BS 7671 Amendment 4: 5 Traps Contractors Will Fall Into

Amendment 4 is expected to take effect in Q3 2026 with a 12-month transition period. During the transition, both Amendment 3 and Amendment 4 are accepted. After the transition, only Amendment 4 applies. Contractors who do not update their practices during the transition period will face non-compliance issues from day one of mandatory enforcement.

Power over Ethernet (PoE) cables carry both data and DC power. The power dissipated in the cable conductors generates heat that reduces the cable’s current-carrying capacity for the data signals and can affect the cable’s rated operating temperature.

Amendment 3 acknowledged PoE heating but left derating largely to the installer’s judgement. Amendment 4 makes PoE derating mandatory through a new regulation aligned with IEC 60364-5-52:2009+A2 Annex E and BS EN 50174-2.

What changes:

• Data cables carrying PoE (IEEE 802.3bt Type 3 and Type 4, up to 90 W per port) must be derated when bundled in cable trays, baskets, or conduits.
• The derating applies to the data cable’s performance category, not to the power delivery system. A bundle of 24 PoE cables in a tray generates significant heat that can push cable core temperatures above the 60°C limit for Category 6A.
• Specific derating tables are provided based on bundle size, cable type (U/UTP, F/UTP, S/FTP), and PoE power level.

The trap: Data cabling contractors who have always been outside BS 7671’s scope now have regulatory obligations under the wiring regulations. An ICT installation with 48 PoE cables bundled in a single tray may require either derating (using fewer cables per bundle), upsizing to a higher category cable, or installing cables in a lower fill ratio than current practice allows.

The ECalPro Cable Sizing Calculator now includes PoE derating factors aligned with Amendment 4 requirements. Use it alongside the Conduit Fill Calculator to verify tray fill ratios.

BS 7671 Section 710 (Medical Locations) receives significant updates in Amendment 4, expanding beyond hospitals to cover a wider range of healthcare settings.

Key changes:

• Group 1 and Group 2 classifications expanded: Dental surgeries, veterinary clinics with general anaesthesia facilities, and cosmetic surgery clinics with sedation capabilities are now explicitly classified. Previously, these were in a grey area that many contractors interpreted as standard commercial installations.
• IT earthing system (IT-M) requirements tightened: The insulation monitoring device (IMD) requirements for Group 2 medical locations now specify faster alarm response times and lower alarm threshold settings, aligned with IEC 60364-7-710 (2021 revision).
• Supplementary equipotential bonding: The maximum impedance between simultaneously accessible exposed-conductive-parts and extraneous-conductive-parts is reduced from the current BS 7671 limit, requiring more attention to bonding conductor sizing and connection quality.
• Emergency power supply requirements: Changeover time classifications (0 s, 0.5 s, 15 s) now include specific requirements for battery-backed LED emergency lighting in medical locations, replacing the general Section 560 provisions.

The trap: A contractor fitting out a dental surgery or aesthetic clinic who treats it as a standard commercial installation will miss the Section 710 requirements entirely. The building control inspection at certification will identify the missing IT system, supplementary bonding, and emergency power provisions — requiring significant rework.

Battery Energy Storage Systems (BESS) have moved from niche technology to mainstream domestic and commercial installations. Amendment 4 introduces specific requirements under the revised Section 722 that go beyond the existing PV and generator provisions.

Key changes:

• Dual isolation: Both AC and DC disconnection means are required at the BESS, accessible without exposure to live parts. The DC isolator must be rated for the maximum battery voltage under all conditions, including fault and charging scenarios where voltage exceeds nominal.
• Overcurrent protection for DC circuits: String protection between battery modules and the inverter requires fuses or breakers rated for DC duty. Standard AC MCBs are not suitable — they cannot extinguish a DC arc. The regulations now explicitly prohibit the use of AC-only protective devices on DC battery circuits.
• Labelling at three locations: Consumer unit, meter position, and BESS enclosure must all carry warning labels specifying the presence of a BESS, the maximum voltage, and the isolation procedure. This is driven by firefighter safety concerns.
• Earthing provisions: Hybrid AC/DC installations require specific earthing arrangements to prevent transferred potentials between the AC supply earth (TN-C-S or TN-S) and the DC battery system earth.

