BS 7671: the IET Wiring Regulations

What BS 7671 is

BS 7671, Requirements for Electrical Installations, is the UK national standard for low-voltage electrical installations in buildings and outdoor locations, up to 1000 V AC and 1500 V DC. It is published jointly by the Institution of Engineering and Technology and the British Standards Institution and is universally known as the IET Wiring Regulations; editions are numbered, and the current one is the 18th. It covers design, selection and erection of equipment, inspection and testing, and the model certificates used to record them.

The book does not stand alone. It is the UK’s implementation of the international IEC 60364 series, adopted in Europe through CENELEC, and its structure and numbering follow that series closely. It is non-statutory, but much of UK law points to it, as described under who enforces it.

The 18th Edition and its amendments

The 2018 edition, Amendment 2 and Amendment 4 each came with a transition of about six months in which the old and new versions could both be used, though not mixed on one job. Amendment 1 could be applied as soon as it was published, and Amendment 3 was a free download to read alongside the Amendment 2 book.

Amendment 2:2022

Available from March 28, 2022, with the previous version usable until September 27, 2022, A2 was a major amendment. The points that affect design:

• Arc fault detection devices (421.1.7). Required for single-phase AC final circuits supplying socket-outlets up to 32 A in higher-risk residential buildings (over 18 m or more than six storeys), houses in multiple occupation, purpose-built student accommodation and care homes; recommended for the same circuits everywhere else.
• Surge protection (443.4.1). Required where an overvoltage could cause serious injury or loss of life, or significant financial or data loss. In all other cases it must be provided unless the owner declares that the possible loss is tolerable.
• 30 mA RCDs for socket-outlets (411.3.3). The rule for socket-outlets up to 32 A was restructured. Where ordinary people, children or vulnerable users are likely to use them there is no exception; elsewhere protection may be omitted only on a documented risk assessment involving a skilled person. Mobile equipment up to 32 A used outdoors is also covered.
• RCD types (531.3.3). Type AC devices may only serve fixed equipment known to have no DC component in its load current.
• Prosumer installations. A new Part 8 with Chapter 82 covers installations with their own generation and storage, such as solar PV and batteries.
• Testing. RCD testing was simplified to a single test at the rated residual current, with a maximum of 300 ms for general non-delay devices, and the old five-times test was dropped.

A corrigendum to A2 followed in May 2023, and among other things it removed safety-service failure from the list of consequences that make surge protection compulsory.

Amendment 3:2024

Published in July 2024, A3 was a narrow, fast-tracked amendment issued as a free PDF to use with the A2 book. Its purpose was installations with more than one source of supply, such as the grid, a solar inverter, a battery or a vehicle that can export. New Regulation 530.3.201 requires the designer to take account of whether each protective device is unidirectional, designed for power flowing from marked supply terminals to load terminals, or bidirectional, safe with power flowing either way. A compact RCBO with electronics may be unidirectional; a device feeding a battery or an inverter circuit generally needs to be bidirectional.

Amendment 4:2026

A4 was published by the IET and BSI on April 15, 2026 as a consolidated book that incorporates all previous amendments, and it becomes the only current version on October 15, 2026. The publishers’ own summary names four headline additions:

• Chapter 57, stationary secondary batteries used for energy storage and to supply installations, including where batteries may be located in dwellings, ventilation, identification and notices, aligned with PAS 63100.
• Section 545, functional earthing and functional equipotential bonding for information and communication technology equipment.
• Section 716, Power over Ethernet: extra-low-voltage DC power carried on data cabling to low-power devices.
• Section 710, medical locations, substantially rewritten, with a new schedule for recording the resistance of supplementary bonding conductors.

Trade coverage of the published book also lists clearer rules on the premature collapse of wiring systems in a fire and on cables in protected escape routes, guidance on buried cables, RCD type selection where devices are in series, and load curtailment. The draft for public comment, which closed in November 2024, had also proposed moving the energy-efficiency guidance of Appendix 17 into a new Chapter 81 and changing the condition-report rules of Chapter 65; we have not confirmed how those landed in the printed book, so check your copy.

What the amendments mean for design and calculations

Most of the numbers you calculate with did not change between amendments. The ones that matter, and how they are derived:

• Earth fault loop impedance. Maximum Zs values for circuit-breakers come from Zs = Cmin × U0 / Ia, with Cmin = 0.95 and U0 = 230 V. For 0.4 s disconnection: B16 needs Ia = 5 × In = 80 A, so Zs ≤ 2.73 Ω, B32 needs Ia = 5 × In = 160 A, so Zs ≤ 1.37 Ω, C32 needs Ia = 10 × In = 320 A, so Zs ≤ 0.68 Ω and D32 needs Ia = 20 × In = 640 A, so Zs ≤ 0.34 Ω. Measured values are compared against about 80% of these, because conductors are cooler during testing.
• RCD-protected TT circuits. RA × IΔn ≤ 50 V, so a 30 mA RCD allows a loop of up to 1,667 Ω in principle.
• Voltage drop. Regulation 525.202 points to the limits in Appendix 4: 3% for lighting and 5% for other uses on a public supply, 6.9 V and 11.5 V at 230 V. See voltage drop.
• Protective conductors. The adiabatic check S ≥ √(I²t) / k uses k values that follow from conductor and insulation temperatures: 115 for copper in 70 °C thermoplastic cable.
• Prospective fault current. For a single-phase supply up to 100 A, the usual design assumption at the origin, unless the distributor states otherwise, is 16 kA.
• Device choice. AFDDs, surge protection, RCD type and bidirectional devices are selection rules rather than calculations, but they change what goes in the consumer unit.

