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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/Voltage Drop Calculations

lesson · 9min · Lesson 12 of 28

mV/A/m tables and how to use them

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

mV/A/m tables and how to use them

Cable Sizing · Lesson 12 · Intermediate

IntermediateReview: professional review pending

Purpose

Use an mV/A/m value only from the table column matching cable, phase system, temperature and impedance assumptions.

Before you beginVoltage-drop purpose · Unit conversion

Learning objectives

  • Interpret mV/A/m
  • Select the correct column
  • Convert millivolts to volts
  • Recognise r/x components

A tabulated mV/A/m value packages conductor impedance and a stated circuit configuration into a convenient design coefficient. It is conditional data, not a material constant.

WHOLE-PATH VOLTAGE-DROP BUDGETWHOLE-PATH VOLTAGE-DROP BUDGETIbloadIndeviceIzIt × applicable factorsphysical method · exact table · declared conditions · all design gates

Core theory

Select the exact cable family, conductor size, single- or three-phase column, conductor operating temperature and AC/DC treatment. For larger AC conductors, resistance and reactance components and power factor may matter.

The route length in the common formula is the one-way circuit length because the table coefficient already embodies its stated circuit arrangement. Do not double it unless the source formula explicitly requires that.

Temperature correction or reduced-load refinement may be permitted by the current Appendix 4 method. Use its stated equations and assumptions rather than inventing a linear adjustment.

Terms, symbols, and units
TermMeaningSymbolUnit
Voltage-drop coefficientTabulated drop per ampere per metrevdmV/A/m
Resistance componentIn-phase impedance contributionrmΩ/m
Reactance componentQuadrature impedance contributionxmΩ/m
ΔV = (vd × Ib × L) / 1000

Use the table's declared one-way length convention and circuit column.

V
Worked exampleA supplied matching table value is 7.3 mV/A/m, Ib = 24 A and one-way length L = 32 m.

Assumptions: Coefficient matches cable and circuit conditions.

  1. Multiply: 7.3 × 24 × 32 = 5606.4 mV.
  2. Convert: 5606.4/1000 = 5.6064 V.
  3. Report: Use 5.61 V without false precision.

Illustrative voltage drop is 5.61 V.

Reasonableness check: More current or length increases drop proportionally within this tabulated method.

Common mistakes
  • Doubling one-way length again
  • Mixing single- and three-phase columns
  • Forgetting division by 1000

Where this appears in practice

The method provides auditable preliminary calculations when table assumptions match the route.

SafetyDo not measure energized conductors merely to validate a classroom coefficient; use competent test methods and suitable instruments.
Local code checkConfirm the current standard and amendments, supply characteristics, equipment voltage tolerance, exact cable standard and manufacturer data, route, terminations, earthing system, protection and local excavation rules. Examples use supplied fictional data only. Isolate and prove dead before cable work; design calculations do not authorise excavation, jointing or energisation.

Knowledge check

Why is mV/A/m not universal for a conductor size?

Its cable, circuit, temperature and impedance assumptions differ. Always cite the exact table and column.

Answer: Its cable, circuit, temperature and impedance assumptions differ. Always cite the exact table and column.

Practical exercise

Trace five supplied coefficients to their cable and circuit headings before calculating any drop.

Summary

  • Coefficient selection comes first
  • Use one-way length as defined
  • AC impedance can matter

Sources and review

  • BS 7671:2018+A4:2026 Requirements for Electrical Installations: IET/BSI; Current edition and corrigenda must be confirmed; United Kingdom
  • Earthing and Bonding FAQs: SWA armour as CPC: IET; Current online guidance; United Kingdom
  • Avoiding danger from underground services (HSG47): Health and Safety Executive; Third 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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