Most electronic equipment manufacturers specify nominal nameplate ratings for their products. Typically, these devices and/or systems are designed to operate within a ±10% voltage window. But voltages can exceed the boundaries of this window, and when they do, most engineers have little flexibility in adjusting voltage supplies. Instead, they end up with processes or equipment that are sensitive to voltage sags. Complications arise when a process involves multiple devices with different voltage requirements. Fortunately, a relatively simple and inexpensive approach involving buck-boost transformers can resolve these voltage mismatch problems and improve sag tolerance.

To give you an idea how voltage windows vary with different equipment, consider these examples:

  • A 230V motor has a 207V to 253V operating window.
  • A computer power supply rated at 120V has an operating window of 108V to 132V.
  • A programmable logic controller (PLC) rated at 115V has a 104V to 127V operating window.


If your goal is to take a piece of equipment and match the voltage with its nameplate rating, install one or more buck-boost transformers at the equipment terminal or the panel feeding the equipment.

As you can see in Fig. 1, a typical buck-boost transformer consists of dual primary and secondary windings. The identical primary windings come in 120V, 240V, and 480V ratings, while the secondary windings come in lower voltages of 12V, 16V, 24V, and 48V.

The Autotransformer Setup

You can field-connect a buck-boost transformer as an autotransformer, as shown in Fig. 2, on page 32. This configuration will decrease (buck) or increase (boost) the supply voltage from 5% to 20%, depending on the way you connect the primary and secondary windings. Only the secondary windings carry load current; therefore, the buck-boost transformer can supply a load rated 10 times higher than the kVA nameplate rating. And although buck-boost transformers are single-phase, you can apply them to most 3-phase equipment by matching three single-phase transformers.

With the autotransformer configuration, you can do two things. First, you can match existing utilization voltages to equipment voltages. For example, pieces of equipment built by overseas manufacturers usually require voltages different from those available in the United States. A buck-boost/autotransformer setup can match this load to an existing power supply. This would be smaller and considerably less expensive than a step-up or step-down isolation transformer.

Second, you can use a buck-boost transformer in the boost configuration to increase the voltage at the terminals of any piece of equipment that seems highly sensitive to voltage sags. Anytime the voltage supplied to such equipment is at or near the low end of the nominal voltage range in the American National Standards Institute's (ANSI) Standard C84.1, the equipment will be more prone to tripping during sags (see Table 1). Simply increasing the utilization voltage at the equipment terminals will help resolve many of these nuisance trips.

An added benefit to using buck-boost transformers is that you can treat only those loads that need it. Consider using this configuration when one of the following situations exist:

  • Equipment nameplate voltages do not match supply voltages available at the point of use. For example, a 230V motor is connected to a 208V supply.
  • Pieces of equipment (e.g., high-intensity discharge (HID) lighting, motor contactors, and other process controllers) drop out during minor voltage sags, while other devices ride through the sags.
  • Pieces of equipment, powered by long cables, shut down during voltage sags.
  • Utilization voltage nears the low end of the Standard C84.1 range. For example, the voltage at the terminals of a 120V motor-control center measures 108V.


An alternative method of modifying utilization voltages is to adjust tap changers on existing step-down service transformers. This method, however, affects terminal voltages throughout the plant, potentially increasing voltages at equipment that do not require higher voltages. Such a wholesale increase in utilization voltage may make loads, which are not vulnerable to voltage sags, vulnerable to overvoltages, possibly resulting in insulation stress, overheating, and component failure.

Selection Steps

Sizing a buck-boost transformer depends on your reason for installing it. If you want to match available voltages with nameplate ratings, follow these selection steps:

Step 1. Measure the available voltage at the equipment terminals.

Step 2. Read the nameplate to determine the voltage rating and current.

Step 3. Subtract the available voltage from the voltage rating to determine a buck-boost value.

