Summary

The 18th Edition of the IET Wiring Regulations (BS 7671:2018) was significantly updated in December 2022 via Amendment 2 (A2). The EV-specific changes were extensive and now appear in Chapter 722 — "Supplies for electric vehicles." This was driven by the rapid growth of home EV charging and the recognition that EV chargepoints present unique electrical safety challenges: high-frequency leakage currents, PME earthing risks, and long continuous high-current loads.

Understanding Chapter 722 is essential for any electrician involved in EV charging installation. The requirements interact with the earthing system of the premises, the consumer unit's protective devices, and the chargepoint's internal electronics in ways that differ from standard high-current appliances.

This article covers the technical BS 7671 requirements. For OZEV registration and grant scheme compliance, see ozev approved installer.

Key Facts

  • Chapter 722 — BS 7671:2018+A2:2022; main EV charging chapter; replaces and expands earlier provisions
  • EV charging modes — Mode 1 (standard socket, UK not recommended), Mode 2 (3-pin socket with in-cable EVSE), Mode 3 (dedicated AC chargepoint — EVSE in wall unit), Mode 4 (DC rapid charging)
  • Dedicated circuit — Regulation 722.311.1: EV chargepoint must be on its own dedicated final circuit
  • MCB type — typically Type B MCB; 32A for 7.4kW (32A × 230V) single-phase; 16A per phase for 11kW 3-phase
  • RCD Type A minimum — standard RCD for AC fault currents; but where DC smooth residual current may exist (most Mode 3 chargepoints), Type B RCD or equivalent protection required
  • Type B RCD — detects AC, AC pulsating DC, and smooth DC fault currents; required where chargepoint electronics may suppress AC component of fault current
  • EV EVSE — Electric Vehicle Supply Equipment; the chargepoint unit; includes protection and control electronics
  • PME restriction — if premises has PME earthing (TN-C-S), the EV chargepoint's exposed conductive parts (and vehicle) cannot be connected to the PME earth terminal without additional protection
  • TT earthing — one solution for PME problem; separate earth rod for EV circuit; disconnects EV from PME earth
  • PEN fault detection — electronic device detecting loss/failure of the PEN conductor; permitted alternative to TT earthing in some scenarios
  • Labelling — the circuit must be labelled at the consumer unit: "Electric Vehicle Chargepoint"
  • EIC — Electrical Installation Certificate required; minor works certificate is NOT sufficient for a new EV circuit

Quick Reference Table: Chapter 722 Protection Requirements

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Scenario Minimum Protection
Mode 3 chargepoint, standard domestic TNS 32A Type B MCB, Type B RCD (or chargepoint provides integral protection)
Mode 3 chargepoint, PME (TN-C-S) earthing As above PLUS either TT earthing for EV circuit or approved PEN fault detection device
Mode 3 chargepoint, TT earthing 32A Type B MCB, RCD (Type A acceptable in fully TT installation)
Mode 4 DC rapid (domestic, unusual) Specialist design; engineer's input required

Detailed Guidance

EV Charging Modes Explained

Understanding charging modes is essential context for the wiring requirements:

Mode 1 — Direct connection to standard 13A socket via standard extension lead or IEC 62196 cable. Not recommended in UK residential due to lack of dedicated circuit, EVSE control, and earth fault protection. BSI PAS 1899 and Chapter 722 effectively discourage Mode 1 for regular home charging.

Mode 2 — 3-pin domestic socket with an in-cable control and protection device (ICCPD/ICPD). The cable contains a basic EVSE unit with earth monitoring. Permitted for occasional use (up to 10 hours charging is typical), but Chapter 722 requires the socket circuit to be checked for suitability before regular Mode 2 use.

Mode 3 — Dedicated EVSE (wall-mounted or post-mounted chargepoint) with dedicated AC supply. This is the standard method for home charging (7.4kW or 22kW). All dedicated home EV chargepoints are Mode 3. Chapter 722 requirements are primarily written for Mode 3.

Mode 4 — DC charging via on-board converter in chargepoint (not in vehicle). Rapid chargers (50kW, 150kW, 350kW) are Mode 4. Not typically in domestic settings.

The Dedicated Circuit Requirement (Reg 722.311.1)

Every Mode 3 EV chargepoint must be on its own dedicated final circuit — a circuit serving only that chargepoint. This means:

  • A separate MCB/RCBO in the consumer unit labelled "Electric Vehicle Chargepoint"
  • The circuit conductor runs from the consumer unit to the chargepoint location without any intermediate junction or shared connection
  • No other loads are connected to this circuit

Why dedicated? EV charging is a prolonged high-current load (32A for 7.4kW, potentially 8–10 hours overnight). The conductor cross-section, fuse/MCB rating, and protective devices are all designed for this continuous load. Sharing the circuit with other equipment (a light fitting, a socket) would violate the circuit design assumptions.

Cable sizing: For a 32A dedicated EV circuit in domestic conditions:

  • 6mm² twin and earth (T&E) is standard for typical run lengths
  • For longer runs (>15m), voltage drop becomes significant; 10mm² may be needed
  • Always check voltage drop does not exceed 3% of supply voltage (6.9V at 230V) from origin to chargepoint terminals

RCD Type B: Why It Matters for EV Chargepoints

Standard household RCDs (Type A) detect AC residual currents and pulsating DC. Modern EV chargepoints contain rectifier circuits that can create a smooth DC component in any fault current. Type A RCDs may not trip under these conditions.

