EV Charger Cable Calculation: A Complete Worked Example

Electric vehicle charger installations are now one of the most common jobs for domestic electricians — and they’re a growing topic in the IET exam. Section 722 of BS 7671 covers the specific requirements for EV charging installations, and it introduces several rules that differ from standard circuit design.

This guide walks through a complete cable sizing calculation for a typical domestic 7.4 kW charger, covering every step from design current to earth fault loop impedance verification.

EV Charger Types

Before calculating anything, you need to know what you’re installing. Most domestic installations use a Mode 3, single-phase, 7.4 kW charge point — often called a “wallbox.”

EV charger types and power requirements

Key Points for Cable Sizing

7.4 kW single-phase = 32A at 230V — this is the standard domestic installation
22 kW three-phase = 32A per phase at 400V — commercial premises
The charger draws 32A continuously for several hours — this is critical for cable sizing

The Continuous Load Problem

Here’s the single most important thing to understand about EV charging: Regulation 722.311 requires EV charging to be treated as a continuous load. This means:

No diversity is applied — the cable must be rated for the full 32A, not a reduced figure
The circuit is assumed to run at full load for 3 hours or more
Standard domestic diversity factors (from BS 7671 Table 4A) do not apply to EV circuits

This is why a 32A EV circuit often needs a larger cable than a 32A cooker circuit — the cooker gets diversity applied, the EV charger does not.

Protection Requirements

Section 722 has specific protection requirements that differ from standard circuits.

EV charger protection requirements

RCD Protection (Regulation 722.531.3.101)

Every EV charging point must have its own dedicated circuit protected by:

A 30 mA RCD — minimum Type A (to detect pulsating DC fault currents)
If the charger can produce smooth DC leakage greater than 6 mA, a Type B RCD is required
Many modern chargers have built-in DC leakage detection, which allows the use of a Type A RCD — check the manufacturer’s documentation

Overcurrent Protection

A 32A MCB or RCBO (Type B curve is standard) on a dedicated circuit
No other loads or socket outlets on the EV circuit
An RCBO is the neatest solution as it provides both overcurrent and RCD protection in one device

PME Earthing (Regulation 722.411.4.1)

EV charging installations on PME (TN-C-S) supplies have additional earthing considerations. The 18th Edition Amendment 2 now permits PME earthing for EV charging, but you should check with the DNO for any local requirements — particularly for outdoor charge points.

Step-by-Step Cable Calculation

Let’s work through a real example.

The Scenario

Charger: 7.4 kW single-phase wallbox (32A)
Cable run: 20 metres from the consumer unit to the garage
Installation method: Clipped direct (Method C), with thermal insulation contact on one side for 2 metres through an insulated wall
Ambient temperature: 30°C (standard)
Earthing system: TN-C-S, Ze = 0.35 Ω
Protective device: 32A Type B RCBO

EV charger cable calculation — worked example

Step 1: Design Current (Ib)

The design current is the maximum current the circuit will carry in normal service.

Ib = P ÷ V = 7400 ÷ 230 = 32.17A

We round this to 32A since the charger is rated at 32A. The fundamental rule is: Ib ≤ In ≤ It.

Step 2: Protective Device Rating (In)

We need In ≥ Ib. A 32A Type B RCBO gives us In = 32A. This satisfies Ib ≤ In.

Step 3: Correction Factors

Now we calculate the overall correction factor to determine the minimum tabulated current rating (It) the cable must have.

Factor	Value	Reason
Ca (ambient temperature)	1.0	30°C ambient is the reference temperature for PVC cables
Cg (grouping)	1.0	Single circuit — no grouping derating needed
Ci (thermal insulation)	0.75	Cable touches thermal insulation on one side (Table 52.2)
Cf (semi-enclosed fuse)	1.0	Using an MCB/RCBO, not a BS 3036 fuse

Overall correction factor = Ca × Cg × Ci = 1.0 × 1.0 × 0.75 = 0.75

Step 4: Minimum Tabulated Current (It)

It ≥ In ÷ CF = 32 ÷ 0.75 = 42.67A

Now look up the current-carrying capacity tables in Appendix 4 of BS 7671. For T&E cable clipped direct (Reference Method C, Table 4D5):

Cable Size	It (Clipped Direct)
4.0 mm²	37A
6.0 mm²	47A
10.0 mm²	64A

4.0 mm² only gives us 37A — that’s less than 42.67A, so it fails. 6.0 mm² gives 47A, which exceeds 42.67A. Select 6.0 mm² T&E.