Step 4. Select a voltage rating for the transformer's secondary windings that are as close as possible to the buck-boost value calculated in Step 3. For example, some PLCs originating from overseas markets are rated at 100V. The difference between the available utilization voltage (120V) and the equipment voltage is -20V. Therefore, select a buck-boost transformer with two 120V primary windings and two 16V secondary windings (field-connected in parallel) to buck the voltage down to about 104V.

(Caution: The buck-boost transformer's load current should not exceed the rating of the secondary windings. If the windings are connected in series, the current rating of the secondary windings is equal to the rating of a single secondary winding. Connecting the windings in parallel doubles the rated current.)

Step 5. Determine the rating of the buck-boost transformer by either using the nameplate data or adding a 10% safety margin to the existing steady-state current draw, as measured with a true rms meter.

Step 6. If the remainder from Step 3 is positive, install the buck-boost transformer in a boost configuration according to the manufacturer's instructions. If the remainder is negative, install the transformer in a buck configuration.

(Caution: Before boosting voltage to the upper end of the Standard C84.1 range, consider the sensitivity of the equipment to different types of overvoltages. Some devices, such, as adjustable-speed drives (ASDs) are highly sensitive to voltage swells and capacitor-switching transients and can trip more frequently when the utilization voltage is boosted to the upper limit of the range.)

When you need to select a buck-boost transformer to increase sag tolerance, follow these four steps and refer to Table 2, on page 32, for an example:

Step 1. Install a monitor at the point of use to record the voltage during normal operation and the times when voltage sags cause the equipment to drop out or malfunction.

Step 2. Determine the amount of voltage boost needed to achieve a utilization voltage at or slightly above the rated voltage, but lower than the maximum voltage in Table 1. For example, HID lighting designed to operate at 208V is often sensitive to voltage sags that do not upset other equipment. Let's say the voltage at a lighting panel measures 195V during normal conditions. During the start-up of a large compressor motor, however, the voltage measures only 172V, causing the lights to go out. To keep the lights on during the motor start-up, you can increase the voltage at the panel by 16V and still be within the C84.1 range for a 208V circuit.

Step 3. Read the nameplates of all the pieces of equipment that connect to the transformer to determine the voltage and current ratings. If you can't obtain this data, measure the current for all loads that connect to the transformer. Add the current ratings or current measurements together, then calculate the kVA load by multiplying the nominal load voltage by the load current.

Step 4. Determine the rating of the buck-boost transformer by either using the nameplate data or adding a 10% safety margin to the existing steady-state current draw, as measured with a true rms meter. Then, multiply the current rating by the required buck or boost voltage in Step 2 to calculate the transformer kVA. (Note: In Table 2, the kVA requirement of the transformer load in Step 3 is much larger than the calculated kVA rating of the transformer.)

Pre-Installation Precautions

As always, there are a number of precautions worth mentioning before installing a buck-boost transformer and assuming all is well. Here's a list of what to watch for:

  • You may need to resize overcurrent protection devices after using a buck-boost transformer because of the increased utilization voltages. Always follow Section 450-4 of the National Electrical Code when doing so.
  • Utilization voltages at equipment terminals may increase during facility shutdowns (e.g., nights, weekends, and holidays). Voltages at the equipment terminals should never exceed the maximum utilization voltages in Table 1. Consider wiring buck-boost transformers so they can be bypassed or turned off during facility shutdowns or low production.
  • Follow the installation instructions and connection diagrams provided by the manufacturer.
  • Check the output voltage and phase rotation of a buck-boost transformer after any electrical modification.
  • Because the kVA rating of a buck-boost transformer will not match the kVA rating of its load, you must clearly indicate any buck-boost modifications on all electrical drawings and panel boards.


Using a buck-boost transformer allows you to cost effectively treat specific loads requiring voltage adjustments and improve sag tolerance. Ultimately, this means you'll provide your facility with the potential for increased equipment reliability and improved process uptime.

Doug Dorr is the business development manager for EPRI PEAC, located in Knoxville, Tenn. You can reach him at ddorr@epri-peac.com.