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Courses/Cable Sizing & Installation/Derating & Correction Factors

lesson · 8min · Lesson 9 of 28

Depth of burial correction (Cs)

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

Depth of burial correction (Cs)

Cable Sizing · Lesson 9 · Intermediate

IntermediateReview: professional review pending

Purpose

Treat buried-cable depth as one part of a coupled ground thermal design.

Before you beginCCC correction factors · Buried services

Learning objectives

  • Define burial inputs
  • Use Cs on its stated basis
  • Recognise soil drying
  • Coordinate ducts and grouping

Buried capacity depends on ground temperature, soil thermal resistivity, depth, grouping, ducts, backfill and drying. Cs cannot be selected from depth alone unless its source explicitly holds the other conditions fixed.

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

Core theory

Confirm the base depth and reference soil/ground conditions behind the chosen cable table. Then apply only corrections whose bases are compatible.

Greater depth often lengthens the heat path, but seasonal ground temperature and moisture can change the outcome; deeper is not an unconditional synonym for worse or safer.

Duct banks, multiple circuits, local heat sources, dry-out and special backfill interact. Complex or heavily loaded routes may require a thermal model and manufacturer data.

Terms, symbols, and units
TermMeaningSymbolUnit
Depth factorCorrection for burial depth relative to a stated baseCsNot applicable
Soil thermal resistivityResistance of soil to heat flowρTK·m/W
Dry-outMoisture loss near a hot cable that increases thermal resistanceNot applicableNot applicable
Worked exampleA route is 0.8 m deep, but the calculation cites only a depth-factor row.

Assumptions: Ground temperature, soil resistivity, duct and grouping are not recorded.

  1. Identify: Depth is known, but several coupled inputs are absent.
  2. Verify basis: Check the base cable table and factor-table assumptions.
  3. Complete: Obtain soil/ground, duct, grouping, backfill and drying evidence before selection.

Depth alone is insufficient to establish buried current capacity.

Reasonableness check: Soil is the cooling medium, so its thermal state is fundamental.

Common mistakes
  • Using Cs without its base depth
  • Ignoring ducts or neighboring circuits
  • Assuming wet soil remains wet

Where this appears in practice

Buried distribution routes need survey data, utility coordination, thermal assumptions and as-built depth records.

SafetyExcavation can strike lethal services. Use current service plans, locating, safe digging and isolation procedures.
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 a burial-depth correction replace soil and grouping checks?

No. It only addresses depth on the source's stated basis.

Answer: No. It only addresses depth on the source's stated basis.

Practical exercise

Create a missing-data register for a fictional duct-bank route before any ampacity is claimed.

Summary

  • Depth is one input
  • Ground assumptions must be explicit
  • Complex routes may need modelling

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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