BACnet/IP or Modbus TCP — when does each one win?

BACnet/IP is the default for HVAC and lighting estates — the schema fits, the discovery story is good, and most BMS vendors implement it natively. Modbus TCP wins where the equipment vendor is electrical-first: power monitoring, switchgear, UPS gear, generators. Pretending one protocol fits everywhere is where projects end up with brittle middleware nobody wants to maintain.

A workable rule

• BACnet/IP for HVAC, lighting, life-safety adjacencies.
• Modbus TCP for power monitoring, switchgear, UPS, generators.
• OPC UA at the OT boundary when you're crossing into process or SCADA estates.

Whichever protocol owns a measurement, name it canonical in the data model. Don't let two protocols carry the same point.