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

Between 2019 and 2021, multiple EV charger installations across the United Kingdom and Europe experienced cable overheating and fires during rapid charging sessions. Investigations by the UK Office for Product Safety and Standards found a recurring pattern: cables had been sized for the charger’s nominal rating but not for the continuous load derating required by BS 7671:2018+A2, Section 722.

A Mode 3 charging circuit operating at 32 A continuously for eight or more hours is a fundamentally different load profile from a socket outlet circuit. Socket outlet circuits assume diversity — users plug in kettles for minutes, not hours. But an EV charger draws its rated current for the entire charging session, which can last overnight. BS 7671 Regulation 722.531.1 therefore requires that EV charging circuits be treated as continuous loads, meaning the cable must be sized for 100% of the rated current with no diversity reduction on individual circuits.

Installers accustomed to domestic socket circuits were applying standard diversity factors to EV circuits — effectively assuming that a 32 A charger would average only 20–25 A over time. The result was cables rated at 32 A being loaded at 32 A continuously, with no thermal margin. In grouped installations where multiple chargers shared cable routes, the combined effect of continuous loading and inadequate grouping derating caused cable temperatures to exceed safe limits, melting insulation and igniting cable tray enclosures.

Design the EV charging installation for a 50-bay commercial car park at a new office building. The installation includes both single-phase and three-phase chargers.

Calculate the full-load current for each charger type:

7 kW single-phase chargers (30 units):

I_b = P / V = 7,000 / 230 = 30.4 A — (Eq. 1)

Standard protective device: 32 A MCB Type C (Type C selected for electronic charger inrush).

22 kW three-phase chargers (20 units):

I_b = P / (√3 × V) = 22,000 / (√3 × 400) = 31.8 A — (Eq. 2)

Standard protective device: 32 A MCB Type C.

Per BS 7671 Regulation 722.531.1, EV charging circuits are classified as continuous loads. The cable must be rated for the full load current without diversity reduction on the individual circuit:

I_design = I_b × 1.0 (continuous load factor) — (Eq. 3)

This means the cable’s derated current-carrying capacity must be at least equal to the protective device rating of 32 A. There is no diversity allowance on a single EV circuit — the charger will draw its rated current for the entire charging session.

Apply derating factors from BS 7671 Appendix C for the individual circuit cables:

Ambient temperature derating (C_a):

BS 7671 reference ambient is 30°C. The enclosed car park reaches 35°C in summer.

From BS 7671 Table C.2, Column: 90°C XLPE, Row: 35°C ambient:

C_a = 0.96

Grouping derating (C_g):

Individual circuit cables run in groups of 6 on perforated cable tray (5 chargers per group, plus a spare). From BS 7671 Table C.3, Row: single layer on perforated cable tray, 6 circuits:

C_g = 0.57

Combined derating factor:

C_total = C_a × C_g = 0.96 × 0.57 = 0.547 — (Eq. 4)

The cable must have a tabulated current rating such that the derated rating exceeds the protective device rating:

I_t ≥ I_n / C_total = 32 / 0.547 = 58.5 A — (Eq. 5)

From BS 7671 Table C.7 (multicore XLPE on cable tray):

Selected individual circuit cable: 10 mm² XLPE copper, for both single-phase and three-phase chargers.

While individual circuits carry full load with no diversity, the aggregate demand from 50 chargers simultaneously is unlikely to equal 50 × rated power. BS 7671 Appendix 12 (informative) and IEC 61851-1 Annex A provide guidance on demand factors for multiple EV chargers:

Total connected load:

P_connected = (30 × 7) + (20 × 22) = 210 + 440 = 650 kW — (Eq. 6)

Applying the demand factor of 0.50 for 50 chargers:

P_demand = 650 × 0.50 = 325 kW — (Eq. 7)

Equivalent three-phase current at 400 V:

I_feeder = 325,000 / (√3 × 400) = 469 A — (Eq. 8)

The feeder from the main switchboard to the EV sub-distribution board must carry 469 A over 40 m. Apply derating for the feeder cable (2 circuits on cable ladder, 35°C ambient):

C_a = 0.96 (35°C, XLPE)

C_g = 0.88 (2 circuits on cable ladder, Table C.3)

C_total = 0.96 × 0.88 = 0.845

I_t ≥ 469 / 0.845 = 555 A — (Eq. 9)

From BS 7671 Table C.7, for multicore XLPE cable on cable ladder:

Selected feeder cable: 300 mm² XLPE copper, 4-core, protected by 500 A MCCB.