How the document is organised

The regulations are numbered by part, chapter, section and regulation, following IEC 60364:

The appendices hold most of the data. Appendix 3 has the time/current characteristics of fuses, circuit-breakers and RCDs. Appendix 4 is the one designers use most: current-carrying capacities for each cable type and installation method, the correction factors for ambient temperature, grouping and thermal insulation, the voltage drop per ampere per metre, and, in section 6.4, the voltage-drop limits. Appendix 6 contains the model certificates and condition reports. You will still see the voltage-drop limits attributed to Appendix 12 in some guides; NICEIC and NAPIT both place them in Appendix 4, section 6.4 of the current edition, reached through Regulation 525.202. We have not been able to check the current title and content of Appendix 12 against the printed A4 book.

How BS 7671 relates to IEC 60364

The IEC writes the international series, IEC 60364. CENELEC adopts its parts for Europe as Harmonization Documents, HD 60364. An HD must be announced in each member country and conflicting national standards withdrawn, but unlike a European Norm it does not have to be adopted word for word: a country may keep its own standard if it is technically equivalent. BS 7671 is the UK’s, which is why its Part 4 chapters and Part 7 sections carry the same numbers as IEC 60364-4-41 or IEC 60364-7-722, while adding UK material such as the rules for ring final circuits with BS 1363 socket-outlets and for PME supplies. The IET describes BS 7671 as based on European standards, which are in turn generally based on international ones.

That shared ancestry is why IEC methods carry over: installation reference methods, TN, TT and IT earthing systems, automatic disconnection and the adiabatic equation work the same way. The UK tables, however, are BS 7671’s own and can differ from the IEC values in detail. See IEC 60364 explained.

Guidance publications

The IET publishes a family of guides that explain how to apply the regulations, and it reports that all of them have been updated for A4:2026:

• On-Site Guide (9th edition), a simplified route with ready-made tables for straightforward installations.
• Guidance Notes 1 to 8, covering selection and erection, isolation and switching, inspection and testing (GN3), fire, shock, overcurrent, special locations and earthing and bonding.
• Electrician’s Guide to the Building Regulations (7th edition) and the Electrical Installation Design Guide (6th edition).

Codes of practice sit alongside these, including the IET Code of Practice for Electric Vehicle Charging Equipment Installation, now in its 5th edition. The guides are not part of BS 7671, but they are what most installers reach for on site.

Who enforces it

BS 7671 has no inspectors of its own. It gets its force from the laws that point to it:

• Building Regulations Part P (England). Requirement P1 covers electrical safety in dwellings. Approved Document P, last revised in 2013, says installations should be designed and installed in accordance with BS 7671; the edition it names is the one current when it was written. Installing a new circuit, replacing a consumer unit, or adding to or altering a circuit in a special location such as the space around a bath or shower is notifiable: it must be certified through a registered competent person scheme, a registered third-party certifier or building control.
• Wales publishes its own Approved Document P under the Welsh building regulations.
• Workplaces. The HSE describes BS 7671 as a widely recognised code of practice and says compliance with it is likely to achieve compliance with the relevant parts of the Electricity at Work Regulations 1989. An installation built to an earlier edition does not automatically fail those regulations.
• Rented homes in England. Under the Electrical Safety Standards in the Private Rented Sector (England) Regulations 2020, landlords must have the electrics checked at least every five years by a qualified person.
• The supply. The Electricity Safety, Quality and Continuity Regulations 2002 govern the distributor’s side, including the conditions for PME earthing.

Which of our calculators support BS 7671

• Cable size: current-carrying capacity from Appendix 4 with the ambient, grouping and thermal-insulation factors, plus the voltage-drop check.
• Earth fault loop impedance: design and measured Zs against the maximum for each circuit-breaker and fuse, from Zs = Cmin × U0 / Ia.
• CPC size: the adiabatic check for circuit protective conductors.
• Prospective fault current: fault current at the origin and along a circuit.
• Voltage drop: the mV/A/m method with the Appendix 4 limits.

These tools were built from data compiled against BS 7671:2018+A2:2022 and A3:2024. Amendment 4’s published summaries mention new guidance on buried cables, including cables in ducts, but no change to the tabulated Zs values. We are checking the A4 book table by table, and any difference will be logged on our corrections page. Values are computed from the published methods and never reproduce the standard’s text.