AC Electrical Supply Requirements: Dedicated Circuit, Isolator Siting, RCD Protection and Earthing

Quick Answer: Under BS 7671:2018+A2:2022 (the 18th Edition IET Wiring Regulations), a fixed air conditioning system requires a dedicated final circuit protected by a Type C MCB (to accommodate motor inrush current), a local means of isolation within sight of the outdoor unit in an appropriate IP-rated enclosure, and 30mA RCD protection for outdoor equipment. Circuit sizing typically ranges from 16A to 32A depending on unit capacity. Part P of the Building Regulations requires notification to Building Control for new circuits in a dwelling.

Summary

Electrical supply errors are one of the most common causes of AC system failures and callouts. Undersized cable, wrong MCB type, missing earth bonding, and incorrectly sited isolators are all recurring problems found on existing installations. Getting the electrical side right from the outset is both a safety requirement and a commercial one — a unit on an undersized circuit or with intermittent protection nuisance-tripping is a warranty headache and a reputational risk.

The 18th Edition (BS 7671:2018+A2:2022, amended 2022) introduced or consolidated several requirements relevant to AC installations: the mandatory 30mA RCD protection for outdoor equipment, updated cable sizing calculation references (Appendix 4), and clarifications around earthing for outdoor metalwork. Installers should work from the current edition, not habits formed under earlier versions.

Part P of the Building Regulations applies to electrical work in dwellings. Adding a new circuit from the consumer unit to an outdoor AC unit is notifiable work unless the installer is registered with a Competent Person Scheme (such as NICEIC, NAPIT, or ELECSA). Unregistered installers must notify Building Control, pay the notification fee, and have the work inspected. In practice, most AC installers subcontract the electrical supply work to a Part P-registered electrician, or ensure their own team carries the relevant Competent Person registration.

Key Facts

Quick Reference Table

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Unit Cooling Capacity Typical Full Load Current Recommended MCB Minimum Cable (short run)
Up to 2.5 kW 8–10A 16A Type C 1.5mm² (check VD)
2.5–5 kW 10–16A 16A Type C 2.5mm²
5–7 kW 16–22A 25A Type C 4mm²
7–10 kW 22–30A 32A Type C 6mm²
10–14 kW 30–40A 40A Type C 10mm²
14–20 kW 40–60A 63A Type C 16mm²
Above 20 kW Manufacturer data required As calculated 3-phase; as calculated
RCD Type Fault Current Detected Suitable for AC Units?
Type AC AC sinusoidal only Not recommended — misses DC faults
Type A AC + pulsating DC Minimum acceptable
Type F AC + pulsating DC + HF Preferred where inverter drives present
Type B AC + DC smooth Required if significant DC component (rare for standard splits)

Detailed Guidance

Circuit Design and Cable Sizing

The design process for an AC circuit starts with the manufacturer data sheet, not with assumptions. Every AC unit has a declared Maximum Current Draw (sometimes labelled Maximum Operating Current or MCA — Minimum Circuit Ampacity). This is the current the unit draws under worst-case operating conditions, not the cooling rated current. MCB and cable selection must be based on this figure.

The cable is then sized per BS 7671 Appendix 4. The key calculation steps are: (1) determine the design current (Ib) from the manufacturer data; (2) select a cable with a current-carrying capacity (Iz) greater than Ib; (3) apply correction factors for grouping, ambient temperature, and installation method (buried, clipped, in conduit, etc.); (4) check that voltage drop over the route length does not exceed 3% of 230V (6.9V); (5) verify that the disconnection time under fault conditions is achievable with the chosen protective device.

For runs exceeding 20–25 metres, voltage drop is often the binding constraint, pushing cable selection up one size above the thermal limit. Document the calculation and keep it with the installation certificate.

XLPE (cross-linked polyethylene) insulated cable is preferred over PVC for outdoor and hot environments — PVC softens above 70°C and can degrade in UV. Where cable runs are exposed outdoors, use UV-stabilised SWA (steel wire armoured) or run in appropriate conduit. SWA cable also provides mechanical protection from accidental damage and is effectively self-earthed via the armour.

MCB Selection: Why Type C Matters

AC compressors are motor loads. When a motor starts from rest, it draws a starting (inrush) current that can be 4–8× the full-load current for a fraction of a second. Modern inverter-driven compressors have reduced inrush compared to fixed-speed designs, but the starting current is still significant.

A Type B MCB has an instantaneous trip threshold of 3–5× its rated current. A 16A Type B trips instantaneously at 48–80A. A compressor drawing 64A at start-up on a 16A circuit will trip a Type B every time it starts. A Type C MCB trips at 5–10× (80–160A for a 16A device), which is above the typical inrush and allows the motor to start reliably.

Using a Type D MCB (10–20× instantaneous trip) is unnecessary and creates a protection blind spot — the MCB would allow dangerous fault currents to flow before tripping. Type C is the correct choice for motor loads.

Where a Main Fuse (Service Head fuse) is close in rating to the MCB, verify discrimination: the MCB should clear a fault before the service fuse operates. With a 100A service fuse and a 32A Type C MCB, discrimination is usually achieved for overcurrents, but check the manufacturer's discrimination curves if in any doubt.

RCD Protection: Type, Rating, and Positioning

The 30mA RCD requirement for outdoor equipment comes from BS 7671 Regulation 411.3.3, which requires 30mA RCD protection for circuits supplying equipment outdoors. There is no option to use a 100mA RCD instead — the 30mA threshold is set to provide personal protection against indirect contact.

