What cable size do I need for a 5kW air conditioning unit?

A 5kW AC unit typically draws 20-22A running current. Use 4mm² cable on a 20A or 25A Type C MCB for most installations. For longer runs (15m+), consider 6mm² to ensure adequate voltage during compressor startup.

Why do AC units need Type C MCBs?

Compressor motors have high inrush current when starting - up to 8 times running current. Type B MCBs (5x trip threshold) often nuisance trip on compressor starts. Type C MCBs (10x) handle this inrush while still providing protection.

Do I need a dedicated circuit for air conditioning?

Yes, AC outdoor units need dedicated circuits from the consumer unit. They cannot share circuits with other loads due to high starting currents and continuous operation. Indoor units may be fed from the outdoor unit or need separate supply depending on model.

What isolation is required for AC units?

A local isolator switch is required adjacent to the outdoor unit for safe maintenance. This should be a rotary isolator rated for the circuit current. IP65 rating minimum for outdoor exposure. The isolator allows disconnection without returning to the consumer unit.

Do air conditioning units need RCD protection?

RCD protection is required for circuits supplying outdoor equipment. 30mA RCD at the consumer unit is standard. Some AC manufacturers specify Type A or Type B RCDs depending on the inverter technology used in the unit.

Can I install a portable AC unit on a normal socket?

Portable AC units up to about 3kW (13A) can use standard 13A sockets. However, they draw significant continuous current and may stress the socket. For regular use, a dedicated socket on its own circuit is recommended.

What about multi-split systems?

Multi-split systems have one outdoor unit serving multiple indoor units. The electrical supply is sized for the outdoor unit capacity. Indoor units are typically powered via interconnecting cables from the outdoor unit, though some need separate supplies. Check manufacturer specifications.

Is planning permission needed for AC outdoor units?

Usually not for domestic installations under permitted development, but there are conditions: maximum 0.6m³ outdoor unit, not on a wall facing a highway, noise limits apply. Listed buildings and conservation areas need permission. Check local requirements.

What about F-Gas regulations?

F-Gas regulations require that refrigerant handling (installation, maintenance, decommissioning) is done by F-Gas certified engineers. The electrical installation can be done by any competent electrician, but refrigerant work needs specialist certification.

Do I need Part P notification for AC installation?

Yes, installing a new circuit for an air conditioning unit is notifiable under Part P. This includes the outdoor unit supply circuit. Work must be done by a registered competent person or inspected by Building Control.

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Air Conditioning Cable Sizing Calculator

Calculate the correct cable size for air conditioning and cooling system installations

Air Conditioning Cable Quick Facts

Domestic split AC units typically 2.5-7kW (11A-30A)
Outdoor unit needs dedicated circuit and local isolator
Type C MCB often required for compressor inrush current
Some units need separate indoor unit power supply
Growing UK market - climate change driving demand

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 AC Installations

Small bedroom unit (2.5-3.5kW)

16A MCB, 2.5mm² cable, Type C recommended

Living room unit (3.5-5kW)

16A or 20A MCB, 2.5mm² or 4mm² cable

Large room/open plan (5-7kW)

20A or 32A MCB, 4mm² or 6mm² cable

Multi-split system (1 outdoor, multiple indoor)

Size for outdoor unit load, indoor units often from FCU

Commercial VRF system

Three-phase usually required, consult manufacturer specs

Complete Climate Control

For year-round heating and cooling, consider an air source heat pump which provides both heating and cooling. Some homeowners combine AC with solar PV systems to offset running costs. For hot tub and pool areas, see our swimming pool electrical guide.

Air Conditioning Cable Sizing

AC cable sizing determines the conductor cross-sectional area (mm²) needed to safely supply air conditioning units. Split systems have an outdoor compressor unit requiring a dedicated circuit and local isolator. Indoor units may be powered from the outdoor unit or need separate supply. Cable must handle compressor inrush current and continuous running load.

AC Electrical Installation Costs (2024)

Typical UK electrical installation costs. Does not include AC unit or F-Gas refrigerant work.

Installation Scenario	Materials	Labour	Total
Single split AC, short run (<8m)	£60-120	£120-220	£180-340
Single split AC, longer run (8-15m)	£100-180	£150-280	£250-460
Outdoor unit on external wall	£80-150	£140-250	£220-400
Multi-split system wiring	£150-300	£250-450	£400-750
Outdoor isolator switch	£30-60	£50-100	£80-160

Prices as of 2024. Full AC installation including refrigerant work is additional.

Compressor Inrush Current

AC compressors draw 4-8 times their running current when starting. A unit with 10A running current may draw 40-80A momentarily on startup. Type C MCBs (10x trip threshold) or Type D (20x) prevent nuisance tripping while maintaining protection. Check manufacturer specifications for MCB requirements.

Split System AC

A split system has an outdoor unit (containing compressor and condenser) connected to an indoor unit (containing evaporator and fan) by refrigerant pipes and electrical cables. The outdoor unit requires dedicated electrical supply. Indoor units may be wall-mounted, ceiling cassette, or ducted.

Inverter AC Technology

Modern inverter AC units vary compressor speed to match cooling demand, rather than cycling on/off. This improves efficiency but creates specific electrical characteristics. Some inverter units require Type B RCDs to detect DC fault currents. Check manufacturer specifications for RCD requirements.

Frequently Asked Questions

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