Alternatively, two parallel 150 mm² cables per phase can be used (each rated 395 A, total 790 A derated capacity = 790 × 0.845 = 668 A — passes with margin). Parallel cables offer easier installation in confined car park ceiling spaces.

BS 7671 Regulation 722.411.4.1 imposes specific requirements on EV charging installations supplied from a TN-C-S (PME) system. The concern is that a fault in the PME neutral outside the installation could cause the earthing terminal to rise to a dangerous voltage — which is then transferred to the metal chassis of a vehicle via the charging cable.

For this installation, the risk assessment per BS 7671 Regulation 722.411.4.1 must consider:

• Open PEN conductor fault: The vehicle chassis could rise to up to 115 V in a worst-case single-phase fault scenario
• Touch voltage exposure: Users may touch the vehicle while standing on a wet concrete floor (low body resistance path)
• Simultaneous contact: User may be in contact with both the vehicle and a grounded metal structure (parking bollard, pipe)

Design decision: Install a TT earthing arrangement for the EV charging circuits, with dedicated earth electrodes and 30 mA RCDs on each circuit. This is the safest approach and is recommended by BS 7671 Regulation 722.411.4.1 Note 2 for outdoor or semi-outdoor installations.

R_A × I_Δn ≤ 50 V — (Eq. 10)

R_A ≤ 50 / 0.030 = 1,667 Ω maximum earth electrode resistance

A single copper-clad earth rod driven to 2.4 m depth in typical soil (100 Ω·m resistivity) achieves approximately 40 Ω — well within the requirement.

The total connected load of 650 kW would require a dedicated 800 kVA transformer if all chargers operated at full power simultaneously. A dynamic load management (DLM) system per IEC 61851-1, Clause 6.2.2 limits the aggregate power draw to the available supply capacity.

DLM design parameters:

With 325 kW available and 50 chargers, the average allocation per charger is:

P_avg = 325 / 50 = 6.5 kW per charger

This exceeds the IEC 61851-1 minimum of 1.4 kW (6 A), so all 50 chargers can operate simultaneously at reduced power. In practice, with only 30–40 vehicles charging at any time (typical daytime office pattern), each active charger receives 8–11 kW — delivering a full charge in 4–6 hours during an 8-hour workday.

BS 7671 Appendix 12 recommends a maximum voltage drop of 3% for EV charging circuits (stricter than the general 5% limit) to ensure charger power electronics operate efficiently.

Feeder cable voltage drop (300 mm², 40 m, 469 A):

From cable data, mV/A/m for 300 mm² XLPE three-phase at 90°C:

mV/A/m = 0.17

ΔV_feeder = 0.17 × 469 × 40 / 1000 = 3.19 V (0.80%) — (Eq. 11)

Individual circuit voltage drop (10 mm², 25 m, 32 A, single-phase worst case):

mV/A/m = 3.8 (10 mm² XLPE, single-phase)

ΔV_circuit = 3.8 × 32 × 25 / 1000 = 3.04 V (1.32%) — (Eq. 12)

Total voltage drop:

ΔV_total = 0.80% + 1.32% = 2.12%

2.12% < 3% limit — PASS

Design summary: 50 × 10 mm² XLPE individual circuits, 300 mm² XLPE feeder, 500 A MCCB, TT earthing with dedicated earth electrodes, dynamic load management capped at 325 kW.

The governing factor is grouping derating on individual circuits. Without proper grouping derating, a 6 mm² cable would have been selected — resulting in continuous operation at 127% of safe capacity, exactly the failure mode that caused the UK charger fires.

The UK EV charger cable fires were caused by a failure to recognise that EV charging circuits are continuous loads with severe grouping derating requirements. The engineering lessons:

• Treat every EV circuit as a continuous load — no diversity reduction on individual charging circuits; the cable must carry 100% of the charger’s rated current indefinitely
• Apply grouping derating rigorously — multiple EV circuits on shared cable routes create significant mutual heating; a grouping factor of 0.57 for 6 circuits means the cable needs nearly double the cross-section
• Assess PME earthing separately — BS 7671 Regulation 722.411.4.1 requires a specific risk assessment for EV charging on PME supplies; TT earthing is often the safest option
• Install dynamic load management — DLM prevents the aggregate demand from exceeding the available supply capacity, avoiding overload of feeder cables and upstream transformers
• Do not use domestic socket circuit rules for EV charging — a 32 A socket outlet circuit assumes intermittent loading; a 32 A EV charger is a continuous 32 A load and must be designed accordingly