Cable Sizing Calculator
Calculate the minimum cable size for any circuit under BS 7671 (UK/Ireland) or AS/NZS 3008 (Australia/New Zealand). Enter your design current, installation method, and correction factors - and get the minimum conductor cross-sectional area in mm².
Minimum cable size
4mm²
Required capacity Iz
32A
Cable rating
36A
4 mm² copper (Method C) carries 36 A
Correction factors applied - Ca (temperature) = 1, Cg (grouping) = 1. Required Iz ≥ Ib ÷ (Ca × Cg) = 32 A.
Free web calculator uses Method C (clipped direct), 70 °C thermoplastic copper reference ampacities. The Voltix app covers methods A1–F, armoured and XLPE cables.
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How cable sizing works
Under BS 7671, the selected cable's current-carrying capacity Iz must be at least the design current Ib after correction factors are applied:
Iz ≥ Ib ÷ (Ca × Cg × Cc)
- Ca- ambient temperature correction factor (Table 4C1). Above 30 °C the factor falls below 1.0, so a larger cable is needed.
- Cg - grouping correction factor (Table 4C3). More circuits bundled together means more heat and a lower factor.
- Cc - thermal insulation / installation factor where applicable.
Worked example
A 32 A circuit is clipped direct (Method C) in a 35 °C ambient, grouped with one other circuit:
- Ca at 35 °C = 0.94, Cg for 2 circuits = 0.80
- Required Iz = 32 ÷ (0.94 × 0.80) = 42.6 A
- 6 mm² carries only 41 A, so it falls short - the next size up, 10 mm² (57 A), is required.
Result:10 mm² copper. Always verify the protective device rating (In) and voltage drop separately.
Frequently Asked Questions
- What installation method should I use for cables clipped to a wall?
- Use Method C (clipped direct to a non-thermally-insulating wall). If cables are enclosed in conduit on the wall, use Method B.
- Do I need to apply grouping factors for a single cable?
- No. Grouping factors only apply when two or more circuits are installed together in the same conduit, trunking, or touching each other.
- What's the difference between Ib, In, and Iz?
- Ib is the design current (what the circuit will actually carry). In is the nominal current rating of the protective device. Iz is the current-carrying capacity of the selected cable. BS 7671 requires Iz ≥ In ≥ Ib.
- Does this calculator cover armoured cable?
- This web calculator covers the most common cable types using Method C reference ampacities. The Voltix app includes tables for SWA, XLPE, and fire-resistant cables across all installation methods.
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