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Sizing a cable for both current and voltage drop

Two independent constraints that must both be satisfied, and recognising which one actually chose the size.

The engineering question

Current-carrying capacity and voltage drop are separate limits. What do you do when they give different answers?

Worked example

The figures below were produced by running the Wire Sizing Calculator with the same inputs — during the build, from its authoritative engine.

Results
Recommended conductor size 10 mm²
Resulting voltage drop 2.2915 %
Ampacity of that size 55 A
Minimum size by ampacity alone 4 mm²
Maximum permitted resistance 2.3958 Ω/km
Assessment Minimum 10 mm² (55 A ampacity) holds drop to 2.3%.
  1. The largest acceptable resistance follows from the drop limit and the route length.
  2. The smallest size with enough ampacity follows from the current alone.
  3. The chosen size is the larger of the two — and derating factors may push it further.

Sanity check

If the recommended size equals the ampacity-only size, the run is short enough that current governs and voltage drop is irrelevant.

Tools for this problem

These are the EngiMetric calculators that compute the numbers in this topic. Each one shows its formula, its units and its limits rather than returning a bare result.

Engineering review required

This topic supports professional engineering work; it does not replace a sealed design. The result depends on site data, the governing standard and engineering judgement, and should be confirmed by a competent engineer before it is built.

References & standards

  • IEC 60364-5-52 — ampacity and voltage drop
  • NEC 310.16 — allowable ampacities

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