Skip to main content
ElectraCore
LearnCalculatorsCircuit DesignerReferences
Saved
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/Full Cable Sizing Design

lesson · 15min · Lesson 25 of 28

End-to-end cable sizing: worked design

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 · 15min
DERATING · TABULATED → EFFECTIVEIndevice→÷ Caambient→÷ Cggrouping→÷ Ciinsul.= Itselect cable with tabulated capacity ≥ It; include all applicable factors
Lesson contents 0%

In this lesson

PurposeCore theoryWorked exampleKnowledge checkSources

In this lesson

PurposeCore theoryWorked exampleKnowledge checkSources
0% read
ElectraCore lesson handout

End-to-end cable sizing: worked design

Cable Sizing · Lesson 25 · Advanced

AdvancedReview: professional review pending

Purpose

Execute a traceable end-to-end cable design in the correct dependency order and iterate until every gate passes.

Before you beginCable sizing workflow · Fire strategy and safety-service basics

Learning objectives

  • Identify the governing function
  • Select evidence on the correct basis
  • Coordinate the complete installed system
  • Record limitations

Cable design is not 'choose amps then check volts.' Load, supply, route, cable, corrections, protection, fault behavior, interfaces and documentation form an iterative evidence chain.

END-TO-END CABLE DESIGN GATESEND-TO-END CABLE DESIGN GATESIbloadIndeviceIzIt × applicable factorsphysical method · exact table · declared conditions · all design gates

Core theory

Define design current/duty, supply/earthing/fault data, route and external influences. Select a candidate from exact tables and apply only compatible factors.

Verify Ib/In/Iz or the applicable overload relationship, voltage drop, ADS/Zs, breaking/conditional capability, adiabatic withstand, terminals, neutral/harmonics, mechanical/fire/environmental suitability and coordination.

Changing size, route, device or material invalidates dependent results. Recalculate, then specify inspection, tests, certification, labels and as-built evidence.

Terms, symbols, and units
TermMeaningSymbolUnit
Design basisControlled assumptions and inputs for the calculationNot applicableNot applicable
Design gateRequirement that must independently passNot applicableNot applicable
IterationRecalculation after a dependent selection changesNot applicableNot applicable
Worked exampleA candidate passes CCC but fails voltage drop.

Assumptions: Only the stated fictional project data apply.

  1. Reject: Thermal success cannot cancel another failed gate.
  2. Revise: Change conductor, route, load distribution or architecture using authoritative data.
  3. Repeat: Re-run thermal, fault, protection, interfaces and documentation checks affected by the change.

The candidate is not acceptable until a revised design passes every applicable gate.

Reasonableness check: The conclusion follows the declared function and evidence rather than a cable nickname.

Common mistakes
  • Selecting by trade name or sheath colour
  • Confusing low smoke with circuit integrity
  • Ignoring supports, joints, routes or terminations

Where this appears in practice

Competent designers coordinate electrical design with the building fire strategy, system standard, product certification and maintainable records.

SafetyFailure of a safety-service circuit during fire can endanger occupants and firefighters. Design, installation, inspection and testing require competent control.
Local code checkConfirm the building fire strategy, authority having jurisdiction, current BS 7671 and application standard (for example BS 5839, BS 5266 or BS 8519), exact tested cable/support/termination system, product certification, route, segregation, protection and documentation. Product names and LSOH labels never substitute for evidence. Isolate and prove dead before work.

Knowledge check

When may a cable be approved after one failed gate?

Never. Revise and reverify every affected dependency.

Answer: Never. Revise and reverify every affected dependency.

Practical exercise

Complete a supplied design worksheet with source citations and an explicit pass/fail row for every gate.

Summary

  • Function determines evidence
  • The tested installed system matters
  • Document every assumption

Sources and review

  • BS 5839-1:2025: Fire detection and fire alarm systems for non-domestic premises: BSI; 2025; United Kingdom
  • Approved Document B: Fire safety: UK Government; Current applicable edition and transition must be confirmed; England
  • Amendment 4:2026 impact on BS 7671: IET; 2026; United Kingdom

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.

Personal study record

Lesson notes

Ready

Notes are stored on this device unless you export them.