Type B RCDs detect:

  • AC residual currents (as Type A)
  • Pulsating DC (as Type A)
  • Smooth DC up to 6mA (additional capability)

For most Mode 3 chargepoints, the chargepoint manufacturer specifies which RCD type is required. Many modern chargepoints incorporate integral Type B RCD protection, in which case the upstream protection only needs to be Type A (or the MCB/RCBO can protect against overcurrent without a separate RCD).

Practical guidance:

  • Always check the chargepoint manufacturer's installation manual to determine the required upstream RCD type
  • If the chargepoint does not specify, fit a 30mA Type B RCD in the consumer unit for the EV circuit
  • RCBOs (combined MCB + RCD in one unit) are widely used for EV circuits; Type B RCBOs are available

PME Earthing and EV Charging: The Key Safety Issue

This is the most complex aspect of Chapter 722. Most UK homes have PME (Protective Multiple Earthing) earthing, also called TN-C-S. In PME, the neutral and earth are combined in the distribution network and separated at the consumer's cut-out (or close to it).

The problem: If the PME PEN conductor develops a fault (break or high resistance), the earth potential at the consumer's premises can rise to dangerous levels relative to true earth. An EV connected to a chargepoint on a PME system would have its chassis energised to this fault potential. If a person simultaneously contacts the EV and true earth (wet ground, metal fencing), they could receive a lethal electric shock.

Chapter 722 solutions:

Option 1: TT earthing for EV circuit Install a separate earth electrode (earth rod) for the EV chargepoint circuit. The EV circuit is completely disconnected from the PME system. The chargepoint is earthed via the separate electrode only. The chargepoint (and EV chassis) is then connected to true earth independent of the PME conductor. This is the simplest solution and the most common for domestic installations.

Option 2: PEN fault detection device A device that continuously monitors the PEN conductor. If it detects a fault (PEN current loss, voltage rise on the earth), it disconnects the chargepoint before dangerous voltages can develop. Devices like the Garo PEN/PME detection units are available. This option requires careful selection of the right device for the network conditions.

Option 3: Separation Use an isolating transformer to provide a separated (IT) supply for the EV circuit. Rarely used for domestic.

How to determine the earthing system: Check the electricity supply meter and cut-out. If the incoming neutral and earth are separate conductors from the DNO's origin, the system is TN-S (no PME concern). If the neutral and earth are combined (typically via a combined neutral/earth link in the meter/cut-out area), it is PME/TN-C-S. The DNO can confirm the earthing arrangement.

Labelling and Documentation

At the consumer unit:

  • The EV circuit MCB/RCBO must be clearly labelled: "Electric Vehicle Chargepoint" or similar
  • The chargepoint location should be noted

Electrical Installation Certificate (EIC): A new EV charging circuit is a new circuit installation — it requires a full Electrical Installation Certificate (EIC) under BS 7671, not a Minor Works Certificate. The EIC includes:

  • Installation details
  • Test results (insulation resistance, earth loop impedance, RCD trip time)
  • Designer/erector/inspector declaration

For OZEV grant installations, the EIC evidence should be retained in the installer's records and may be requested by OZEV during audits.

Frequently Asked Questions

Can I install a 7.4kW EV chargepoint on a 100A supply?

Yes. A 32A EV chargepoint consumes 7.4kW — approximately 32% of a 100A supply capacity. This is entirely feasible provided the consumer unit has capacity for an additional 32A circuit and the incoming supply cable/cut-out is rated for the combined maximum demand. For most standard domestic supplies, this is not an issue. For older properties with 60A cut-outs, check the supply capacity before installing.

Does the consumer unit need replacing for an EV installation?

Not necessarily. If the existing consumer unit has a spare way for a 32A MCB/RCBO and the main switch capacity is adequate, the consumer unit does not need replacing. If the consumer unit is a split-load with only a single RCD protecting all circuits, you may need to add Type B RCD protection specifically for the EV circuit. If the consumer unit is old (pre-2016, wired in rubber/PVC without modern protection), consult a qualified electrician about whether upgrade is needed.

Is a 3-pin socket acceptable for regular EV charging?

For occasional top-up charging (a few times a week, overnight), Mode 2 charging via a 3-pin socket with an ICCPD cable is technically permitted. However, Chapter 722 requires assessment of the socket circuit (condition, rating, earth fault loop impedance) before regular Mode 2 use. For daily commuter charging, a dedicated Mode 3 chargepoint is strongly recommended. Mode 2 via a 13A socket is not recommended for long regular sessions.

What is the maximum length of cable run for a 32A EV circuit on 6mm²?

At 32A continuous load, the voltage drop limit is 3% × 230V = 6.9V. For 6mm² twin and earth copper, resistance is approximately 7.41 mΩ/m (two conductors, live and neutral). Maximum length at 32A: 6.9V / (32A × 7.41mΩ/m × 2) ≈ approximately 14.5m. For runs longer than this, upsize to 10mm². Always calculate for the actual route length, not straight-line distance.

Regulations & Standards