Worked Examples

Complete worked examples showing real-world electrical calculations step-by-step, with standard clause references at every stage.

Worked Example: Residential Cable Sizing — Multi-Standard

Step-by-step worked example sizing a cable for a residential cooker circuit. Includes full derating factor calculations, voltage drop verification, protective device coordination, and a multi-standard comparison across AS/NZS 3008, BS 7671, and IEC 60364.

14 min read

Worked Example: Fire-Resistant Cable Sizing for a 67m Vertical Riser — The Grenfell Tower Cable Question

Step-by-step calculation for sizing fire-resistant cables in a 24-storey residential tower riser. Covers BS 7671 Regulation 422.2, vertical cable derating, and why standard PVC cables are inadequate above 3 storeys.

18 min read

Worked Example: Emergency Generator Feeder Cable Sizing — The Piper Alpha Lesson

Complete worked example sizing the feeder cable from a 500 kVA emergency diesel generator to an essential services switchboard, including engine cranking current allowance, black-start transients, and the mistake that left Piper Alpha without power.

20 min read

Worked Example: Voltage Drop Calculation for a 2 km Industrial Feeder — The 2003 Northeast Blackout

Complete voltage drop calculation for a 2 km 415 V industrial feeder. Shows why the simplified mV/A/m method fails at this distance, how reactive impedance dominates, and the cascading failure mechanism that blacked out 55 million people.

18 min read

Worked Example: Motor Circuit Cable Sizing for a 75 kW Escalator Drive — The King's Cross Fire Investigation

Full motor circuit design for a 75 kW escalator drive: cable sizing, starter selection, protection coordination, and voltage drop at starting current. Inspired by the 1987 King's Cross Underground fire investigation.

22 min read

Worked Example: Transformer Protection Coordination Study — The Sayano-Shushenskaya Dam Disaster

Complete transformer protection coordination study: fault current calculations per IEC 60909, protection device grading from transformer to final circuit, and how inadequate protection coordination contributed to the worst hydroelectric disaster in history.

22 min read

Worked Example: Arc Flash Hazard Assessment for a 480 V Motor Control Centre — The Texas City Refinery Explosion

Complete IEEE 1584-2018 arc flash calculation for a 480 V MCC: incident energy, arc flash boundary, PPE category selection, and why the 2005 Texas City refinery explosion changed how we assess electrical hazards in process industries.

20 min read

Worked Example: DC Battery System Short-Circuit Calculation — The Fukushima Daiichi Lesson

Complete short-circuit calculation for a 110 V DC battery system feeding emergency loads. Covers battery internal resistance, cable impedance, DC fault current, and why Fukushima lost all DC power within 8 hours of the tsunami.

18 min read

Worked Example: Maximum Demand Calculation for a Commercial Building — The 1998 Auckland CBD Power Crisis

Complete maximum demand calculation for a 20-storey commercial office building per AS/NZS 3000. Includes diversity factors, load classification, transformer sizing, and the cascading cable failures that blacked out Auckland's CBD for 5 weeks.

20 min read

Worked Example: Lightning Protection Risk Assessment for a Fuel Storage Facility — The Buncefield Explosion

Complete IEC 62305 lightning protection risk assessment for a fuel storage depot. Covers risk calculation, protection level selection, rolling sphere method, and why the 2005 Buncefield explosion raised fundamental questions about lightning protection in hazardous areas.

18 min read

Worked Example: Power Factor Correction for an Industrial Plant — The Victorian Black Saturday Bushfire

Complete power factor correction calculation for a 2 MW industrial plant. Covers capacitor bank sizing, harmonic resonance check, cable loss reduction, and how poor power factor contributed to line losses and conductor heating during the 2009 Black Saturday bushfires.

18 min read

Worked Example: Cable Tray Fill and Fire Barrier Design for a 24-Storey Residential Tower — The Grenfell Tower Cable Riser Question

Step-by-step cable tray fill calculation for a high-rise residential riser per IEC 61537 and BS 7671. Covers tray fill ratio, grouping derating, fire barrier spacing per BS 8519, structural loading, and emergency circuit segregation — lessons from the Grenfell Tower fire.

16 min read

Worked Example: Earthing Grid Design for a 6,400 MW Hydroelectric Powerhouse — The Sayano-Shushenskaya Dam Disaster

Complete earthing grid design per IEEE 80 and IEC 60364-5-54 for a large hydroelectric powerhouse. Covers soil resistivity modelling, Schwarz's formula, step and touch potential calculations, and grid conductor sizing — lessons from the 2009 Sayano-Shushenskaya disaster.

18 min read

Worked Example: Conduit Fill Calculation for a 38-Storey Office Building Riser — The One Meridian Plaza Fire

Complete NEC Chapter 9 conduit fill calculation for a high-rise office building electrical riser. Covers conductor area lookup, conduit sizing, fill percentage verification, jamming ratio, and ampacity adjustment — lessons from the 1991 One Meridian Plaza fire that killed 3 firefighters.

