BS 7671:2018 Amendment 4 (April 2026) — Key Changes for Electrical Engineers

Amendment 4 to BS 7671:2018 (cited as BS 7671:2018+A4:2026) is the fourth and likely final amendment to the 18th Edition of the IET Wiring Regulations before the anticipated 19th Edition. Published by BSI and the IET, Amendment 4 takes effect on 15 April 2026.

A six-month transition period runs from 15 April 2026 to 15 October 2026. During this period:

• Installations may be designed and installed to either BS 7671:2018+A3 or BS 7671:2018+A4.
• From 16 October 2026, all new electrical installation work must comply with Amendment 4.
• Existing installations are not required to be upgraded unless they are materially altered or extended.

Amendment 4 addresses three areas where the 18th Edition needed updating to keep pace with the rapid growth of renewable energy, electric vehicles, and fire safety requirements: solar photovoltaic systems, EV charging infrastructure, and arc fault detection devices (AFDDs).

Chapter 72 (previously Section 712) has been extensively rewritten to address the modern reality of UK solar installations. The 2018 text was based on IEC 60364-7-712:2017, but the UK solar market has evolved significantly since then, particularly with the growth of battery-coupled PV systems and export-limiting inverters.

Key changes in Chapter 72:

The rapid shutdown requirement is perhaps the most impactful change. Drawing on experience from the US (NEC 690.12) and from UK fire service feedback, Regulation 72.537 requires that PV systems on buildings provide a means of reducing DC conductors to a safe voltage (≤120 V DC) within 30 seconds of activating the rapid shutdown device. This effectively mandates module-level power electronics (optimisers or microinverters) or string-level rapid shutdown devices for most rooftop installations.

Section 722, first introduced in Amendment 2 (2022), receives significant updates in Amendment 4 to address lessons learned from the rapid deployment of EV chargers across the UK.

The major changes are:

• Regulation 722.311 (Maximum demand): Revised diversity factors for EV charging circuits in multi-point installations. The A3 diversity of 0.8 for 5–10 charging points has been reduced to 0.6, based on real-world usage data from UK charge point operators showing that simultaneous charging at full rated power is less common than originally assumed. This reduces upstream cable and switchgear sizing requirements for car parks and fleet depots.
• Regulation 722.531 (PME earth): Clarified requirements for TN-C-S (PME) earthed supplies. Amendment 4 now explicitly permits PME earthing for Mode 3 and Mode 4 chargers with additional earth electrode or earth mat, removing the previous ambiguity that led some DNOs to require TT earthing for all EV installations.
• Regulation 722.411.4 (RCD protection): Type B RCDs (or Type A + 6 mA DC detection) remain mandatory for Mode 3 chargers, but Amendment 4 adds a new note clarifying that chargers with built-in DC fault detection that has been independently certified to IEC 62955 may use a Type A RCD externally.
• Regulation 722.55 (Cable management): New requirement for cable management systems (overhead gantries or ground-mounted posts) to be rated for the mechanical stresses of repeated cable deployment, with reference to BS EN 61851-1:2019 connector durability ratings.

EV charging diversity factors (Section 722, Table 72A):
  Number of charge points    A3 diversity    A4 diversity
  1                          1.00            1.00
  2-4                        0.90            0.85
  5-10                       0.80            0.60
  11-20                      0.70            0.50
  21-50                      0.50            0.40
  50+                        0.40            0.35

The reduced diversity factors reflect data from over 50,000 UK public and workplace charge points monitored over 2023–2025. Peak simultaneous utilisation rarely exceeds 40% even at busy locations, largely because charging sessions are spread across arrival times and many vehicles charge at reduced power once they reach 80% state of charge.

Arc Fault Detection Devices (AFDDs) to BS EN 62606 have been a contentious topic since the 18th Edition introduced them as a recommendation in Regulation 421.1.7. Amendment 4 makes AFDDs mandatory for specific installation types, while retaining them as recommended for others.

