A wallbox is quickly bought – and almost as quickly installed wrongly. Three mistakes keep recurring, and all three get expensive. The good news: all three are completely avoidable with a little planning.
Mistake 1: Not reported or not approved
Under Germany's § 19 NAV: charging equipment up to 12 kVA must be reported to the grid operator; above that – practically every 22 kW box – it needs his approval before installation. Skip this and you risk a removal order and fines, and in the event of damage the insurer digs in too: an unregistered, improperly built installation is a gift for any benefit reduction. Registration is usually handled by the registered electrical contractor – who is mandatory anyway, because wallbox installation is no DIY project.
Mistake 2: Cross-section by gut feeling instead of calculation
An 11 kW wallbox draws a continuous 16 A on three phases – continuous load over hours, not the usual intermittent household duty. On long feeders to the garage, voltage drop therefore often decides, not heating: 15 m works with 5×2.5 mm², from about 25 m 5×4 mm² belongs in the wall. Wired too thin means: permanent heat in the cable, unnecessary losses with every single charge – and in the extreme case scorch marks. If you are laying new cable, go one size up right away; the extra cost of a few euros per metre pays for itself over decades of charging losses. Charging time, cross-section and registration duty in one pass are computed by the EV Charger Calculator (Charging Time, Cable, Fusing), ampacity in detail by the Cable Cross-Section Calculator.
Mistake 3: Skimping on protection
Every wallbox needs its own circuit with a matching breaker and RCD protection. The crux is DC fault current: during charging, DC fault currents can occur that blind a normal type A RCD – it then no longer trips reliably for other faults in the house either. What is needed is therefore either a type B RCD or – the usual route – a wallbox with integrated DC fault current monitoring (6 mA) plus a type A RCD. What is out of the question: clamping the wallbox onto an existing socket circuit or charging permanently via a household socket – a domestic plug is simply not built for 16 A continuous load.
The short pre-installation checklist
- Report/get approval (≤ 12 kVA report, > 12 kVA approve – the electrician handles it)
- Calculate the feeder: cross-section by length and voltage drop, dedicated circuit
- Clarify protection: type B RCD or box with DC monitoring + type A, matching breaker
- Consider load management: with several boxes or a weak service connection
Tick off these four points and you charge worry-free for decades – with clean paperwork at the grid operator and the insurer.
