In industrial and utility power systems, control transformers play a critical role in supplying power to control circuits and instrumentation. One of the most important characteristics of a control transformer is its operating voltage. Despite the high or medium voltages often present in the overall system, the transformer itself operates at a low secondary voltage, which is why control transformers are classified as low voltage (LV) devices.
Here, we explain this voltage relationship is essential for proper system design, integration, and safety.
Control Transformers Are Classified as Low Voltage Devices
By definition, a control transformer is used to step down voltage from a system’s primary level to a much lower, safer voltage for use in control and instrumentation circuits. These secondary voltages are typically:
- 24V AC
- 120V AC
- 208V AC
- 240V AC
Because the output voltage remains under 1,000 volts, control transformers fall under the low voltage (LV) classification, regardless of the voltage level on the primary side. This output powers critical components such as relays, contactors, PLCs, sensors, and HMI interfaces.
What Happens When Used in MV or HV Systems?
It’s a common misconception that if a transformer is installed in a medium voltage (MV) or high voltage (HV) environment, the transformer itself must be MV or HV. In reality, control transformers always operate at low voltage on the secondary side. The difference lies in the primary voltage rating required to interface with the rest of the system.
Here’s how that plays out:
Primary Voltage Inputs May Be LV, MV, or HV:
- Low Voltage (LV) Systems:
Primary voltages in the range of 208V, 480V, or 600V are common. The transformer steps this down to 120V or 24V for control. - Medium Voltage (MV) Systems (1 kV to 35 kV):
Control transformers are built with primary windings rated for voltages like 4.16 kV or 13.8 kV. The transformer still outputs LV—commonly 120V or 240V—to support control logic. - High Voltage (HV) Systems (>35 kV):
In HV utility environments, the transformer may interface with HV switchgear or instrument transformers. Even here, it performs the same core function: providing safe, isolated low-voltage power to control, protection, and monitoring equipment.
Proper Sizing and Rating Needed
Even when part of a medium or high-voltage system, a control transformer is still a low-voltage component in its output and function. It must be sized and rated appropriately for the primary voltage of its environment, but its role is consistent: deliver safe, low-voltage power to control circuits with high reliability and electrical isolation.
Real-World Application Scenarios
- Industrial Plants: Facilities with 4.16 kV or 13.8 kV MV distribution systems use control transformers with MV-rated primaries to step down to 120V AC for motor controls and interlocks.
- Utilities and Substations: In HV substations, control transformers interface with voltage transformers or breakers and support SCADA, protective relays, and alarms—all of which require low-voltage supply for reliable operation.
- OEM and Skid-Mounted Equipment: Manufacturers often build MV-capable control transformers into modular equipment, allowing seamless integration with MV supply while still providing the standard LV control power.
Summary
A control transformer is typically classified as low voltage (LV) because it is designed to step down voltage for control circuits, instrumentation, or industrial applications. These transformers usually operate at secondary voltages below 1 kV, commonly in the range of 24V, 120V, 208V, or 240V.
However, if a control transformer is used in a medium voltage (MV) or high voltage (HV) system, it is part of a larger electrical system but still operates at LV levels within the control circuit. The primary voltage of such a transformer could be in the MV range (e.g., 4.16 kV, 13.8 kV) or even HV in utility applications, but its function remains as a low-voltage power supply for control equipment.
Need Help Sourcing a Specific Control Transformer?
At VCM Solutions, we provide application-specific transformer solutions engineered for performance, compliance, and long-term dependability. Whether you’re designing for LV, MV, or HV systems, our team ensures your control transformer selection is aligned with electrical codes, safety standards, and operational demands.
Contact VCM Solutions today to consult with a technical expert.