CEC Part I · Appendix B (3% branch / 5% total guideline)

Voltage Drop Calculator

Check a run's length against its conductor size — flags anything over the commonly cited 3% branch-circuit / 5% total guideline.

Conductor Size (Copper)

Load Current (A)

One-Way Run Length (ft)

Circuit Voltage

Copper, single-phase

VOLTAGE DROP

— V

PERCENT DROP

— %

Result: Enter your run details to check against the 3%/5% guideline.

Long conductor runs lose voltage to resistance — the longer and thinner the wire, the more it loses.
Excess voltage drop shows up as dim lights, sluggish motors, and appliances running hotter or less efficiently than they should.
The commonly cited guideline is no more than 3% drop on a branch circuit, or 5% total from service to load — this tool checks your run against that.
If your run comes back over the guideline, the fix is a heavier gauge wire, not a bigger breaker.