Electrical Cable Sizing Calculator

The cable size depends on the heat pump's power rating, supply voltage, cable run length, installation method, and ambient temperature. A 9.5 kW single-phase heat pump on a 30-metre run typically needs 6 mm² or 10 mm² cable. Use the calculator above with your specific parameters to get an accurate result compliant with BS 7671 / IEC 60364.

BS 7671 recommends a maximum voltage drop of 3% for lighting circuits and 5% for other circuits. Most heat pump installations should target 3% to 4% voltage drop to ensure reliable compressor starting and efficient operation. Higher voltage drops can cause compressor start failures and reduced performance.

Steel Wire Armoured (SWA) cable is commonly used for outdoor heat pump installations because it provides mechanical protection and can be buried directly without additional conduit. However, the choice depends on your installation method. Cables clipped to an external wall may use non-armoured cable in suitable trunking or conduit instead.

Derating factors reduce the current-carrying capacity of a cable based on installation conditions. Key factors include ambient temperature (cables in hotter environments carry less current), installation method (cables in conduit carry less than those clipped direct), and cable grouping. Multiply the base current capacity by each applicable derating factor to find the actual capacity.

The MCB (miniature circuit breaker) rating must be equal to or greater than the design current but not exceed the cable's current-carrying capacity after derating. For heat pumps with compressor motors, a Type C or Type D MCB is often recommended to handle inrush currents. Always verify with the heat pump manufacturer's installation manual.