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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 · 8min · Lesson 11 of 28

Why voltage drop matters: Reg 525

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
VOLTAGE DROP ALONG A RUN230Vlength L · resistance mV/A/mload230−VdVd = (mV/A/m × Ib × L) ÷ 1000
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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

Why voltage drop matters: Reg 525

Cable Sizing · Lesson 11 · Intermediate

IntermediateReview: professional review pending

Purpose

Allocate voltage-drop limits across an installation so equipment receives a suitable voltage under credible load.

Before you beginOhm's law · Cable thermal selection

Learning objectives

  • Explain voltage drop
  • Separate supply variation from installation drop
  • Allocate a design budget
  • Check equipment performance

Voltage at a load is lower than at the circuit origin when current flows through conductor impedance. Excessive drop can impair lighting, motors, controls and protective operation even where cable temperature is acceptable.

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

Core theory

Section 525 and Appendix 4 guidance must be read in the current edition. Common percentage figures are design guidance with qualifications, not universal statutory supply-quality guarantees.

The installation designer controls drop from the origin to the utilization point; upstream supply variation and equipment tolerance are separate inputs. Allocate the total budget across distribution and final circuits instead of spending it twice.

Use credible design current, route length, conductor temperature and AC impedance. Starting, inrush, constant-power behavior and sensitive loads may require a transient or manufacturer-specific check.

Terms, symbols, and units
TermMeaningSymbolUnit
Voltage dropDifference in voltage between two points under loadΔVV
Design budgetAllocated maximum drop for a route or circuitNot applicable% or V
Utilization pointPoint where equipment consumes electrical energyNot applicableNot applicable
ΔV% = 100 × ΔV / Un

State the nominal-voltage basis and whether the value covers the whole installation path.

%
Worked exampleA supplied design has 3.1 V drop in a distribution circuit and 5.4 V in its final circuit on a 230 V basis.

Assumptions: Drops are compatible and at the same design condition.

  1. Add: Total installation drop = 3.1 + 5.4 = 8.5 V.
  2. Percent: 100 × 8.5/230 = 3.70%.
  3. Assess: Compare with the allocated current-edition limit and equipment operating requirements.

Total illustrative drop is 8.5 V or 3.70% of 230 V.

Reasonableness check: Both series route sections contribute to the load-terminal reduction.

Common mistakes
  • Applying a final-circuit allowance twice
  • Ignoring distribution-circuit drop
  • Treating voltage drop as the only cable check

Where this appears in practice

Long lighting, motor, EV, heating and remote-building supplies often become voltage-drop governed.

SafetyLow terminal voltage can cause stalled motors, contactor chatter and overheating. Investigate causes under safe test procedures.
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

Does a thermally adequate cable automatically satisfy voltage-drop requirements?

No. Thermal capacity and delivered voltage are independent design gates.

Answer: No. Thermal capacity and delivered voltage are independent design gates.

Practical exercise

Allocate a supplied total drop budget between a distribution circuit and three final circuits, stating load assumptions.

Summary

  • Voltage drop affects equipment
  • Allocate the whole path
  • Check steady and transient needs

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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