Unit
Cable run
Result
What this does not cover
Ratings are for one circuit in 30 °C air with no grouping or insulation, from BS 7671 Appendix 4 (Table 4D5 for twin & earth, Tables 4D4A/4D4B for 70 °C SWA — conservative for XLPE SWA, with a 6% margin added to its voltage drop). It does not apply correction factors for heat, grouping or insulation, and it does not check earth fault loop impedance or disconnection time. A qualified electrician must design and certify the circuit to BS 7671.
AUX also asks for: a 30 mA high-speed RCD suited to inverter equipment, an isolator with at least 3 mm contact separation on all poles, and — for heat pumps — a separate dedicated circuit for the backup heater and for any tank immersion.