Heat Pump Electrical Calculator
Calculate cable size, MCB, and supply requirements for air source heat pumps
Reference Info & Formulas
5-8kW: Single phase usually OK
10-12kW: Check supply capacity
16kW+: Three phase recommended
Type C MCB for compressor starting current
Isolator within sight of outdoor unit
Type A RCD minimum
Check DNO supply adequacy
MCS installer certification required
Data: BS 7671:2018+A2:2022, MCS Standards
For guidance only. The responsibility for any electrical installation lies with the qualified person carrying out the work. Always verify calculations independently and apply professional judgement.
How This Calculator Works
Heat pump circuit design accounts for continuous compressor loads, high starting currents, and coordination with the existing supply capacity.
The calculator sizes the circuit based on the heat pump's maximum operating current from the manufacturer's data sheet, not the nominal kW rating. Compressor motors are treated as continuous loads with significant inrush current on start-up, requiring careful MCB type selection and cable sizing.
I = P / (V × PF) [single-phase]- I
- = Full load current (A)
- P
- = Heat pump rated input power (W)
- V
- = Supply voltage — 230V single-phase
- PF
- = Power factor — typically 0.85-0.95 for heat pump compressors
For three-phase: I = P / (√3 × V × PF), where V = 400V
After determining the full load current, the calculator selects a cable size from Table 4D5 (T&E) or Table 4D4A (SWA for outdoor runs), applies correction factors for ambient temperature, grouping, and thermal insulation, then verifies voltage drop is within the 5% limit. A Type C MCB is selected to handle compressor inrush without nuisance tripping.
Quick Reference — ASHP Circuit Sizing
Typical air source heat pump circuit requirements
BS 7671 Table 4D5, manufacturer data| Heat Pump Rating | Supply | Current Draw | Cable Size (Method C) | MCB |
|---|---|---|---|---|
| 5kW | Single-phase | 22A | 4mm² T&E (37A) | 25A Type C |
| 8kW | Single-phase | 35A | 6mm² T&E (47A) | 40A Type C |
| 12kW | Single-phase | 52A | 10mm² T&E (64A) | 63A Type C |
| 12kW | Three-phase | 17A per phase | 2.5mm² (24A) | 20A Type C |
| 16kW | Three-phase | 23A per phase | 4mm² (32A) | 25A Type C |
| 20kW | Three-phase | 29A per phase | 4mm² (32A) | 32A Type C |
Values assume Method C (clipped direct), 30°C ambient, PF 0.85, no grouping or insulation derating. Always verify against the specific manufacturer's maximum current rating.
Quick Reference — Supply Adequacy
Typical domestic maximum demand with heat pump
IET Guidance Note 1, Appendix H| Load | Demand (A) | With Diversity |
|---|---|---|
| Existing domestic load (diversified) | 40-60A | — |
| 8kW ASHP (single-phase) | 35A | No diversity applied |
| 12kW ASHP (single-phase) | 52A | No diversity applied |
| 7kW EV charger | 32A | No diversity applied |
| Immersion heater (3kW backup) | 13A | May apply diversity with ASHP |
Heat pumps and EV chargers are continuous loads — no diversity reduction. A maximum demand calculation should be performed before installation to confirm supply adequacy.
Practical Notes
Supply Overload Risk
Type C MCB — Not Type B
DNO Notification May Be Required
Frequently Asked Questions
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