HVAC Control Transformer BankHVAC Control Transformer Bank - This bank of transformers is being used to power several VAV box controllers. One of many VAV boxes is powered by one of these transformers. In most VAV box applications, the power for the control system is at the box and a properly sized transformer for VA load is used to power the controller. In this application, the VAV boxes were not fan-powered and line voltage was not available in the boxes. So a central transformer bank was used and a wire for control power was pulled to each VAV box.

HVAC Control Transformer Bank

Using the manufacturer’s rating of the DDC controller and the power needed to drive all the actuators (VA rating listed on the actuators) the entire load was calculated and the appropriately sized transformers were used. The proper circuit design is necessary to prevent the overload of the transformer.  In this case, the actuators and other loads are added to the entire circuit load and then the transformers are assigned to a specific amount of loads in the circuit based on transformer VA Rating.

If the load demand for the transformer exceeds the transformer’s VA rating the controllers may not work properly and the transformer will overheat and eventually fail. With the number of transformers in this panel, a small panel fan was necessary to remove the heat from inside the panel. Transformers generate a lot of heat and it is important to have a method to keep them cooler. The panel fan was nothing more than the same fans that keep desktop PCs cooled off.

HVAC Control Transformer Bank - Circuit Protection

Additionally, each panel with control transformers inside them had a central fuse to protect the transformers from being blown in case of a dead short. As transformers can easily be blown with dead shorts it is prudent to have fuse protection in the circuit especially when the circuit is so complex and long stringing through a plenum to each VAV Box. Of course, all the wire was in conduit but that does not mean a dead short is not possible. It just means the wire is protected somewhat.

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HVAC Control Transformer Bank