The trap: Electricians experienced with PV installations who treat a battery retrofit as “just another inverter circuit” will miss the DC-specific protection requirements, the dual isolation requirement, and the three-location labelling. The BESS-specific requirements in Section 722 are separate from and additional to the PV requirements in Section 712.

Amendment 4 updates the provisions for functional earthing of ICT (Information and Communication Technology) equipment, aligning with BS EN 50310 and addressing the increasing prevalence of high-frequency interference from switch-mode power supplies, LED drivers, and power electronics.

Key changes:

• Separation of functional earth and protective earth: For installations with significant ICT loads (server rooms, network equipment rooms, broadcast facilities), Amendment 4 provides clearer guidance on when a separate functional earth conductor is required versus when the combined protective/functional earth approach is acceptable.
• High-frequency bonding: New provisions address the bonding mesh requirements for ICT rooms, where high-frequency noise on earthing conductors can disrupt sensitive equipment. The mesh bonding approach per BS EN 50310, Section 8 is now referenced directly from BS 7671.
• EMC consideration: The existing Regulation 332 on EMC is expanded. Installations with variable speed drives, large UPS systems, or extensive LED lighting must now consider the common-mode noise current path through the earthing system. This may require additional filtering or isolation at the ICT equipment.

The trap: Contractors who install server rooms and network closets using standard commercial earthing practices will not meet the new functional earthing requirements. A standard TN-C-S earthing arrangement without additional measures can introduce unacceptable noise levels on the ICT earth, causing equipment malfunctions that are difficult to diagnose after the fact.

Amendment 3 introduced Regulation 443.4, requiring surge protective devices (SPDs) in most new installations where the consequences of an overvoltage could affect human life, public services, or commercial/industrial activity. Amendment 4 expands the scope further.

Key changes:

• SPD requirements extended to alterations: Previously, SPDs were required for new installations. Amendment 4 extends the requirement to significant alterations and additions to existing installations, specifically when a new consumer unit is installed or when the existing installation is modified to add circuits serving sensitive equipment.
• Coordination with PV and BESS: SPDs on both the AC and DC sides of PV and BESS installations are now explicitly required, not just recommended. The DC-side SPD must be rated for the maximum open-circuit voltage of the PV array or battery string.
• Type 1 SPD requirements clarified: Installations with external lightning protection systems (LPS) require Type 1 SPDs. Amendment 4 clarifies the test method distinction between Type 1 (10/350 μs) and Type 2 (8/20 μs) SPDs, addressing confusion in the market about products labelled as “Type 1+2 combined.”
• SPD disconnection device requirements: The backup protection for the SPD (typically a dedicated MCB or fuse) must be coordinated with the SPD’s maximum backup fuse rating. Amendment 4 makes this coordination requirement explicit and provides reference tables.

The trap: A contractor replacing a consumer unit in an existing domestic installation — previously a straightforward job — now triggers the SPD requirement. The cost of adding an SPD module (typically £50–100 for a Type 2 SPD plus a dedicated MCB way) must be included in the quotation. Quoting without it means either absorbing the cost or facing a non-compliance issue at testing.

Additionally, PV and BESS installers who have been fitting AC-side SPDs only will need to add DC-side SPDs to their standard specification.

1. Review the IET Guidance Notes. Updated Guidance Notes covering Amendment 4 changes will be published alongside the amendment. Guidance Note 3 (Inspection and Testing) and Guidance Note 7 (Special Locations) are particularly relevant.
2. Update standard specifications and quotation templates. Add SPD provisions to all consumer unit replacement quotes. Add BESS-specific items to PV/battery quotes. Include PoE derating notes in data cabling specifications.
3. Check medical location classifications. If you work in dental, veterinary, or cosmetic surgery fitouts, verify whether Section 710 now applies to your typical projects.
4. Attend manufacturer CPD sessions. SPD manufacturers, BESS inverter suppliers, and test equipment manufacturers will all offer training on the new requirements.
5. Use calculation tools that are updated for Amendment 4. The ECalPro Cable Sizing Calculator includes PoE derating and BESS DC circuit provisions aligned with Amendment 4.

Standards referenced: BS 7671:2018+A4 (Regulations 332, 421.1.7, 443.4, Sections 710, 712, 722), IEC 60364-5-52:2009+A2, IEC 60364-7-710, BS EN 50174-2, BS EN 50310.