Heat pump cable size calculator
Size the supply cable for an air-to-water or air-to-air heat pump from the breaker rating in its manual, the length of the run and how the cable is installed. The calculator below starts with a 20 A single-phase unit on a 20 m run.
Copper conductors, IEC 60364-5-52 Annex B current-carrying capacities at 30 °C, temperature factors from Table B.52.14 (rounded up to the next 5 °C step) and grouping factors for bunched circuits from Table B.52.17. The breaker is the smallest standard rating at or above the load current, and the cable must carry at least that rating after derating (Ib ≤ In ≤ Iz). Not covered here: buried cables (method D), aluminium, fault-loop disconnection times and short-circuit checks. Guidance only: check the standard, the cable maker's data and your national wiring rules. The installer remains responsible for the design.
Size the cable to the breaker in the manual
Heat pump manuals give a maximum current (often called MCA, minimum circuit ampacity) and a maximum or recommended breaker or fuse. Enter that breaker rating as the load in amps. The calculator then picks the same breaker and a cable that carries it after derating, which covers the compressor at full output.
Typical sizes
20 m of copper PVC cable clipped direct (method C), not grouped, 3% voltage drop. An outdoor unit's cable on a sunny wall can run hotter than the room, so the last column uses 40 °C.
| Breaker in the manual | Supply | 30 °C | 40 °C (sunny outside wall) |
|---|---|---|---|
| 16 A | 1-phase 230 V | 2.5 mm² | 2.5 mm² |
| 20 A | 1-phase 230 V | 4 mm² | 4 mm² |
| 25 A | 1-phase 230 V | 4 mm² | 4 mm² |
| 32 A | 1-phase 230 V | 6 mm² | 6 mm² |
| 10 A | 3-phase 400 V | 1.5 mm² | 1.5 mm² |
| 16 A | 3-phase 400 V | 1.5 mm² | 2.5 mm² |
| 20 A | 3-phase 400 V | 2.5 mm² | 2.5 mm² |
| 25 A | 3-phase 400 V | 4 mm² | 4 mm² |
Also check
- Breaker curve. Inverter heat pumps start softly; older fixed-speed compressors draw a large inrush and often need a C or D curve breaker. The manual says which.
- Isolator. Most manuals ask for a local isolator next to the outdoor unit.
- Backup heater. Air-to-water units often have an electric backup heater with its own supply and breaker. Size that circuit separately.
- Outdoor cable. Use a cable rated for UV and outdoor use, and buried runs need factors for ground installation (method D), which this calculator doesn't cover.
Questions
What size cable for a heat pump?
It depends on the breaker the manual asks for. For a 20 A single-phase heat pump on a 20 m run clipped direct, the calculator gives 4 mm²; for 16 A three-phase, 1.5 mm². Enter your unit's breaker rating and length above.
Should I size the cable for the running current or the breaker?
For the breaker. The cable has to carry the breaker rating after derating so the breaker protects it (Ib ≤ In ≤ Iz). Sizing only for the running current can leave the cable unprotected.
Does a heat pump need its own circuit?
Yes, manufacturers ask for a dedicated circuit for the outdoor unit, and often a separate one for the backup heater.
Can I rely on the result without checking?
No. It is guidance based on IEC 60364. Follow the heat pump's installation manual and your national wiring rules. The installer remains responsible for the design.
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