Engineering notes
Choose a DDC from the I/O count
Size a DDC from analog and digital points, spare capacity and the protocol. Do not buy by cabinet size.
Count points before you pick a box
A controller is sized by analog in, analog out, digital in and digital out — not by the size of the enclosure. Buy a small CPU and you add modules later at a higher unit price, or you run out of a family that no longer stocks that rack.
What to count
- Every sensor and every valve on the drawings
- Dry contacts: breakers, pumps, fans, generators
- Spare 10–15% on each type — not one spare on the whole panel
Then lock the family (modules must belong to it) and the trunk: BACnet/IP, MS/TP or Modbus. BMS equipment · 24 V supply