BS 7671 Amendment 4: What's Changing in the Orange Book (2026)

BS 7671:2018+A4:2026 — the fourth amendment to the 18th Edition of the IET Wiring Regulations — publishes on 15 April 2026. It becomes mandatory for all new electrical installations from 15 October 2026.

This is not a full rewrite. It is a targeted amendment addressing specific gaps that the electrical industry has been dealing with since Amendment 3 published in 2022. The most significant changes affect Power over Ethernet (PoE) cable derating, buried duct installation data, and battery energy storage systems (BESS).

I will update this article on April 15 when the Orange Book publishes and I can reference the exact clause numbers and table values. Until then, this is based on IET announcements, industry consultation documents, and my own experience implementing the existing Amendment 3 provisions.

What We Know So Far

The IET has confirmed these areas of change for Amendment 4:

1. Power over Ethernet (PoE) Cable Derating — New Cp Factor

This is arguably the most consequential change for commercial building design. PoE technology has evolved from 15.4W per port (IEEE 802.3af) to 90W per port (IEEE 802.3bt Type 4). When you bundle dozens of PoE-carrying data cables in a containment system, the cumulative heat generation is no longer negligible.

Amendment 4 introduces a Cp correction factor that must be applied when sizing cables in bundles where PoE power is present. The factor depends on the PoE class (Type 1 through Type 4) and the number of cables in the bundle.

This matters because a cable tray carrying 96 x Cat6A cables each delivering 60W of PoE generates over 5.7 kW of heat. That heat raises the ambient temperature inside the containment, reducing the current-carrying capacity of every cable in the tray. Without the Cp derating, cables operate hotter than their rated temperature and insulation life degrades.

2. New Appendix 4 Data for Buried Duct Installations

The existing Appendix 4 data for cables in buried ducts has known gaps. Amendment 4 adds new installation configurations and thermal data for common buried duct arrangements that were previously not covered, forcing engineers to use conservative estimates or manufacturer-specific data.

The new data is expected to cover additional duct spacings, depths, and soil thermal resistivity combinations. This will give engineers more precise derating factors instead of having to apply worst-case assumptions.

3. Battery Energy Storage Systems (BESS)

With the rapid adoption of domestic and commercial battery storage, Amendment 4 addresses specific installation requirements for lithium-ion and other battery chemistries. This includes isolation, ventilation, fire resistance, and circuit protection requirements.

4. ICT and Functional Earthing

Updated guidance on functional earthing for information and communication technology (ICT) equipment, reflecting the reality that modern buildings have significantly more electronic loads requiring clean earth references.

5. Medical Locations

Alignment with updated medical location requirements, reflecting changes to the IEC 60364-7-710 standard for healthcare facilities.

6. Section 559 — Luminaires and Lighting

Updates to luminaire installation requirements, likely reflecting the near-complete transition to LED lighting technology and the associated thermal characteristics.

The Transition Timeline

There is a 6-month dual-edition period:

• 15 April 2026: Amendment 4 publishes. Engineers may design to either A3 or A4.
• 15 October 2026: Amendment 4 becomes mandatory. All new installations must comply with A4.
• Existing installations designed and certified to Amendment 3 remain compliant. There is no requirement to retrospectively upgrade.

For engineers working on projects that span the transition date, the design edition should be agreed with the client and recorded in the project documentation. In practice, most designers will adopt A4 as soon as the Orange Book is available, because it is always safer to design to the latest edition.

What This Means for Cable Sizing

If you regularly size cables for commercial buildings with PoE networks, Amendment 4 will change your results. The Cp derating factor reduces the effective current-carrying capacity of cables in bundled PoE installations. In some cases, this may mean upsizing cables or splitting containment runs.

ECalPro's cable sizing calculator already includes the PoE Cp factor framework with indicative values based on CENELEC/IEC guidance. On 15 April 2026 — the same day the Orange Book publishes — we will update to the actual BS 7671 A4 values.

Who Supports Amendment 4?

What You Should Do Now

1. Read the Orange Book when it publishes on April 15. Focus on Section 716 (PoE) and Appendix 4 (buried ducts).
2. Review current projects spanning the October 15 compliance date. Decide which edition to design to.
3. Check your cable sizing tools — ensure they support the Cp derating factor for PoE installations.
4. Enrol in our free 5-day email course covering all 12 confirmed Amendment 4 changes — practical guidance, not marketing.

I will update this article on 15 April 2026 with confirmed clause numbers, table values, and worked examples from the published Orange Book.

Disclaimer: Details in this article are based on IET announcements and industry consultation documents as of March 2026. Specific clause numbers and table values will be confirmed upon publication of BS 7671:2018+A4:2026 on 15 April 2026.