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Educational use: ElectraCore supports learning and preliminary checks. It does not replace a competent electrician or engineer. Verify results, equipment data, and current local regulations before installation or live work.
Courses/Cable Sizing & Installation/Derating & Correction Factors

lesson · 9min · Lesson 6 of 28

Ambient temperature correction (Ca)

Course syllabusCourse overview
01Current-Carrying Capacity
  1. ReadHow CCC tables work: Appendix 4
  2. ReadReference method A: enclosed in conduit
  3. ReadReference method B: clipped direct
  4. ReadReference methods C, E, F, G
  5. exerciseCCC selection problems
02Derating & Correction Factors
  1. ReadAmbient temperature correction (Ca)
  2. ReadGrouping correction factor (Cg)
  3. ReadThermal insulation factor (Ci)
  4. ReadDepth of burial correction (Cs)
  5. exerciseApplying multiple correction factors
03Voltage Drop Calculations
  1. ReadWhy voltage drop matters: Reg 525
  2. ReadmV/A/m tables and how to use them
  3. ReadCalculating voltage drop for single-phase
  4. ReadThree-phase voltage drop
  5. exerciseVoltage drop problems set
04Armoured Cables
  1. ReadSWA construction: layers and materials
  2. ReadUnderground cable installation methods
  3. ReadSWA as protective conductor?
  4. ReadXLPE vs PVC insulation
  5. exerciseSWA sizing exercise
05Mineral Insulated (MICC) Cable
  1. ReadMICC construction and applications
  2. ReadFire performance cables: FP200, LSOH
  3. ReadCable selection for life safety systems
  4. quizFire cable quiz
06Full Cable Sizing Design
  1. ReadEnd-to-end cable sizing: worked design
  2. ReadDocumenting the cable schedule
  3. ReadCommon errors and how to avoid them
  4. quizFinal assessment
Lesson · 9min
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In this lesson

PurposeCore theoryWorked exampleKnowledge checkSources

In this lesson

PurposeCore theoryWorked exampleKnowledge checkSources
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ElectraCore lesson handout

Ambient temperature correction (Ca)

Cable Sizing · Lesson 6 · Intermediate

IntermediateReview: professional review pending

Purpose

Apply ambient-temperature correction from the correct air or ground table and cable temperature rating.

Before you beginCCC tables · Temperature scales

Learning objectives

  • Identify reference ambient
  • Select Ca
  • Separate air and ground bases
  • Avoid unsupported interpolation

A cable's thermal margin depends on the difference between its permitted conductor temperature and its surroundings. Ca is table-, insulation- and environment-specific.

CORRECTION-FACTOR THERMAL MODELCORRECTION-FACTOR THERMAL MODELIbloadIndeviceIzIt × applicable factorsphysical method · exact table · declared conditions · all design gates

Core theory

First establish whether the base table assumes ambient air or ground temperature and its reference value. Then use the correction table for the cable's permitted operating temperature and protective arrangement.

Ambient is the credible local temperature around the cable, including enclosures and heat sources, not necessarily the room thermostat or weather average.

Do not linearly interpolate or extrapolate unless the governing data permits it. Near temperature limits, a different cable, route or engineered calculation may be necessary.

Terms, symbols, and units
TermMeaningSymbolUnit
Ambient temperatureTemperature of the medium surrounding the installed cableθa°C
Temperature factorCorrection for ambient differing from referenceCaNot applicable
Conductor limitMaximum applicable conductor operating temperatureNot applicable°C
Iz = It × Ca (when Ca is the only applicable factor)

The factor and base table must share the same conditions.

A
Worked exampleA supplied air table gives It = 52 A and the applicable supplied Ca = 0.94.

Assumptions: Cable/table temperature bases match; No other factor applies.

  1. Apply: Iz = 52 × 0.94.
  2. Calculate: Iz = 48.88 A.
  3. Retain: Use 48.9 A for the design comparison, without rounding upward.

Illustrative corrected capacity is 48.9 A.

Reasonableness check: A hot-ambient factor below one reduces capacity.

Common mistakes
  • Using an air factor for buried cable
  • Using room temperature despite a hot enclosure
  • Assuming all 90 °C cables permit 90 °C terminals

Where this appears in practice

Boiler rooms, roof spaces, plant rooms and sun-heated containment require credible local temperature evidence.

SafetyHigh ambient and hot surfaces can cause burns and hidden insulation damage; isolate before inspection.
Local code checkUse the current standard, exact cable construction and manufacturer data, declared installation geometry, conductor loading, ambient or ground conditions, and competent design procedure. The numbers in worked examples are supplied fictional table data, not universal ratings. Cable work must be isolated, proved dead, tested and certified as required locally.

Knowledge check

May Ca be selected before the cable temperature rating and base table are known?

No. Those conditions determine the applicable correction table.

Answer: No. Those conditions determine the applicable correction table.

Practical exercise

Choose Ca from three supplied table extracts and justify why the other two are inapplicable.

Summary

  • Reference basis matters
  • Ambient must be local and credible
  • Do not invent interpolation

Sources and review

  • BS 7671:2018+A4:2026 Requirements for Electrical Installations: IET/BSI; Current edition and corrigenda must be confirmed; United Kingdom
  • Guidance Note 6: Protection Against Overcurrent: IET; Current edition must be confirmed; United Kingdom
  • Electricity at Work Regulations 1989: HSR25: Health and Safety Executive; Current online edition; Great Britain

Editorial review date: 2026-08-22. Professional electrical review is pending.

Educational material for learning and preliminary checks. Verify current local requirements and exact equipment instructions. This lesson does not replace competent professional work.

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