BS 7671 Amendment 4: Power over Ethernet Cable Derating

PoE Thermal Effects

Power over Ethernet delivers DC power alongside data through structured cabling. While individual cable power levels are modest (up to 90 W per cable under IEEE 802.3bt Type 4), the cumulative heating effect in large cable bundles can be significant. When dozens of PoE cables are bundled together in trunking or cable trays, the total heat dissipation can raise conductor temperatures above their rated limits.

Cable Bundle Heating

Amendment 4 introduces derating factors for bundled PoE cables, acknowledging that heat dissipation is restricted when cables are tightly packed. The derating depends on the number of cables in the bundle, the power level per cable, the installation method, and the ambient temperature. Cables in enclosed trunking require more aggressive derating than those on open cable trays.

Derating Considerations

Designers must consider whether PoE cables can maintain their data transmission performance at elevated temperatures. Category 6A cables, commonly used for PoE, have an operating temperature limit that may be reached in large bundles. The amendment provides guidance on maximum bundle sizes, minimum cable spacing, and the relationship between power delivery and cable performance grades.

IEEE 802.3bt and BS 7671

IEEE 802.3bt (4-pair PoE) defines four Types: Type 1 (15.4 W), Type 2 (30 W), Type 3 (60 W), and Type 4 (90 W). Amendment 4 aligns BS 7671 requirements with these power levels, ensuring that UK wiring regulations keep pace with the PoE standards used by network equipment manufacturers. The combination of IEEE power classifications and BS 7671 derating factors gives designers a complete framework.

Practical Impact

For most small PoE installations (fewer than 12 cables), the derating impact is minimal. The requirements become critical in enterprise environments — office buildings with hundreds of PoE access points, hospitals with PoE-powered medical devices, and smart buildings with extensive IoT sensor networks. In these cases, cable tray sizing, ventilation, and routing must account for PoE thermal loads.