Thermal insulation factor
Ci
A correction factor for a cable that meets thermal insulation, which holds its heat in. A cable surrounded by insulation for more than 0.5 m is taken at half its clipped-direct rating, and a shorter stretch gets a smaller cut. If the already describes the insulation, don't apply Ci on top.
Using Thermal insulation factor in context
Reviewed
A distinction worth remembering
A useful revision comparison is a short cable in open air against the same load supplied over a long route through insulation. Installation conditions, voltage drop and fault protection can change the selection. Learn the meaning of each input before treating a calculation as a design decision.
Questions to take further
- How can I revise Thermal insulation factor beyond the definition?
- Explain what Thermal insulation factor describes, compare it with a related term and use it in a topic question. State any assumptions and what evidence you would need before applying the idea to real work.
Sources
- Open University: relationships between electrical quantities — The relationship between voltage, current and resistance and the limits of an ohmic model.
- IET: current BS 7671 edition and updates — BS 7671:2018+A4:2026 and the main subject areas updated by Amendment 4.
- IET: earthing and bonding FAQs — Protective bonding, earthing arrangements and loop impedance.
Where Thermal insulation factor fits in the learning path
The Design Calculations section covers these units:
- Sizing a Cable, Every Factor
- Volt Drop and Zs in Depth
- Fault Current, the Adiabatic Check and Device Choice
- Submains, Three-Phase Boards and Demand
Curriculum mapping: C&G 2365 Unit 305.
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- Cable SizingInstallation
2. Try recalling, then explore practice
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- Cable Selection — quiz previewPlus · Installation
3. Return to the meaning
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Recall a related term
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Factors used with cable-rating data to account for conditions such as temperature, grouping and thermal insulation. The appropriate table, installation method and calculation must be used together; not every factor is necessarily a reduction. Cable selection also needs coordination with the protective device and checks such as voltage drop and fault protection, rather than a comparison with load current alone.
Correction factors
Read the Correction factors entryCompare related terms
Open a meaning to compare its role with Thermal insulation factor, or follow its full entry.
Correction factors
Factors used with cable-rating data to account for conditions such as temperature, grouping and thermal insulation. The appropriate table, installation method and calculation must be used together; not every factor is necessarily a reduction. Cable selection also needs coordination with the protective device and checks such as voltage drop and fault protection, rather than a comparison with load current alone.
Read the Correction factors entryAmbient temperature factor — Ca
A correction factor for the air temperature around a cable. The tables assume 30 °C in air, so hotter air gives a factor below 1 and leaves the cable less room for its own heat, while cooler air gives one above 1. The figure comes from BS 7671's Appendix 4 and depends on the cable's insulation as well as the temperature.
Read the Ambient temperature factor entryGrouping factor — Cg
A correction factor for cables run together. Loaded cables warm each other up, so each one can carry less than it could on its own. The figure comes from BS 7671's Appendix 4 and depends on how many circuits are grouped and how they're laid out.
Read the Grouping factor entryReference method
A described way of installing a cable, such as clipped direct or in conduit on a wall, that the current ratings are built on. How easily heat escapes changes what a cable can carry, so the cable's construction picks the table and the method picks the column. If the method changes along the route, size for the stretch that sheds heat worst.
Read the Reference method entryA definition is a start. Using it in a real question is where it sticks.