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Calculator · E-507

Feeder voltage drop calculator

Voltage drop on a commercial feeder or a long branch run at 120/208 V or 347/600 V, with copper or aluminum conductors and parallel sets, in metres.

Feeder voltage drop · E-507Metric · SI
sets
Large feeders often run two or more conductors per phase in parallel.
m
A
The calculated load current, not necessarily the breaker rating.
°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:

  • Three phase: drop = √3 × length × current × resistance per metre.
  • Single phase: drop = 2 × length × current × resistance per metre (out and back).
  • Parallel sets divide the resistance: two sets of 350 kcmil carry the current like one conductor of twice the area.

Resistance comes from 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 and 1 kcmil = 0.5067 mm².

The last row shows why buildings distribute at 600 V: the same kVA at 600 V draws about a third of the current, and the percentage drop falls to about one eighth. See 120/208 V vs 347/600 V.

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. Check the manufacturer’s data.
  • DC resistance only. On large feeders, reactance (especially in steel conduit) and power factor add to the drop, so this result is a floor, not a design value.
  • The Canadian Electrical Code limits voltage drop in Rule 8-102 and sets conductor ampacity, correction factors and termination ratings elsewhere. The designer or electrician sizes the feeder to those rules. This calculator does not size conductors for installation.

Questions people ask

How do I calculate voltage drop on a three-phase feeder?

Multiply √3 by the one-way length, the current and the conductor’s resistance per metre at its operating temperature. Enter the run above; the resistance used is shown so you can check it against the cable data.

Why run parallel conductors?

Above a certain size, two smaller conductors per phase are easier to pull and terminate than one very large one, and together they have less resistance. Parallel sets must match in length, material and size.

Is aluminum acceptable for commercial feeders?

Aluminum feeders are common in commercial buildings when sized for aluminum’s higher resistance and terminated with rated lugs and proper torque. The upsizing shows up directly in this calculator.

Does voltage drop matter at 600 V?

Much less than at 208 V for the same power, which is why long runs to rooftop units, parkade levels and outbuildings are often fed at 600 V and transformed down near the load.

Sources

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