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BS 7671 Thermal Insulation Derating Factors for Cables Passing Through Fire-Stopped Penetrations

Kai Tai Fok

I'm interested in hearing how others apply BS 7671 thermal insulation derating factors in practice when cables only pass through a relatively short section of thermal insulation or fire stopping.

BS 7671 Appendix 4 provides derating factors for cables surrounded by thermal insulation, and in some cases the resulting reduction in current-carrying capacity can be significant. However, many building installations only have cables passing through localised fire-stopped penetrations, fire collars, fire pillows, or fire batt systems, typically over a short distance compared with the total cable run.

My question is:

Do you apply the thermal insulation derating factor to the entire cable length and its current-carrying capacity, or do you consider the effect to be more localised?

From a heat transfer perspective, the highest cable temperature will occur within the insulated/fire-stopped section, but heat can also be dissipated longitudinally along the conductor and sheath into adjacent cooler sections of cable. This suggests the thermal behaviour may not be equivalent to a cable being continuously surrounded by thermal insulation over its entire length.

Has anyone come across:

  • Guidance from BS 7671, IEC 60364, ERA reports, manufacturers, or DNOs on this specific situation?
  • Any accepted industry practice for short fire-stopped penetrations?
  • Thermal modelling, testing, or engineering assessments that justify a less conservative approach than applying the full derating factor to the complete circuit?

I'd be interested in both the theoretical basis and how designers, contractors, and reviewers typically approach this in practice.

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  • In my opinion, a cable passing through a short fire-stopped penetration should not automatically be considered thermally equivalent to a cable continuously surrounded by insulation.
    From a heat transfer perspective, the fire-stopped section creates a local thermal bottleneck and will likely be the hottest point on the circuit. However, conductor heat is not confined to that point alone. Longitudinal heat flow through the conductor, insulation and sheath allows dissipation into the adjacent cooler cable sections. Therefore, a cable traversing a 100 mm fire batt will not exhibit the same thermal profile as a cable embedded in insulation for several metres.

    In practice, I would assess:

    • Length of cable within the fire-stopping material.
    • Circuit loading relative to cable capacity.
    • Number of grouped cables within the penetration.
    • Thermal properties of the fire-stopping product.
    • Available design margin.

    For typical fire batt, fire pillow or sleeve penetrations of short length, I would generally regard the effect as localised rather than circuit-governing, provided the circuit is not operating close to its thermal limits. Where the enclosed length approaches or exceeds the 0.5 m threshold, or where the circuit is heavily loaded, I would be more inclined to apply the Appendix 4 derating factors or undertake a detailed assessment.

    I would be interested to know whether anyone has access to IEC 60287 calculations, ERA test reports, manufacturer data or fire-stop testing that quantifies conductor temperature rise through short fire-stopped penetrations, as this would help bridge the gap between the simplified assumptions of BS 7671 and actual installation conditions.

Reply

  • In my opinion, a cable passing through a short fire-stopped penetration should not automatically be considered thermally equivalent to a cable continuously surrounded by insulation.
    From a heat transfer perspective, the fire-stopped section creates a local thermal bottleneck and will likely be the hottest point on the circuit. However, conductor heat is not confined to that point alone. Longitudinal heat flow through the conductor, insulation and sheath allows dissipation into the adjacent cooler cable sections. Therefore, a cable traversing a 100 mm fire batt will not exhibit the same thermal profile as a cable embedded in insulation for several metres.

    In practice, I would assess:

    • Length of cable within the fire-stopping material.
    • Circuit loading relative to cable capacity.
    • Number of grouped cables within the penetration.
    • Thermal properties of the fire-stopping product.
    • Available design margin.

    For typical fire batt, fire pillow or sleeve penetrations of short length, I would generally regard the effect as localised rather than circuit-governing, provided the circuit is not operating close to its thermal limits. Where the enclosed length approaches or exceeds the 0.5 m threshold, or where the circuit is heavily loaded, I would be more inclined to apply the Appendix 4 derating factors or undertake a detailed assessment.

    I would be interested to know whether anyone has access to IEC 60287 calculations, ERA test reports, manufacturer data or fire-stop testing that quantifies conductor temperature rise through short fire-stopped penetrations, as this would help bridge the gap between the simplified assumptions of BS 7671 and actual installation conditions.

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