Engineering notes
24 V for a DDC panel: add the load before you buy
Add controller, sensor, valve and relay current before you buy a 24 V supply. A short supply leaves valves mid-stroke.
A panel can have enough modules and still die on the supply
Controllers, 0–10 V sensors, modulating valves and interposing relays usually run on 24 V. Buying a supply that “fits the box” without adding milliamps leaves valves mid-stroke and sensors jumping when several valves pull at once.
- Controller and module current from the maker’s sheet
- Each active sensor (0–10 V, loop-powered 4–20 mA)
- Each actuator: holding and inrush — use the larger number
- Relays, lamps, a modem if they share the same supply
Add them, then apply about 1.3 spare. 24 V AC and 24 V DC are not interchangeable if the valve sheet names one. Split valve power from CPU and sensor power when the valve current is large. Interposing relays