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Courses/Cable Sizing & Installation/Derating & Correction Factors

lesson · 10min · Lesson 7 of 28

Grouping correction factor (Cg)

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 · 10min
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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

Grouping correction factor (Cg)

Cable Sizing · Lesson 7 · Intermediate

IntermediateReview: professional review pending

Purpose

Apply grouping correction from the actual number, loading and arrangement of circuits or cables.

Before you beginHeat transfer · Circuit diversity

Learning objectives

  • Count the correct grouping units
  • Describe contact and spacing
  • Use permitted load assumptions
  • Avoid double correction

Adjacent loaded cables warm one another. Cg depends on what is grouped, how it is arranged, and which circuits are materially loaded, not simply the number visible.

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

Core theory

Use the applicable table definition: it may count circuits, multi-core cables or loaded conductors. Record whether cables touch, are spaced, lie in layers, share containment or are buried together.

Diversity, intermittent duty or lightly loaded conductors may be recognised only through the exact standard/design method and defensible load profile. Do not omit a circuit because it is usually off.

Some tabulated capacities or special methods already embody an arrangement. Confirm the base before multiplying Cg so grouping is neither omitted nor counted twice.

Terms, symbols, and units
TermMeaningSymbolUnit
Grouping factorCorrection for mutual heatingCgNot applicable
Loaded circuitCircuit contributing material thermal load under the applicable design caseNot applicableNot applicable
TouchingCable surfaces in contact in the declared arrangementNot applicableNot applicable
Worked exampleA supplied table gives It = 47 A and Cg = 0.80 for the declared group.

Assumptions: The count and arrangement match; No other factor applies.

  1. Correct: Iz = 47 × 0.80.
  2. Calculate: Iz = 37.6 A.
  3. Compare: Compare 37.6 A with the applicable Ib/In coordination rules.

Illustrative grouped capacity is 37.6 A.

Reasonableness check: Mutual heating reduces capacity relative to the reference arrangement.

Common mistakes
  • Counting conductors when the table counts circuits
  • Assuming diversity without evidence
  • Applying grouping twice

Where this appears in practice

Tray, trunking, risers and multi-circuit trenches need an as-installed grouping schedule.

SafetyOpening loaded containment can expose live parts and hot cables. Isolate and verify before intrusive 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

Does 'six cables' uniquely select Cg?

No. Cable/circuit definition, loading, spacing and arrangement are also required.

Answer: No. Cable/circuit definition, loading, spacing and arrangement are also required.

Practical exercise

From a supplied tray drawing, identify grouping units, formation, credible load case and missing evidence.

Summary

  • Count by the table definition
  • Arrangement matters
  • Load assumptions require evidence

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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