Mandatory AFDD installation types (from 16 October 2026):

Recommended (not mandatory) AFDD installations:

• Domestic dwellings (single occupancy) — recommended for bedrooms and living rooms
• Offices and retail premises — recommended for final circuits in areas with combustible furnishings
• Industrial premises — recommended for circuits supplying portable equipment

The distinction between mandatory and recommended reflects a risk-based approach. The mandatory categories are all situations where either the occupants are particularly vulnerable (sleeping, reduced mobility, children) or the building construction poses an elevated fire propagation risk (timber frame with concealed voids).

Amendment 4 does not change the fundamental cable sizing methodology in Appendix 4 or the current rating tables. The cable sizing impact comes indirectly through:

• Reduced EV diversity factors: Lower diversity means lower maximum demand for multi-charger installations, which reduces the required submain and distribution cable sizes. A 20-charger car park designed to A3 with 0.70 diversity might have been sized for 14 × 7.4 kW = 103.6 kW. Under A4 with 0.50 diversity, the design load drops to 10 × 7.4 kW = 74 kW — a 29% reduction in submain capacity.
• Solar PV DC cable sizing: The new Chapter 72 provisions provide specific methodology for DC cable sizing that accounts for solar radiation heating. For roof-mounted DC cables, an additional temperature adder of 15–25 K above ambient is specified depending on the cable's proximity to the roof surface. This may require larger DC cable sizes than previous practice.
• AFDD voltage drop: AFDDs introduce a small additional voltage drop (typically 0.2–0.5 V at rated current). For long circuits already near the voltage drop limit, this may need to be accounted for. Regulation 525.1.1 (Note) now explicitly requires AFDD voltage drop to be included in the circuit voltage drop calculation.

Voltage drop calculation including AFDD:
  V_drop_total = V_drop_cable + V_drop_AFDD + V_drop_RCD + V_drop_MCB

  Where V_drop_AFDD is typically:
    16 A circuit:  ~0.2 V
    32 A circuit:  ~0.4 V
    63 A circuit:  ~0.8 V

  BS 7671 Table 4Ab limits (unchanged in A4):
    Lighting:     3% of 230 V = 6.9 V
    Other uses:   5% of 230 V = 11.5 V

Beyond the three headline areas, Amendment 4 includes several other updates:

• Regulation 411.3.3 (Earth fault loop impedance): Updated maximum Zs values for some MCB types to reflect changes in manufacturer specifications. Type C 32 A MCBs in a TN-S system: Zs reduced from 0.72 Ω to 0.68 Ω.
• Section 710 (Medical locations): Alignment with BS EN 60364-7-710:2012+A1:2024. Updated requirements for Group 2 medical locations, including more specific IT system monitoring requirements.
• Part 6 (Inspection and testing): New Regulation 643.7 requiring insulation resistance testing of PV DC circuits to be performed under specific conditions (minimum irradiance level or using a DC source). This addresses the long-standing issue of PV insulation resistance testing giving misleading results at low irradiance.
• Regulation 421.1.201: New requirement for consideration of fire propagation characteristics of cables in escape routes. Cables in escape routes should be either low-smoke zero-halogen (LSZH) to BS EN 60332-3 or enclosed in fire-resistant trunking.
• Chapter 82 (Energy efficiency): Updated guidance on power factor correction for EV charging installations and large PV systems. New informative annex on demand-side response integration.

ECalPro is committed to supporting Amendment 4 from its effective date. Our implementation timeline:

Specific features being updated for Amendment 4:

• Cable sizing calculator: Updated EV diversity factors in Table 72A, AFDD voltage drop inclusion, solar PV DC cable temperature adder.
• Voltage drop calculator: AFDD voltage drop now included as a device drop input field when BS 7671 is selected.
• Maximum demand calculator: Updated EV charging diversity table with A4 values.
• Report output: Updated standard citation to "BS 7671:2018+A4:2026" with specific regulation references for all new provisions.