Power Supplies for New York BAS Retrofits

Power supplies for New York BAS retrofits provide the low-voltage control power that controllers, sensors, and interposing relays need, in a prewired enclosure rather than as a build to be assembled in the field. Functional Devices, Inc. manufactures RIB® (Relay-In-a-Box®) prepackaged power supplies alongside its relay and current-sensor lines. On retrofit work in New York’s older buildings, where panel space and clean control power are often constraints, a prepackaged power supply can meaningfully reduce the amount of field work necessary to complete a job.

What a Prepackaged Power Supply Replaces

Building automation systems run on low-voltage control power; controllers, sensors, and the coils of interposing relays all need a stable 24V supply. In the field, this means mounting a separate transformer and breaker, wiring the breaker into the transformer primary, and landing the secondary on the control circuits. A prepackaged supply delivers the same result in one enclosure, with transformer and breaker included. On a retrofit where the existing panel is crowded and a ground-up rebuild isn’t feasible, consolidation is the practical solution.

RIB® Prepackaged Power Supplies

The PSH500AB10-LVC is a 500 VA supply with five isolated 100 VA, 24 Vac Class 2 secondary outputs and a low-voltage compartment (LVC). The main breaker and switch are on the outside, with a convenience outlet on its own 4A breaker. Each secondary circuit has its own 4A circuit breaker, an on/off switch, an LED indicator, and a pluggable terminal. Its two-piece lid separates the high-voltage side from the low-voltage circuits.

The PSMN24SB4 adds prepackaged circuit-breaker protection for Class 2 loads and mounts individually or snaps next to RIBMN-series relays and PSMN-series power supplies in a panel.

Selecting a supply depends on the total current the connected controllers, sensors, and relay coils will draw, and the number of isolated secondary circuits the panel needs.

Budgeting Secondary Circuits on a Retrofit

Isolated secondary circuits are advantageous both during commissioning and later service. Each secondary on a PSH500AB10-LVC is independently switched and breakered, so a device group can be isolated for testing without dropping power to the rest of the panel. On a retrofit where a controller, a set of sensors, and a bank of relay coils are added at different stages, each group can be energized and verified independently as it goes in.

The secondary circuits on the low-voltage side are Class 2, which sets limits on power and governs how the low-voltage wiring is run and separated from line-voltage conductors. In this case, the separation is physical: Class 2 circuits sit apart from the high-voltage side, so a technician commissioning or troubleshooting the control wiring is not working next to line voltage. In an older New York panel where space is tight and existing wiring is dense, that separation reduces both complexity and risk.

Headroom is another consideration. Sizing a supply with only marginal capacity above the connected load leaves no room for a later addition, but control systems tend to grow as a building adds monitoring and staged equipment, especially under New York City's Local Law 97. Specifying a supply with capacity beyond the current load, and secondary circuits beyond the current device count, keeps the next addition from requiring a second supply.

Frequently Asked Questions

How do I size a prepackaged power supply?

Add up the current drawn by the connected controllers, sensors, and relay coils. The supply should then be sized above that combined load, plus additional headroom, with enough isolated secondary circuits to support the panel layout. The PSH500AB10-LVC provides 500 VA across five isolated 100 VA Class 2 circuits, each with its own breaker.

Why use a prepackaged power supply instead of a separate transformer?

A prepackaged power supply includes the transformer and breaker in one prewired enclosure, which removes the extra step of assembling a separate transformer, breaker, and interconnecting wiring in the field.

What does a low-voltage compartment do?

The low-voltage compartment (LVC) keeps Class 2 secondary circuits physically separated from anything high-voltage, so a technician can safely access the control circuits for commissioning or troubleshooting.

What does Class 2 mean for a power supply?

Class 2 refers to a power source that is limited in the power it can deliver, so that it does not present a fire or shock hazard under normal conditions. Class 2 circuits are governed by lighter wiring rules than line-voltage circuits. The secondary circuits in RIB® prepackaged power supplies are Class 2, and building automation controllers and sensors are designed to run on Class 2 power.

Why is BAS control power 24V?

24V is low enough for Class 2 treatment, which allows for lighter wiring rules and reduces shock risk for a technician working in a live panel. It is also the voltage that most controllers, sensors, and relay coils are built to accept, so a single supply can feed a mixed set of field devices.

Can I run multiple devices from one power supply?

Yes, within the supply's rated output. Add up the current drawn by all connected controllers, sensors, and relay coils, and keep the total within the supply's capacity. A supply with multiple isolated secondary circuits, such as the PSH500AB10-LVC, lets each device group run on its own breakered circuit. If one breaker is triggered, the others will not be affected.