Q: What size cable does an air-source heat pump need?

Size from the heat pump's electrical input (not heat output). Full load current is kW × 1000 ÷ 230 (single-phase) or ÷ (√3 × 400) (three-phase); the cable is then sized so its derated rating meets the MCB. An 8 kW single-phase unit draws about 35 A and typically needs 6 mm² T&E with a 40 A Type C MCB — the calculator gives the exact figures.

Q: What RCD does a heat pump require?

A Type A RCD as a minimum ; some inverter-driven heat pumps with electronic drives require a Type B RCD to detect smooth DC fault currents. Always follow the manufacturer's installation instructions, and fit an isolator within sight of the outdoor unit.

Question 1

What size cable does an air-source heat pump need?

Accepted Answer

Size from the heat pump's electrical input (not heat output). Full load current is kW × 1000 ÷ 230 (single-phase) or ÷ (√3 × 400) (three-phase); the cable is then sized so its derated rating meets the MCB. An 8 kW single-phase unit draws about 35 A and typically needs 6 mm² T&E with a 40 A Type C MCB — the calculator gives the exact figures.

Question 2

Why does a heat pump need a Type C MCB?

Accepted Answer

The compressor draws a large inrush (locked-rotor) current at start — typically 3–6× full load current. A Type C MCB tolerates this without nuisance tripping, where a Type B (tripping at 3–5×) would trip. Inverter-driven compressors have softer starts; always check the manufacturer data.

Question 3

Does a heat pump need a three-phase supply?

Accepted Answer

Most domestic heat pumps run on single phase, but units above roughly 8 kW may exceed what the single-phase supply can comfortably carry — check the maximum demand and consult the DNO, who may require a three-phase supply or a supply upgrade.

Question 4

What RCD does a heat pump require?

Accepted Answer

A Type A RCD as a minimum; some inverter-driven heat pumps with electronic drives require a Type B RCD to detect smooth DC fault currents. Always follow the manufacturer's installation instructions, and fit an isolator within sight of the outdoor unit.

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