Choosing a conductor that carries the current without overheating and delivers the voltage the load needs.
Cable selection is two tests, and you take whichever gives the bigger conductor. Current-carrying capacity protects the cable from itself. Voltage drop protects the load from being undersupplied.
On short runs the current rating usually decides. On long runs voltage drop almost always decides, and by a wide margin — which is why a 20 A circuit at the end of a 40 m run can need a cable two or three sizes larger than the breaker suggests.
Ib, the current the circuit will actually carry. Everything here depends on it.
It ÷ (Ca × Cg × Ci × Cf) — ambient temperature, grouping, thermal insulation and the protective device. This is the calculation installers do first and it is not yet built.
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To BS 7671, IEC 60364 or NEC limits, with the Appendix 4 mV/A/m figure alongside.
Both tests together, against NEC ampacity and the terminal temperature rule.
Diameter, cross-section, resistance and ampacity for any AWG size.
The adiabatic check, S = √(I²t)/k. Decides the minimum conductor a fault can pass through without damage. Not yet built.
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The published figures these calculators are built from, with every value sourced.
Voltage drop for DC, single-phase and three-phase runs in copper or aluminium, sized in mm² or AWG, to BS 7671, IEC 60364 or NEC limits — with the BS 7671 Appendix 4 mV/A/m figure shown alongside.
AWGDiameter, cross-section, resistance and current rating for any AWG size — with separate free-air and in-conduit ampacity, because they differ by a factor of two.
AWGWhat size wire you need, from the load, the run length and the conditions — sized against both NEC ampacity and voltage drop, with the terminal temperature rule that stops 90 °C wire giving 90 °C ampacity.
3 calculators in this category. Every one is free to use on ElectroCalculators, with no account required, and every result cites the standard it came from.