16 min read

Worked Example: Emergency & Standby Power System for a 1,000-Occupancy Entertainment Venue — The Cocoanut Grove Fire

Step-by-step emergency power system design for a large nightclub venue. Covers NEC 700/701/702 load classification, generator sizing, ATS rating, battery backup duration, and why the 1942 Cocoanut Grove fire led to modern emergency lighting codes.

19 min read

Worked Example: Harmonic Analysis & Neutral Conductor Sizing for a 2 MW Data Center — The Nordic Neutral Burnout

Step-by-step harmonic analysis for a data center with non-linear server loads. Covers THD calculation, triplen harmonic neutral current, K-factor transformer derating, IEEE 519 TDD limits, and why a 2019 Nordic data center lost 40% capacity to neutral conductor failure.

20 min read

Worked Example: 3,200 A Busbar System Design for a Railway Station Main Switchboard — The King's Cross Busbar Question

Step-by-step busbar sizing for a major underground station switchboard. Covers current density, temperature rise, short circuit withstand, electromagnetic force, joint resistance, and why the 1987 King's Cross fire investigation found decades of unchecked busbar overloading.

20 min read

Worked Example: Resilient LV Network Design for a Coastal Community — Hurricane Maria, Puerto Rico

Complete LV distribution network design for a 200-dwelling coastal community per IEC 60364-5-52. Includes ADMD calculation, transformer sizing for redundancy, ring-main topology, voltage drop across the network, fault level verification, and protection coordination. Based on the lessons of Hurricane Maria (2017), the longest blackout in US history.

22 min read

Worked Example: Distance Protection Settings for a Transmission Interconnector — The 2003 Italy-Switzerland Blackout

Complete distance protection relay setting calculation for a 380 kV transmission interconnector per IEC 60255-121. Includes Zone 1/2/3 reach settings, time grading, load encroachment boundary check, power swing blocking, and coordination with adjacent line protection. Based on the 2003 blackout that left 56 million Italians without power.

24 min read

Worked Example: Medium Voltage Cable Sizing for a Hospital Feeder — The Gorakhpur Tragedy

Complete IEC 60287 medium voltage cable sizing calculation for an 11 kV hospital feeder. Covers thermal circuit model, soil thermal resistivity correction, voltage drop at MV, short circuit withstand, and screen earthing — and why the 2017 Gorakhpur hospital tragedy exposed the consequences of degraded MV infrastructure.

19 min read

Worked Example: Museum Lighting Design with Emergency Lighting — The Jodrell Bank First Light Pavilion

Complete lighting design calculation for a museum gallery space using the lumen method per BS EN 12464-1. Covers room index, utilisation factor, maintenance factor, UGR glare rating, emergency lighting spacing per BS 5266-1, and LENI energy assessment — and why the 2022 Jodrell Bank First Light Pavilion commissioning problems exposed the limits of standard calculation methods on curved geometry.

18 min read

Worked Example: HVAC Electrical System Sizing for Dual-Mode Heating/Cooling — The Texas Winter Storm Uri Disaster

Complete HVAC electrical sizing calculation for a commercial building with heat pump and auxiliary resistance heating. Covers NEC 440 motor circuits, NEC 424 resistance heating, summer vs winter demand comparison, feeder sizing, and disconnect requirements — and why the 2021 Texas storm proved that designing only for summer cooling loads is a fatal oversight.

20 min read

Worked Example: EV Charger Cable Sizing for a 50-Bay Commercial Car Park — The UK Continuous Load Fires

Complete EV charging infrastructure calculation for a 50-bay commercial car park. Covers BS 7671 Section 722 continuous load derating, demand factor application for multiple chargers, PME earthing considerations, and dynamic load management design.

19 min read

Worked Example: PV String Sizing and Protection for a 500 kWp Commercial Rooftop — The Walmart Yuba City Fire

Step-by-step PV string configuration, overcurrent protection, and DC cable sizing for a 500 kWp commercial rooftop solar installation. Covers NEC 690 1.56 factor, temperature-corrected string voltage, and arc fault risk assessment.

20 min read

Worked Example: 315 MVA Power Transformer Sizing and Overload Assessment — The 2020 Mumbai Grid Collapse

Step-by-step power transformer sizing, loss calculation, efficiency, temperature rise, and insulation ageing analysis for a 400/220 kV, 315 MVA receiving station transformer. Covers IEC 60076-7 overload capacity, Arrhenius insulation ageing, and loss-of-life from sustained overloading.

21 min read

Worked Example: Cable Pulling Feasibility for a 185mm² Feeder at an Open-Pit Mine

Step-by-step cable pulling tension calculation for a 185mm² Cu 3C+E XLPE feeder through a 150m cable route with two 90° bends at a copper-gold mining operation. Covers pulling tension, SWBP, jam ratio, and conduit fill per NEC and IEEE 1185.

15 min read

Worked Example: Industrial Motor Feeder — 75 kW DOL Motor, 150 m Run

Step-by-step motor feeder cable sizing for a 75 kW direct-on-line motor with a 150 m cable run under four international standards. Covers starting current, voltage drop at starting, derating factors, and multi-standard comparison across AS/NZS 3008, BS 7671, IEC 60364, and NEC.

