Ambient 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.
Using Ambient temperature 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 Ambient temperature factor beyond the definition?
- Explain what Ambient temperature 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 Ambient temperature 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.
See this section in the full curriculum
This section is on the Plus path. Lesson availability depends on content review and your learning path.
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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 Ambient temperature 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 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 entryThermal 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 reference method already describes the insulation, don't apply Ci on top.
Read the Thermal insulation factor entryTabulated current — It
The current a cable can carry under the reference conditions in BS 7671's Appendix 4 tables, before any adjustment. To choose a cable, divide the device's rating by the correction factors that apply, then find a cable whose tabulated figure is at least that. Multiply the chosen cable's It by installation factors such as Ca, Cg and Ci, and you've got its capacity as installed, Iz.
Read the Tabulated current entryA definition is a start. Using it in a real question is where it sticks.