EV Charger Cable Sizing Calculator — BS 7671 Section 722

This calculator sizes the cable for an EV charger installation based on BS 7671 18th Edition Section 722 and the IET Code of Practice for Electric Vehicle Charging Equipment Installation. It checks both current carrying capacity and voltage drop.

The sizing process

Step 1 — Design current. EV charging is a continuousload (BS 7671 Reg 722.311), so the circuit is sized at 100% of the charger output with no diversity. A 7kW charger is protected by a 32A device, so the design current is taken as Ib = 32A (not the back-calculated 7000 ÷ 230 = 30.4A, which would undersize the circuit). A 3.6kW charger uses 16A; a 22kW three-phase charger uses 32A per phase.

Step 2 — Current capacity (Reg 433.1.1).Coordinate Ib ≤ In ≤ Iz: the cable’s current-carrying capacity Iz must be ≥ the protective-device rating In (32A), not merely the design current. Allowing for derating, the required tabulatedcapacity is It ≥ In ÷ Cf. With the IET Code of Practice rating factor Cf = 0.75 (cable in contact with thermal insulation on one side), It ≥ 32 ÷ 0.75 = 42.7A, which rules out 4mm² (37A) and selects 6mm² (47A) for a typical 7kW charger.

Step 3 — Voltage drop.VD = (mV/A/m × current × length) ÷ 1000. Must not exceed 11.5V (5% of 230V) single-phase, or 20V (5% of 400V) three-phase.

The calculator’s capacities are BS 7671 values for flat twin & earth at 30°C ambient on a single circuit. Clipped-direct ratings come from Table 4D5 (Reference Method C). Conduit and trunking ratings come from Table 4D2A (Reference Method B, one two-core cable, single phase), because Table 4D5 tabulates no conduit or trunking column at all: it carries only the thermal-insulation methods, Method A and Method C. Using 4D2A here is standard practice and is the conservative choice. For grouped circuits, higher ambient or thermal insulation, lower the rating factor to derate; for a 22kW three-phase charger use 4-core SWA and confirm its capacity against the armoured tables.

Where this calculator stops

Table 4D5 runs from 1mm² to 16mm², which is the whole range flat twin and earth is manufactured in. If a long run or a heavy derating pushes the circuit past 16mm², the calculator refuses instead of naming a bigger twin and earth: no such cable exists to buy, and no published table supports a rating for one. A circuit that big belongs on armoured cable rated from Table 4D4A and designed by a qualified electrician.

Buried runs are refused for the same reason. Table 4D5 has no direct-burial column because flat twin and earth is not rated for burial. An underground route to a driveway or detached garage needs armoured cable, laid at the correct depth with marker tape, sized against the armoured tables.

EV-specific requirements (Section 722)

• Dedicated circuit: EV charger must be on its own dedicated circuit from the consumer unit
• RCD protection: 30mA, Type A minimum (Type B if no built-in DC protection)
• PME earthing: Risk assessment required. TT earthing may be needed for outdoor chargers
• Load management: May be required if supply capacity is limited (smart chargers can reduce output)
• Cable type: SWA for outdoor/buried runs, twin & earth for internal clipped routes

Always consult a qualified electrician (Part P registered) for EV charger installations. The installer must notify Building Control and provide an Electrical Installation Certificate.

Working figures for this are collected in our Part S EV Charging Requirements reference table.