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Calculator · Drawing E-502

Voltage drop calculator

Voltage drop and percentage for a run of copper or aluminum conductor at 120, 240, 208, 347 or 600 V, with the length in metres. Useful for garage feeds, hot tubs, EV chargers and long branch circuits.

Voltage drop · E-502Metric
m
A
°C
Resistance rises with temperature. 75 °C is a common design assumption.
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Estimates only. Conductor sizing for installation must follow the Canadian Electrical Code and be done by a qualified person.

CCT 01How it works

Voltage drop is the conductor’s resistance multiplied by the current, over the path the current travels:

  • Single phase: drop = 2 × length × current × resistance per metre (out and back).
  • Three phase: drop = √3 × length × current × resistance per metre.

Resistance comes from the conductor’s cross-section and resistivity. Annealed copper at 100% of the International Annealed Copper Standard has a conductivity of 58 m/(Ω·mm²) at 20 °C and a temperature coefficient of 0.00393 per °C1. Cross-sections follow the AWG definition (a 36 AWG wire is 0.127 mm across, and each step changes the diameter by the 39th root of 92) and 1 kcmil = 0.5067 mm².

CCT 02Assumptions and limits

  • Aluminum is taken as EC-grade conductor at 61% of IACS conductivity with a temperature coefficient of 0.00403 per °C. These are typical values; check the manufacturer’s data.
  • DC resistance only. Reactance matters for large conductors at high current and is ignored here, as is the small extra resistance of stranding.
  • The Canadian Electrical Code limits voltage drop in Rule 8-102, and an electrician sizes the conductor to that rule, to the load’s ampacity and to the breaker. This calculator does not size conductors for installation.

Questions people ask

What wire size do I need for a detached garage in Calgary?

It depends on the load, the distance, the conductor and how it is installed, and it must match the breaker. Use this calculator to see how distance affects voltage, then have an electrician size the feeder. Feeding a garage needs a permit and a locate before you trench. See hot tubs, garages and subpanels.

Why does voltage drop matter?

Motors run hotter, heaters deliver less heat, EV chargers charge slower and LED drivers can misbehave when the voltage at the load sags. Long runs at high current are where it shows up.

Is aluminum wire acceptable for a feeder?

Aluminum conductors are common for services and larger feeders when sized and terminated correctly. Aluminum branch-circuit wiring in older homes is a different issue; see aluminum wiring.

Sources

  1. Copper Wire Tables, Handbook 100U.S. National Bureau of Standards (now NIST) · 1966