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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 13 of 28

Calculating voltage drop for single-phase

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
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

Calculating voltage drop for single-phase

Cable Sizing · Lesson 13 · Intermediate

IntermediateReview: professional review pending

Purpose

Calculate single-phase voltage drop and test it against an explicitly allocated design limit.

Before you beginmV/A/m tables · Single-phase circuits

Learning objectives

  • Calculate ΔV
  • Convert to percent
  • Compare with budget
  • Revise a design

A single-phase calculation is only defensible when current, length and coefficient share the same route and design condition.

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

Core theory

Use design current appropriate to the load, not automatically the protective-device rating. Account for harmonic-rich or constant-power loads where the standard or equipment behavior requires it.

Compare the accumulated origin-to-load drop with the applicable current-edition recommendation and the equipment's declared voltage range. A pass against one does not prove the other.

If the result fails, options include larger conductors, shorter route, higher distribution voltage, load redistribution or local transformation. Follow this with every thermal and fault check again.

Terms, symbols, and units
TermMeaningSymbolUnit
Single-phase circuitCircuit with line and return-current pathNot applicableNot applicable
Route lengthOne-way installed length used by the selected table methodLm
Terminal voltageVoltage available at the load terminalsNot applicableV
Uload ≈ Uorigin − ΔV

An approximate steady-state relation; confirm source voltage and equipment behavior.

V
Worked exampleUsing supplied vd = 4.4 mV/A/m, Ib = 31 A, L = 45 m and a 230 V basis, find drop and percentage.

Assumptions: Matching table coefficient; Steady design load.

  1. Drop: ΔV = 4.4×31×45/1000 = 6.138 V.
  2. Percent: 100×6.138/230 = 2.6687%.
  3. State: Report about 6.14 V and 2.67%, then compare with the allocated limit.

Illustrative drop is 6.14 V, or 2.67% on a 230 V basis.

Reasonableness check: The result is below the roughly 8.5 V example from a longer/higher-coefficient path.

Common mistakes
  • Using In when Ib is required
  • Comparing only the final-circuit segment
  • Rounding before the decision

Where this appears in practice

Single-phase feeders, socket circuits and heating loads require whole-path budgeting.

SafetyAn unexpectedly large measured drop can indicate loose or damaged connections with fire risk; isolate and investigate competently.
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

What current belongs in the basic table calculation?

The applicable design current for the stated load case. Protective-device rating has a different design role.

Answer: The applicable design current for the stated load case. Protective-device rating has a different design role.

Practical exercise

Screen two supplied conductor coefficients, then rerun all affected design checks for the selected option.

Summary

  • Keep inputs on one basis
  • Assess volts and percent
  • A redesign changes other checks

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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