Adding EV charging to an existing building without upgrading the main supply is one of the most common challenges in 2025 real estate. The solution is load management: dynamically limiting charger output based on total building consumption — and KNX plus Modbus is the right tool for this.

The problem: shared supply capacity

A typical EU apartment block has a 63A or 100A three-phase main supply. Add five 11kW EV chargers and you potentially draw 55kW — easily exceeding the main fuse. The solution is dynamic load management: each charger's output is limited so the total never exceeds the available capacity.

How KNX load management works

1. A Modbus energy meter on the main supply measures real-time current draw (all three phases). 2. The KNX system (via Modbus gateway or Home Assistant) reads the current draw. 3. Available headroom = main fuse (e.g. 100A) minus current load. 4. This headroom is distributed across active chargers. If three chargers are active and headroom is 36A, each gets 12A max. 5. Commands are sent to chargers via their Modbus or OCPP interface: set maximum current to X.

Charger protocols: Modbus vs OCPP

Most professional EV chargers support Modbus RTU/TCP for load management (Easee, ABB Terra, Alfen, Keba). Modbus register 1000 (typically) accepts the maximum current command. The KNX-Modbus gateway or HA sends the command. Some chargers also support OCPP — a cloud-based protocol for billing and fleet management. For simple load management, Modbus is simpler and more reliable.

Example: MDT SCN-RT4.01 + Modbus

In ETS, configure the MDT Modbus gateway to read the main energy meter (register 3004 for current phase 1, Schneider iEM3155). Set up a KNX logic block (MDT Logikmodul) to calculate available headroom: (100A − measured_current) / active_chargers. Write the result to a KNX DPT 9.021 object. The gateway translates this to a Modbus write to the charger's current-limit register.

Home Assistant alternative

If you use HA: the energy_management blueprint reads the Modbus meter, calculates headroom, and sends Modbus writes to chargers. More flexible than KNX logic, easier to modify. Less deterministic than dedicated hardware.

Multiple chargers in an apartment block

For 10+ chargers, consider a dedicated load management system (AMPECO, Charge Point Management System) that uses OCPP and handles billing as well. For 2–5 chargers, KNX/Modbus/HA is cost-effective and reliable.