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: Determine the charger current. A 7kW charger on a 230V supply draws 7000 ÷ 230 = 30.4A. The MCB must be rated at 32A.

Step 2: Select the smallest cable whose current carrying capacity (per BS 7671 tables for the installation method) is ≥ the MCB rating.

Step 3: Check voltage drop: VD = (mV/A/m × current × length) ÷ 1000. Must not exceed 11.5V (5% of 230V) for power circuits.

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.