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Courses/Cable Sizing & Installation/Current-Carrying Capacity

lesson · 8min · Lesson 3 of 28

Reference method B: clipped direct

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 · 8min
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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

Reference method B: clipped direct

Cable Sizing · Lesson 3 · Intermediate

IntermediateReview: professional review pending

Purpose

Correct the legacy 'Method B: clipped direct' label and select data from the actual mounting arrangement.

Before you beginReference-method classification

Learning objectives

  • Recognise the title mismatch
  • Describe clipped-direct heat transfer
  • Use current tables
  • Handle route transitions

In commonly used current BS 7671 Appendix 4 mappings, clipped direct is represented by Reference Method C, while Method B describes different conduit/trunking arrangements. The retained title must never be used as design evidence.

CURRENT-CARRYING CAPACITY WORKFLOWCURRENT-CARRYING CAPACITY WORKFLOWIbloadIndeviceIzIt × applicable factorsphysical method · exact table · declared conditions · all design gates

Core theory

A cable fixed directly to a surface can usually lose heat more readily than the same cable surrounded by insulation, but cable spacing, surface, bundling, covering and construction remain material.

Use the exact current table's installation description and notes. Standards and course labels can change; physical evidence and cited source must be recorded.

Clips, supports, bending radius, external influence and damage protection are separate installation requirements; a thermal rating does not prove mechanical adequacy.

Terms, symbols, and units
TermMeaningSymbolUnit
Clipped directCable supported directly on a surface in the arrangement specified by the applicable tableNot applicableNot applicable
Method CCommon current-table designation associated with clipped-direct arrangements; verify exact editionNot applicableNot applicable
Route transitionChange in installation condition along a cableNot applicableNot applicable
Worked exampleA schedule says 'Method B / clipped direct.' What should the reviewer do?

Assumptions: The project uses current BS 7671 data.

  1. Flag: The letter and physical description appear inconsistent.
  2. Inspect: Confirm actual mounting, spacing, covering and cable type.
  3. Recalculate: Use the table/column matching that verified arrangement and update the record.

Treat the schedule as an inconsistency, not permission to choose either rating.

Reasonableness check: A method label cannot override contradictory physical evidence.

Common mistakes
  • Perpetuating the title mismatch
  • Assuming open view means free air
  • Ignoring bundled sections

Where this appears in practice

Design review should compare schedules, drawings, photographs and as-installed routes.

SafetyDo not remove clips or covers on energised or unsupported circuits; isolate and manage mechanical damage risk.
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

Is clipped direct safely selected from the letter B in this lesson title?

No. Use the exact physical description and current source; commonly it maps to Method C.

Answer: No. Use the exact physical description and current source; commonly it maps to Method C.

Practical exercise

Audit a fictional schedule for description-to-method mismatches and write corrective RFIs.

Summary

  • The legacy title is misleading
  • Clipped direct commonly maps to C
  • Verify edition and geometry

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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