Type A RCDs are the minimum required — they detect both AC fault currents and pulsating DC fault currents, which can occur in circuits with rectified loads (such as inverter-driven compressors). Older Type AC RCDs detect only AC fault currents and are not suitable. In new installations, Type F is a good choice: it detects AC, pulsating DC, and high-frequency components, providing robust protection for modern inverter equipment.

RCDs can be incorporated into the MCB (RCBO — Residual Current Breaker with Overcurrent protection) at the consumer unit, giving both overload and RCD protection in a single device. This is the cleanest solution and avoids the nuisance-trip risk associated with a single RCD protecting multiple circuits. Where a 30mA RCBO is used at the board, a separate RCD in the circuit is not required.

Time-delayed RCDs (S-type) are used for discrimination in installations where a consumer unit RCD feeds multiple circuits — but a 30mA time-delayed RCD provides no additional personal protection benefit; the 30mA limit is about fault current magnitude, not time.

Local Isolator: Specification and Siting

BS 7671 Regulation 537.2.2.6 requires a means of switching off for mechanical maintenance to be provided that can be operated by ordinary persons (non-electrically skilled) and is visible from the equipment it controls. For an outdoor AC unit, this means a rotary isolator or similar device mounted on the external wall near the unit, clearly labelled as the AC isolator.

The isolator must:

Do not use a standard indoor-rated IP2X enclosure for an outdoor isolator — water ingress will cause corrosion, intermittent contact, and potential arcing.

The "within sight" requirement means visible without moving to a different room or location. An isolator inside the building, even near a window, does not satisfy this requirement if an engineer servicing the outdoor unit cannot see it. In practice, mounting the isolator on the external wall within 1–2 metres of the outdoor unit is standard.

Earthing Arrangements

The most common earthing system in UK homes is TN-C-S (also called PME — Protective Multiple Earthing), where the neutral and protective earth conductors are combined in the supply cable and separated at the consumer unit. The meter tails bring both live and combined neutral/earth into the property; the earth is taken from the neutral via a terminal provided by the Distribution Network Operator (DNO).

In a TN-C-S system, connecting outdoor metalwork — including the outdoor AC unit frame — to the PME earth is generally acceptable per BS 7671. However, BS 7671 Regulation 544.1.1 notes that PME earth conductors should not be used as the sole means of earthing for certain outdoor installations. In practice, for a standard domestic AC installation, bonding the outdoor unit to the circuit protective conductor (CPC) of the dedicated circuit is correct and compliant.

For TT systems (where the earth is provided by a local earth electrode rather than via the supply neutral), the outdoor earth can be less reliable. An RCD provides the primary fault protection in TT systems. Confirm the system type before designing the earthing arrangement.

In commercial premises, the earthing arrangements may be more complex, particularly in older buildings with TN-S (separate earth and neutral throughout the supply), or in premises served by private substations. The main earthing conductor and main bonding conductors must be verified as adequate for the additional earth fault current path before connecting the AC circuit.

Frequently Asked Questions

Can I connect the AC unit to an existing ring final circuit rather than running a new circuit?

No. A split AC unit should have a dedicated final circuit. Ring final circuits are designed for general socket outlets and the loads plugged into them. Adding a motor load like an AC compressor introduces inrush current that can cause nuisance tripping of the RCD protecting the ring. More fundamentally, the AC system deserves reliable, isolated protection — if the ring trips for any reason, you lose the AC. A dedicated circuit also makes fault-finding and isolation straightforward.

Why do some AC outdoor units have a 3-pin plug when delivered? Can I just use a standard socket?

Some small (under 2.5 kW) domestic AC units are supplied with a standard 13A plug for flexibility in positioning. This is not best practice for a permanent installation. BS 7671 recommends that fixed equipment has a dedicated circuit with appropriate protection. Using a standard socket means sharing the ring with other loads, no dedicated overload protection, no local lockable isolator, and no guarantee of Type C protection. For anything other than a very small temporary or portable unit, hardwire to a dedicated circuit.

The outdoor unit is in the customer's garden on a concrete pad. Does the earthing arrangement change?

For equipment installed on the ground outdoors and reachable by a person who might also be in contact with general earthed metalwork (fences, pipes, etc.), BS 7671 requires that all exposed metalwork is bonded to the protective conductor. The main risk is simultaneous contact with the outdoor unit and an earthed fence or pipe — a potential difference under fault conditions could be dangerous. Ensure the circuit CPC is connected to the unit's earth terminal and that all accessible metalwork is bonded.

What Part P route do I need to follow for a new AC circuit in a domestic property?

If you are registered with a Competent Person Scheme (NICEIC, NAPIT, ELECSA, or similar), you self-certify. You issue an Electrical Installation Certificate (EIC) and notify your scheme, which notifies the Local Authority on your behalf. If you are not registered, you must notify Building Control before starting work, pay the relevant fee, and arrange for the work to be inspected and certified. Building Control will issue a completion certificate after a satisfactory inspection.

Is a 3-phase supply needed for a 12 kW outdoor unit?

Not always — it depends on the unit. Many single-phase 230V inverter splits are rated up to 14 kW and draw well within single-phase supply limits. However, some manufacturers specify 3-phase for units above 10–12 kW. Check the unit's electrical data plate and data sheet. If 3-phase is required and the property only has a single-phase supply, a new 3-phase supply connection must be arranged with the DNO — this adds significant cost and programme time.

Regulations & Standards