Power supplies for BAS retrofits provide 24 V control power in buildings that were not designed for controls. Chicago’s commercial stock spans over a century of construction, and distribution voltage varies by building, so the power for every controller, actuator, and relay added in a retrofit must be taken from the building’s existing distribution. Functional Devices, Inc. manufactures prepackaged power supplies for building automation, including multi-tap models built for this situation.
Control Power in a Retrofit
In new construction, control power can be designed to fit standard voltage requirements: the panel schedule accounts for it from the start. However, in a retrofit, it has to be derived from circuits that are already in the building. In each location where new devices cluster, before any point is wired, it must be determined which existing circuit supplies the 24 V.
Distribution Voltage in Chicago’s Older Stock
Chicago’s commercial stock of buildings was built across a century, and its power distribution reflects that. The circuit available near a new controller may be 120 Vac in one electrical closet, 277 Vac from a lighting panel on another floor, and 480 Vac in the mechanical room. With a fixed-input transformer, each of those locations is a separate selection: a different part, ordered and stocked separately, and a separate opportunity for a mismatch.
Matching the Power Supply to the Building
Two product characteristics remove most of the location-by-location constraints. The first is the multi-tap primary: the PSH series accepts 120, 240, 277, or 480 Vac on the same transformer, so the supply installed in a 120 V closet and the one installed on a 277 V lighting panel are the same stocked item, just connected at different taps.
The second is the packaging. A control power circuit assembled from components requires a transformer, circuit protection, and an enclosure, all sourced, mounted, and wired separately. A prepackaged supply contains the transformer, breakers, and outputs in one enclosure and mounts as a unit.
Sizing the Power Supply by Output Count
Each output on a PSH series supply is a Class 2, 100 VA, 24 Vac circuit. Each controller, actuator, or relay coil assigned to a circuit has a rated draw in VA; the draws on a circuit, when added together, must stay within the 100 VA rating. The panel’s device count determines how many circuits are required. A supply with more outputs than the initial build requires can then have leftover circuits for any points added later, which retrofit work tends to produce.
PSH Series Models
- The PSH500A provides five Class 2, 100 VA, 24 Vac outputs from the multi-tap primary, in a NEMA 1 metal enclosure. It is sized for multi-controller panels such as VAV retrofit work, where several devices draw from one location.
- The PSH300A provides three 100 VA Class 2 outputs from the same 120–480 Vac input range, for smaller panel builds.
- The PSH500AB10-LVC adds a low-voltage compartment that separates the Class 2 wiring from the high-voltage side of the supply. Each secondary circuit is isolated, with its own 4 A breaker, on/off switch, LED indicator, and pluggable terminal, so an individual circuit can be worked on or verified without opening the high-voltage side.
Frequently Asked Questions About BAS Power Supplies
What voltage do BAS devices run on?
Most BAS controllers, actuators, and relay coils in commercial work run on 24 V power, stepped down from the building’s distribution voltage by a control power supply or transformer.
What does a multi-tap primary do?
A multi-tap primary accepts more than one input voltage on the same transformer: the PSH series taps at 120, 240, 277, or 480 Vac. The installer lands the tap that matches the available circuit, so one stocked part covers locations with different distribution.
What is a Class 2 output on a power supply?
The PSH series outputs are UL Class 2 circuits at 100 VA each. Class 2 is a power-limited circuit classification, and the 100 VA output rating is the sizing number for the devices the circuit will feed.
Why separate low-voltage and high-voltage wiring in the supply?
The PSH500AB10-LVC’s low-voltage compartment keeps the Class 2 circuits behind their own cover, apart from the high-voltage side. A technician commissioning or troubleshooting a control circuit works in the low-voltage compartment without exposing the primary wiring.
Where can I buy BAS power supplies in Chicago?
RIB® prepackaged power supplies are stocked by authorized Functional Devices distributors in Illinois. A distributor can confirm availability of a specific model and output count for the panel.
Source BAS Power Supplies in Illinois
Functional Devices manufactures the prepackaged power supplies used for control power in BAS retrofit work. To specify a supply, browse the power supply lineup or find an authorized Functional Devices distributor in Illinois.