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Courses/Cable Sizing & Installation/Derating & Correction Factors

lesson · 9min · Lesson 8 of 28

Thermal insulation factor (Ci)

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

Thermal insulation factor (Ci)

Cable Sizing · Lesson 8 · Intermediate

IntermediateReview: professional review pending

Purpose

Assess cable contact with thermal insulation using the exact prescribed case or engineered method.

Before you beginReference methods · Grouping

Learning objectives

  • Describe insulation contact
  • Recognise route-length effects
  • Select Ci lawfully
  • Reject a universal 0.5 rule

Thermal insulation restricts heat escape. The result depends on cable construction, enclosure, insulation contact and length; there is no universal Ci = 0.5 for every insulated route.

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

Core theory

Record whether insulation surrounds, covers or touches the cable, whether it is in conduit, and the affected length. Match that condition to the current standard's reference method, factor or calculation rule.

A method-specific tabulated value may already represent an insulated wall. Adding Ci automatically could double-derate; conversely, using open-route data can dangerously omit insulation.

Where the real geometry is outside prescribed cases, obtain manufacturer or engineering data rather than extending a rule of thumb.

Terms, symbols, and units
TermMeaningSymbolUnit
Thermal insulation factorApplicable correction for a specified insulation conditionCiNot applicable
SurroundedHeat escape restricted around the cable as defined by the sourceNot applicableNot applicable
Affected lengthLength of route subject to the insulation conditionNot applicablem
Worked exampleA designer writes Ci = 0.5 but records no insulation geometry or length.

Assumptions: No source clause is cited.

  1. Reject: The factor has no demonstrated applicability.
  2. Survey: Record enclosure, contact, surrounding material and affected length.
  3. Rework: Select the exact prescribed case or engineered data and check whether the base method already includes it.

The calculation is incomplete; 0.5 cannot be treated as universal.

Reasonableness check: Different heat paths cannot share one unconditional factor.

Common mistakes
  • Always applying 0.5
  • Ignoring short insulated sections
  • Double-derating an insulated-wall method

Where this appears in practice

Retrofit insulation can invalidate an existing cable's original heat path and must be assessed before covering routes.

SafetyDo not disturb building insulation without checking live services, asbestos and access hazards; isolate affected circuits.
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

When is Ci = 0.5 valid?

Only when the exact governing rule and installation conditions prescribe it. It is not a universal insulation factor.

Answer: Only when the exact governing rule and installation conditions prescribe it. It is not a universal insulation factor.

Practical exercise

Compare three insulation cross-sections and identify whether a method, factor or engineered calculation is needed.

Summary

  • Describe the heat path
  • Check the base method
  • No universal insulation factor

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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