Low voltage supplies for HVAC and BAS panels step down line voltage to a usable control voltage, typically 24 VAC or 24 VDC. Proper sizing is essential for reliable performance across the relays, controllers, and sensors that depend on them.
Selecting and applying the right control transformer or power supply is one of the more consequential decisions in a BAS or HVAC panel build. Get it right and the panel runs reliably without issue. Size it short or apply it incorrectly and the symptoms—chattering relays, controller resets, drifting sensors—can be time-consuming to track down. This guide covers the fundamentals: what these devices do, how to size them, and what to watch out for in the field.
What Control Transformers and Power Supplies Actually Do
In most commercial control systems, line voltage (120/208/240/277 VAC) must be stepped down to a safer, usable control voltage. There are two ways this happens, depending on the application:
Control Transformer (AC to AC): Steps down line voltage to 24 VAC for thermostats, relays, contactors, and many controllers
Power Supply (AC to DC): Converts line voltage (or 24 VAC) to regulated 24 VDC for controllers, sensors, and networked devices that require stable DC
Functional Devices offers both categories, designed for panel builders, contractors, and BAS techs who need consistent output, straightforward mounting, and dependable protection. Browse the full transformer and power supply lineup to find the right fit for your application.
AC vs. DC in the Real World
Both voltages often live side by side in the same panel, serving different purposes.
A voltage feed of 24 VAC is common for legacy HVAC controls, zone valves, and relay coils. It's widely compatible and easy to distribute. A voltage feed of 24 VDC is preferred for modern BAS controllers, sensors, and communication modules, offering tighter regulation and better performance for electronics.
A typical panel might use a control transformer to feed 24 VAC loads alongside a separate DC power supply for the controller and network devices. Keeping them distinct in your design and labeling helps avoid cross-connection issues down the line.
Sizing Control Transformers: Getting VA and Current Right
Undersizing is the fastest path to nuisance problems. Oversizing isn't dangerous, but it wastes space and budget. The goal is to size with enough headroom to handle steady-state loads and inrush.
Step 1: Add Up the Loads
For AC systems, total the VA (volt-amps) of each device. For DC systems, total the current (amps).
Example (24 VAC):
- Thermostat: 3 VA
- Two relays at 2 VA each: 4 VA
- Damper actuator: 10 VA
- Total: 17 VA
Example (24 VDC):
- Controller: 0.5 A
- Sensors (4 × 0.05 A): 0.2 A
- Communication module: 0.2 A
- Total: 0.9 A
Step 2: Add a Safety Margin
A good rule is 25–30% headroom for future devices and inrush.
- AC example: 17 VA × 1.3 ≈ 22 VA → select a 40 VA transformer
- DC example: 0.9 A × 1.3 ≈ 1.2 A → select a 24 VDC, 1.5 A or 2 A supply
Step 3: Consider Inrush and Diversity
Actuators, contactors, and some controllers draw more current at startup. If multiple devices can start simultaneously, account for that spike. It’s wise to size the transformer VA to any specified inrush VA. When in doubt, step up one size.
Step 4: Check Temperature Rise and Enclosure Limits
Heat is a real derater. In tight panels or high ambient conditions, choose a higher VA/current rating or provide ventilation. Both transformers and power supplies lose effective capacity as temperatures climb.
UL Listing and Why It Matters for Control Transformers
Specifying UL listed control transformers isn't just a box to check—it reduces risk and simplifies the approval process.
UL listing confirms the product meets recognized safety standards for insulation, temperature rise, and construction. Inspectors and AHJs expect listed components in control panels, and listed products perform consistently across installations. Functional Devices designs transformers and power supplies to meet these requirements, which helps keep commissioning straightforward.
Primary Voltage and Tap Selection
Not every jobsite delivers the same primary voltage. Common inputs include 120, 208, 240, and 277 VAC.
A single-primary transformer is simpler but less flexible if site voltage changes. A multi-tap primary gives you one transformer with multiple input options—useful for panel builders who standardize SKUs across projects.
Correct tap selection matters. Mis-tapping a transformer can push secondary voltage out of range, causing overheating or under-voltage issues that are difficult to diagnose after the fact. For a deeper look at common wiring pitfalls, the 4 Essential Things to Know About Transformers post covers several of the mistakes that come up most often in the field.
Distribution: One Larger Transformer or Several Smaller Ones?
Both approaches work. The right choice depends on layout, load types, and serviceability.
One larger transformer means fewer components and simpler wiring, but it might create a single point of failure. Long secondary runs can also introduce voltage drop.
Multiple smaller transformers make it easier to isolate loads and troubleshoot, but add components and panel space.
A common best practice is to separate "dirty" loads (actuators, contactors) from "clean" electronics (controllers, sensors). Use a dedicated DC supply for electronics and keep AC loads on their own transformer.
Voltage Drop and Wire Sizing
Even at 24 volts, distance matters. Keep secondary runs as short as practical, and upsize conductors for longer runs to limit drop. Measure under load during commissioning—a system that reads 24 VAC at no load can sag to 21–22 VAC when everything energizes.
For DC systems, voltage drop can affect communication reliability, which is one reason many BAS designers prefer regulated DC for controllers and network devices.
Protection and Grounding
Protecting the transformer and the loads it feeds requires attention at both ends of the circuit.
For primary protection, use a fuse or breaker sized to the transformer rating. On the secondary side, inline fusing for branch circuits prevents a single fault from taking down the entire control bus—the transformer's built-in breaker is there to protect the transformer, not the load. For grounding, follow manufacturer guidance. Proper grounding reduces noise and improves stability, especially in mixed AC/DC panels.
Mounting and Panel Workflow
Mounting style and labeling have a real impact on how long installation takes and how easy the panel is to service later.
Choose mounting styles that match your build: foot mount, DIN rail, or enclosure-ready units. Prewired assemblies reduce field terminations and the likelihood of wiring errors. Clear labeling of primary taps, secondary outputs, and AC vs. DC circuits saves time for anyone who works on the panel after initial installation.
Functional Devices components are built with panel workflow in mind: consistent terminals, readable markings, and configurations that match real BAS panel layouts.
Common Control Transformer Mistakes to Avoid
- Undersizing VA/current and skipping headroom
- Mixing AC and DC circuits without clear separation
- Ignoring inrush for actuators and contactors
- Long secondary runs without accounting for voltage drop
- Skipping secondary fusing and turning small faults into larger outages
- Incorrect primary tap selection
Each of these tends to show up later as intermittent problems: hard resets, nuisance trips, or devices that work inconsistently.
Control Transformer Selection Checklist
Before finalizing your bill of materials, confirm:
- Voltage type: 24 VAC, 24 VDC, or both
- Total load: VA (AC) or amps (DC), with 25–30% margin
- Primary voltage: 120/208/240/277 VAC and tap selection
- Environment: ambient temperature, enclosure space, ventilation
- Protection: primary breaker/fuse and secondary branch fusing
- Distribution Plan: single vs. multiple supplies/transformers
- Compliance: UL listed components
Work through those seven points, and the panel should behave the way it's supposed to.
Control Transformer and Power Supply Solutions from Functional Devices
Functional Devices supports contractors and panel builders with a broad range of control transformers and power supplies designed for BAS and HVAC applications—reliable output, clear labeling, and installation-friendly configurations.
Need help selecting the right transformer or DC supply for your panel? Use the Functional Devices Distributor Locator to find a supplier near you, or contact Functional Devices directly for product selection guidance.