22 min read

Worked Example: Solar PV String Cable — 10 kW Rooftop Inverter

Step-by-step DC string cable and AC output cable sizing for a 10 kW residential rooftop solar system under three standards. Covers voltage rise vs voltage drop, temperature coefficients, string configuration, and the NEC 1.56 factor.

20 min read

Worked Example: Data Centre UPS Output — 500 kVA UPS with Parallel Redundancy

Step-by-step UPS output cable sizing for a 500 kVA UPS system with N+1 parallel redundancy and heavy non-linear load. Covers harmonic neutral current, triplen harmonics, THD calculation, and why the neutral can carry more current than the phase conductors.

24 min read

Worked Example: Hospital Essential Supply — Operating Theatre Distribution Board

Step-by-step submain cable sizing from MSB to an operating theatre distribution board feeding critical surgical equipment. Covers medical IT systems, earth fault loop impedance, equipotential bonding, and how AS/NZS 3009, BS 7671 Section 710, and IEC 60364-7-710 force different cable selections.

22 min read

Worked Example: Mining Trailing Cable — 500 m Dragline Supply at 11 kV

Step-by-step medium-voltage trailing cable sizing for a 35 MW dragline excavator in an open-cut coal mine. Covers soil thermal resistivity, cyclic loading, IEC 60287 thermal circuit method, and why mine spoil conditions force 2-3x cable upsizing compared to standard tables.

26 min read

Worked Example: Earth Fault Loop Impedance Verification for a TN-S System per BS 7671 Chapter 41

Step-by-step earth fault loop impedance verification for a 32 A Type B MCB circuit in a TN-S system. Covers conductor resistance at 20°C, temperature correction to 70°C, Zs calculation, and comparison against BS 7671 Table 41.3 maximum values.

12 min read

Worked Example: Voltage Dip During Motor Starting for a 55 kW DOL Motor

Step-by-step calculation of voltage dip at motor terminals during direct-on-line starting of a 55 kW induction motor. Covers starting current, source impedance from fault level, cable impedance, and percentage voltage dip verification against the 15% limit.

12 min read

Worked Example: Sizing a Power Factor Correction Capacitor Bank for an Industrial Site

Step-by-step calculation for sizing a power factor correction capacitor bank to improve power factor from 0.72 to 0.95 lagging at a 500 kVA industrial site. Covers reactive power calculation, capacitor bank selection, cable sizing, fuse rating, and harmonic considerations.

12 min read

Worked Example: Stationary Battery Sizing for a 30-Minute UPS Backup per IEEE 1185

Step-by-step battery sizing calculation for a 48 V DC UPS system providing 12 kW backup for 30 minutes. Covers duty cycle analysis, discharge current calculation, cell sizing from manufacturer curves, aging and temperature correction factors, and final Ah capacity determination per IEEE 1185.

12 min read

Worked Example: Conduit Fill Calculation for a Mixed-Cable Conduit per NEC Chapter 9

Step-by-step conduit fill calculation for a mixed-size cable installation in EMT conduit per NEC/NFPA 70:2023. Covers conductor cross-sectional areas from NEC Chapter 9 Table 5, total occupied area, 40% fill limit for more than 2 conductors, and trade size selection from Table 4.

10 min read

Worked Example: Cable Sizing for a 75 kW, 400 V Motor per AS/NZS 3008.1.1:2025

Complete cable sizing calculation for a 75 kW three-phase motor circuit per AS/NZS 3008.1.1:2025. Includes FLA calculation, derating cascade for ambient temperature and grouping, voltage drop verification, and earth conductor sizing per Table 5.1.

14 min read

Worked Example: Voltage Drop for a 200 m Office Lighting Circuit per BS 7671:2018

Complete voltage drop calculation for a 200 m single-phase office lighting circuit per BS 7671:2018. Demonstrates initial failure at 2.5 mm², upsizing to 4 mm², and full verification using Table 4D2B impedance values.

12 min read

Worked Example: Arc Flash Calculation for a 415 V MCC per IEEE 1584-2018

Complete arc flash incident energy calculation for a 415 V motor control centre using the IEEE 1584-2018 method. Covers arcing current estimation, incident energy calculation, arc flash boundary, and PPE category selection.

14 min read

Worked Example: Maximum Demand for a 5-Storey Commercial Building per AS/NZS 3000:2018

Complete maximum demand calculation for a 5-storey commercial building per AS/NZS 3000:2018. Includes area-based load estimation, demand factors per Table 2.1, ADMD calculation, and MSB sizing at 415 V.

14 min read

Worked Example: Maximum Short-Circuit Current at an LV Switchboard per IEC 60909

Complete short-circuit current calculation at a low-voltage switchboard per IEC 60909-0:2016. Covers transformer impedance with KT correction, busbar impedance, total fault impedance, and initial symmetrical short-circuit current determination.

14 min read

See how standards differ?

Compare the same scenario across AS/NZS 3008, BS 7671, IEC 60364, and NEC — see exactly where the results diverge and why.

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