What cable size do I need for an air source heat pump?

Most domestic ASHPs (5-12kW) need 4mm² or 6mm² cable on a 16A-32A circuit. The exact size depends on the heat pump's electrical rating and cable run length. Always check manufacturer specifications and calculate voltage drop for your installation.

What MCB rating for an air source heat pump?

MCB rating depends on the heat pump's maximum current draw. Typical ratings: 5-8kW units = 16A or 20A, 8-12kW units = 20A or 32A. Type C MCBs are often required due to compressor inrush current. Check manufacturer documentation for specific requirements.

Do air source heat pumps need RCD protection?

Yes, 30mA RCD protection is required under BS 7671 for circuits supplying outdoor equipment. Type A RCD is minimum, but some manufacturers specify Type B for heat pumps with DC components. Check the installation manual for RCD requirements.

Why do some heat pumps need Type C MCBs?

Heat pump compressors have significant inrush current when starting - up to 6-10 times running current. Type C MCBs have higher trip thresholds than Type B, preventing nuisance tripping during compressor starts while still providing protection.

Can I connect a heat pump to an existing circuit?

No, heat pumps must have a dedicated circuit from the consumer unit. They cannot share circuits with other loads. The circuit must be protected by appropriate MCB and RCD, with an isolator switch near the outdoor unit.

What is the Boiler Upgrade Scheme grant?

The BUS provides £7,500 towards ASHP installation in England and Wales. Requirements: MCS-certified installer, property must have valid EPC, replacing fossil fuel heating. The installer claims the grant - you pay the reduced price. Funding runs until 2028.

Do I need three-phase for a heat pump?

Most domestic heat pumps up to 12kW work on single-phase supply. Larger units (12kW+) may require three-phase. Check if your property has three-phase available - if not, DNO application and significant cost for upgrade. Many manufacturers offer single-phase options to avoid this.

How far can the outdoor unit be from the consumer unit?

Typically 10-20m is ideal. Longer runs increase voltage drop and cable cost. Very long runs (30m+) may need 10mm² cable. Also consider refrigerant pipe length limits between outdoor and indoor units (usually 20-30m maximum).

What isolator switch is needed for heat pumps?

A rotary isolator switch must be installed adjacent to the outdoor unit for safe maintenance. This should be IP65 rated minimum for outdoor exposure. The switch allows isolation without returning to the consumer unit.

Is MCS certification required for heat pump installations?

MCS certification is required to claim the Boiler Upgrade Scheme grant. Even without the grant, MCS ensures quality installation, valid warranty, and compliance with manufacturer requirements. MCS installations also qualify for reduced VAT (0%) on the system.

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Air Source Heat Pump Cable Sizing Calculator

Calculate the correct cable size for air source heat pump (ASHP) installations

£7,500 GRANT

Boiler Upgrade Scheme Available

MCS-certified installations qualify for the £7,500 Boiler Upgrade Scheme grant in England and Wales. The grant is deducted from your installation cost - no upfront payment required. Check eligibility with your installer.

ASHP Cable Quick Facts

Most domestic ASHPs need 16A-32A single-phase supply
Typical cable size: 4mm² or 6mm² depending on run length
£7,500 Boiler Upgrade Scheme grant available (MCS required)
Outdoor unit typically within 15m of consumer unit
Three-phase may be needed for larger 12kW+ systems

Cable Size Calculator

BS 7671:2018+A2:2022 compliant

1. Load Current

2. Cable Length (meters)

3. Installation Method

Lighting circuit (3% voltage drop limit)

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Common ASHP Installations

5-8kW ASHP (small home)

16A MCB, 2.5mm² or 4mm² cable for runs under 15m

8-12kW ASHP (average home)

20A or 32A MCB, 4mm² or 6mm² cable

12-16kW ASHP (large home)

May need 3-phase supply, consult manufacturer specs

Long cable run to outdoor unit (20m+)

Check voltage drop - likely need 6mm² minimum

ASHP with integrated hot water cylinder

Check total electrical load - may need larger circuit

Complete Your Low-Carbon Home

Heat pumps work brilliantly alongside other renewable technologies. Pair with solar PV and battery storage to power your heat pump with free electricity. Add an EV charger for complete electrification. Heat pump systems often include backup immersion heaters for hot water boosting.

Air Source Heat Pump Cable Sizing

ASHP cable sizing determines the conductor cross-sectional area (mm²) needed to safely supply an air source heat pump. Most domestic ASHPs draw 8-25A depending on size and model. The outdoor unit location and cable run length are critical factors - longer runs need larger cables to maintain voltage within acceptable limits for compressor operation.

ASHP Electrical Installation Costs (2024)

Typical UK electrical installation costs. Does not include heat pump unit or plumbing work.

Installation Scenario	Materials	Labour	Total
ASHP electrical connection, short run (<10m)	£80-150	£150-280	£230-430
ASHP electrical connection, medium run (10-20m)	£150-280	£200-350	£350-630
ASHP electrical connection, long run (20m+)	£250-400	£280-450	£530-850
Consumer unit upgrade for ASHP	£150-300	£200-350	£350-650
Three-phase supply upgrade	£500-1500	£800-2000	£1300-3500

Prices as of 2024. Three-phase upgrade costs vary significantly by DNO and location.

Boiler Upgrade Scheme (BUS)

The Boiler Upgrade Scheme provides £7,500 grants towards air source heat pump installation in England and Wales. To qualify, the property must have an EPC, and the heat pump must be installed by an MCS-certified installer. The grant is deducted from the installation cost - no upfront payment required from homeowners.

ASHP Electrical Requirements

Air source heat pumps have specific electrical requirements beyond simple cable sizing. These include: inrush current capacity (Type C MCB often needed), voltage stability (±10% tolerance typically), dedicated circuit with appropriate RCD, and local isolator switch. Some units have specific requirements for earth impedance.

Heat Pump COP and Electrical Load

Coefficient of Performance (COP) indicates heat pump efficiency - a COP of 3 means 3kW heat output per 1kW electrical input. This means a 12kW heat output system may only draw 4kW electrically at design conditions. However, COP varies with outdoor temperature, and backup heating may increase electrical load.

Frequently Asked Questions

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