Calculate EV charging time plus supply cable cross-section, fusing and RCD requirements for 3.7 to 22 kW wallbox installations.
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Sizing a wallbox installation: charging time, cable and protection
This calculator answers the three questions of every wallbox project: how long does charging take, what cable goes into the wall, and how must the circuit be protected?
Realistic charging time
Charging time is the energy to be replenished divided by charging power – plus charging losses, which are easily forgotten. Example: a 60 kWh battery from 20 to 80 % is 36 kWh net. With 10 % losses the car draws about 39.6 kWh from the grid. On an 11 kW wallbox that takes 3 h 36 min; on 3.7 kW single-phase more than 10 hours.
The supply cable: continuous load is the point
A wallbox draws its current constantly for hours – unlike almost any other household load. The feed is therefore sized generously: typically max. 1 % voltage drop and a cross-section that handles continuous load. For an 11 kW box on a 15 m run the calculator recommends 2.5 mm²; longer runs quickly require 4 or 6 mm². At 22 kW (32 A continuous) 6 mm² is the minimum.
Protection
Besides the MCB, every charging point needs residual-current protection that also detects smooth DC fault currents: either an RCD type B or – the usual route – an RCD type A combined with the wallbox's built-in 6 mA DC monitoring.
FAQ
Why does my car only charge at 11 kW on a 22 kW box? The vehicle's onboard charger limits AC charging power – many models max out at 11 kW AC.
Can the wallbox share an existing cooker circuit? No – a charging point needs its own dedicated, unbranched circuit with its own protection.
