Calculate The Cable Size [best] Jun 2026

From tables, 2.5 mm² copper (24 A) is borderline; 4 mm² (32 A) is safe.

Whether a cable is in a conduit, on a tray, or clipped to a wall changes its "breathability". The formula for required ampacity ( Izcap I sub z ) is: 4. Check for Voltage Drop calculate the cable size

For our 21.7 A load, 2.5 mm² might be borderline. If derating factors apply (e.g., 0.8 total), we need 21.7 / 0.8 = 27.1 A design current → step up to 4 mm². From tables, 2

Using your local electrical code (NEC in US, BS 7671 in UK, AS/NZS 3008 in Australia), look up the required cross-sectional area. Check for Voltage Drop For our 21

[ I = \fracP\sqrt3 \times V_L \times \textPower Factor ]

First, calculate the full-load current of the circuit.