Heat Pump Voltage Drop Calculator
Calculate voltage drop for air source and ground source heat pump circuits
Heat Pump Quick Facts
- Domestic ASHP typically 5-16kW requiring 20-40A single-phase supply
- Larger systems (12kW+) often benefit from three-phase for balanced loading
- Compressor starting current can be 3-6× running current - check manufacturer specs
- Defrost cycles cause temporary current spikes - allow headroom in cable sizing
- MCS certification requires BS 7671 compliance including voltage drop limits
Values from BS 7671 Table 4D1B (PVC copper at 70°C)
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Common Heat Pump Scenarios
5kW ASHP - outdoor unit 10m from CU
22A with 4mm² (11 mV/A/m). At 10m: (11 × 22 × 10) ÷ 1000 = 2.42V (1.05%) ✓
9kW ASHP - typical domestic (15m)
32A with 6mm² (7.3 mV/A/m). At 15m: (7.3 × 32 × 15) ÷ 1000 = 3.50V (1.52%) ✓
12kW ASHP - long garden run (25m)
40A with 10mm² (4.4 mV/A/m). At 25m: (4.4 × 40 × 25) ÷ 1000 = 4.40V (1.91%) ✓
16kW three-phase ASHP (30m)
25A per phase with 6mm². At 30m 3Ø: (7.3 × 25 × 30 × 0.866) ÷ 1000 = 4.74V (1.18% on 400V) ✓
GSHP with multiple compressors
Size for maximum simultaneous demand. 20kW system may draw 35A+ at startup. Consider soft start or staged compressor operation.
Complete Your Renewable Energy Setup
Combining heat pump with solar PV? Great combination for reducing running costs. For EV charger installations alongside your heat pump, check your total property demand. Use our cable sizing calculator for current capacity checks.
Heat Pump Electrical Requirements
Heat pumps contain compressor motors that draw significant current - typically 20-40A for domestic air source units. Unlike resistive heaters, compressor motors have inrush current on startup (3-6× running current) and may require Type C or D MCBs to prevent nuisance tripping. The outdoor unit location means cables often run externally, requiring SWA or protected installation methods.
Heat Pump Electrical Installation Costs (2024)
Typical UK costs for electrical installation only. Excludes heat pump unit and plumbing.
| Installation Scenario | Materials | Labour | Total |
|---|---|---|---|
| Simple ASHP circuit (<15m) | £100-200 | £200-350 | £300-550 |
| Standard installation (15-25m) | £180-350 | £300-500 | £480-850 |
| Long run or buried cable | £300-550 | £400-700 | £700-1250 |
| Three-phase installation | £400-700 | £500-800 | £900-1500 |
| CU upgrade/new way | £150-350 | £150-300 | £300-650 |
Prices as of 2024. BUS grant (£7,500) covers part of total installation cost.
ASHP vs GSHP Considerations
Air Source Heat Pumps (ASHP) locate the compressor unit outdoors with refrigerant lines to indoor unit. Ground Source Heat Pumps (GSHP) have the compressor indoors with ground loop connections. GSHP may have longer cable runs to plant room. Both require careful voltage drop calculation as compressor efficiency reduces with low voltage supply.
MCS and BUS Grant Requirements
The Microgeneration Certification Scheme (MCS) is required for Boiler Upgrade Scheme (BUS) grants of £7,500 for ASHP or £7,500 for GSHP. MCS installation standards mandate BS 7671 compliance including voltage drop limits. MCS installers must provide EIC certificates and design documentation showing compliance.
Defrost Cycle Impact
ASHPs periodically reverse refrigerant flow to defrost the outdoor coil in cold weather. This defrost cycle temporarily increases current draw as the compressor works harder. While brief (typically 5-10 minutes), frequent defrost cycles in very cold weather mean the heat pump operates at elevated current more often. Allow margin in cable sizing for this operating mode.
Frequently Asked Questions
Related Electrical Calculators
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