Step 5: Voltage Drop Check

Regulation 525 limits the voltage drop to 5% of the nominal voltage for power circuits, which is:

Max VD = 230 × 0.05 = 11.5V

From Table 4D5, the mV/A/m value for 6.0 mm² T&E is 7.3 mV/A/m.

VD = (mV/A/m × Ib × L) ÷ 1000 = (7.3 × 32 × 20) ÷ 1000 = 4.67V

4.67V is well within the 11.5V limit — PASS.

Step 6: Earth Fault Loop Impedance (Zs)

Finally, verify that the earth fault loop impedance allows the protective device to disconnect within the required time.

Zs = Ze + (R1 + R2)

From the resistance tables (at 20°C):

R1 for 6.0 mm² = 3.08 mΩ/m
R2 for 2.5 mm² CPC = 7.41 mΩ/m
R1 + R2 per metre = 10.49 mΩ/m

For 20 metres: (R1 + R2) = 10.49 × 20 ÷ 1000 = 0.21 Ω

Zs = 0.35 + 0.21 = 0.56 Ω

The maximum Zs for a 32A Type B MCB (Table 41.3, 0.4s disconnection) is 1.37 Ω. Our calculated Zs of 0.56 Ω is well within the limit — PASS.

EV charger worked example — all steps

When to Use SWA Cable

For outdoor or underground cable runs, T&E is not suitable. You should use:

SWA (Steel Wire Armoured) cable — for buried or exposed outdoor runs
Minimum depth: 500 mm below ground (with cable tiles or warning tape)
SWA provides its own mechanical protection and can serve as the CPC (the steel wire armour)
Common choice: 6.0 mm² 3-core SWA (or 2-core + armour as earth)

For runs that are entirely inside the building (e.g., consumer unit to an internal garage), clipped T&E is acceptable.

Common Exam Scenarios

Scenario	Cable Size	Notes
7.4 kW, 15 m, clipped direct, no derating	6.0 mm² T&E	4.0 mm² is too small (37A < 42.67A with Ci)
7.4 kW, 30 m, clipped direct, no insulation	6.0 mm² T&E	Check voltage drop: 7.3 × 32 × 30 = 7.0V (OK)
7.4 kW, 40 m, clipped direct	10.0 mm² T&E	VD with 6.0 mm²: 9.3V — tight. Consider 10.0 mm²
7.4 kW, outdoor underground	6.0 mm² SWA	Must use SWA for external/underground runs
22 kW three-phase, commercial	6.0 mm² 5-core SWA	32A per phase, check per-phase VD

Quick Reference Summary

Parameter	Value
Standard domestic EV charger	7.4 kW, 32A, single-phase
Protective device	32A Type B RCBO (30 mA, Type A minimum)
Diversity factor	1.0 (no diversity — continuous load)
Typical cable (internal)	6.0 mm² T&E (6242Y)
Typical cable (external)	6.0 mm² SWA
Maximum voltage drop	5% of 230V = 11.5V
Key BS 7671 section	Section 722

Key Regulations

Section 722 — Electric vehicle charging installations (entire section)
Reg. 722.311 — Continuous load, diversity not applicable
Reg. 722.411.4.1 — Earthing arrangements for PME supplies
Reg. 722.531.3.101 — RCD protection requirements (Type A minimum)
Reg. 722.433 — Overcurrent protection
Table 4D5 — Current-carrying capacity for T&E cables (Reference Method C)
Reg. 525 — Voltage drop limits

Practice and Further Study

EV charger cable calculation combines Part 4: Protection for Safety with Part 5: Selection and Erection and Section 722 of BS 7671. Test your knowledge:

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