7.4 kW EV Charger Cable Size: BS 7671 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.”

 

 

Key Points for Cable Sizing

Charger Type	Power	Current	Notes
7.4 kW single-phase	7.4 kW	32A at 230V	Standard domestic installation
22 kW three-phase	22 kW	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:

 

Key Regulation: Reg 722.311 requires EV charging circuits to be treated as continuous loads — no diversity is applied, and the cable must be rated for the full load current.

 

Continuous Load Implication	Detail
No diversity	The cable must be rated for the full 32A, not a reduced figure
Assumed full-load duration	The circuit is assumed to run at full load for 3 hours or more
Standard diversity excluded	Domestic diversity factors (from BS 7671 Table 4A) do not apply to EV circuits

 

Insight: 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.

 

 

RCD Protection (Regulation 722.531.3.101)

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

 

Condition	Required RCD Type
Standard EV charging point	30 mA Type A RCD (minimum) — detects pulsating DC fault currents
Charger produces smooth DC leakage greater than 6 mA	Type B RCD required
Charger has built-in DC leakage detection	Type A RCD permitted — check 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

Parameter	Value
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) for 18 metres, then 2 metres through an insulated stud wall with the cable touching the inner wall surface (Installation Method 102)
Ambient temperature	30°C (standard)
Earthing system	TN-C-S, Ze = 0.35 Ω
Protective device	32A Type B RCBO

 

 

Step 1: Design Current (Ib)

The design current is the maximum current the circuit will carry in normal service. For a Mode 3 EV charger, the charger’s own control electronics regulate the charging current to its nameplate rating — in this case 32A. This is the design current we use, not the theoretical P ÷ V figure.

 

Ib = 32A (charger nameplate rating)

 

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 (32A ≤ 32A).

 

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)	1.0	No factor — the insulated-wall section is rated from its own column of Table 4D5 (Reg 523.9)
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 × 1.0 = 1.0

 

Important: There is no correction factor for a cable that touches insulation on one side. Regulation 523.9 says that where a cable in a thermally insulated wall is in contact with a thermally conductive surface on one side, its current-carrying capacity is tabulated in Appendix 4. You pick the right column instead of multiplying by a factor. The Ci factors (0.88 down to 0.50) are only for a cable totally surrounded by insulation, and they apply to the clipped-direct rating.

 

Step 4: Minimum Tabulated Current (It)

It ≥ In ÷ CF = 32 ÷ 1.0 = 32A

 

Now look up the current-carrying capacity for flat T&E in Table 4D5 of BS 7671 Appendix 4. The cable runs in two conditions, and the most onerous part of the route decides the size. So check both columns:

 

Cable Size	Clipped direct (Method C, col 8)	Insulated stud wall, touching inner surface (Method 102, col 4)
4.0 mm²	37A	27A
6.0 mm²	47A	35A
10.0 mm²	64A	47A

 

4.0 mm² is fine clipped direct (37A), but only gives 27A through the insulated wall — less than 32A, so it fails. 6.0 mm² gives 35A in the wall, which meets 32A. Select 6.0 mm² T&E.

 

Exam tip: If the cable went through the wall in conduit instead, Reference Method A (Table 4D5 col 6) applies: 6.0 mm² is rated 32A, which still meets It = 32A.

 

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.

 

 

When to Use SWA Cable

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

 

SWA Cable Specification	Detail
Cable type	SWA (Steel Wire Armoured) — for buried or exposed outdoor runs
Minimum burial depth	500 mm below ground (with cable tiles or warning tape)
Mechanical protection	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	4.0 mm² T&E	37A ≥ 32A (Method C); VD: 11 × 32 × 15 = 5.3V (OK)
7.4 kW, 30 m, clipped direct, no insulation	6.0 mm² T&E	4.0 mm² drops 11 × 32 × 30 = 10.6V — inside 11.5V with little margin; 6.0 mm²: 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

Regulation	Requirement
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:

• Part 4 — Protection for Safety quiz
• Part 5 — Selection and Erection quiz

 

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