimates only, for planning. Electrical work is regulated; follow the Canadian Electrical Code and local inspection requirements. Not a substitute for a licensed electrician's design.