Control Transformer Selection Tips

Control Transformers Definition:

Control Transformers are designed to provide a stable secondary voltage at high initial energizing or inrush currents typically 5 to 15 times normal operating load current and are virtually always single phase. They are also called “CPTs”, Control Power Transformers, High Inrush Transformers, Step-Down and Power Transformers.

Control Transformer Selection Tips:

1.  If existing replacement try to obtain the manufacturers part number

2. If competitive part number is not available or the unit is not available you will need the following information:

  • Primary Voltage (input): the voltage available from the electrical distribution system and its operational frequency, which is connected to the transformer supply voltage (H) terminals. Single Phase primaries are typically expressed as: 120V, 208V, 230V, 277V, 480V. The voltage potential measured between the two wires you intend to connect to the transformer determines the primary voltage connections chosen.
  • Secondary Voltage (output): the voltage required for load operation, which is connected to the transformer load voltage (X) terminals. Single Phase secondaries are typically expressed as: 24V, 120V, 240V
  • The following values are necessary to determine the load rating:
    • Inrush VA is the product of the load voltage (V) multiplied by the current (A) that is required during startup. It is calculated by adding the inrush VA requirements  of all devices (contactors, timers, relays, pilot lights, solenoids, etc.), which will be energized together. Inrush VA requirements are best obtained from the component manufacturer.
    • Sealed VA, also called Steady State VA, is the product of the load voltage (V) multiplied by the current (A) that is required to operate the circuit after initial start-up or under normal operating conditions.  It is calculated by adding the sealed VA requirements of all electrical components of the circuit that will be energized at any given time. Sealed VA requirements are best obtained from the component manufacturer.
    • Volt-Amps (VA) & AC Watts
    • Kva, equal to (Volts X Amps)X1000

Tip: If amp load and voltage is known then Volts X Amps = VA. If VA and Volts are known then VA /_ _Volts = Amps, AC Watts = VA X Power Factor

3. Sizing the transformer

  • Take all the components that may be energized simultaneously and add their listed (they are always listed by the mfr) energizing currents (Inrush VA) and then add to that the running current of any device that may be already operating. Take that value and find a comparable value on the Regulation Data Chart (below) under the 90% column. Alternately, the √((inrush VA)² _+ (sealed VA)²) formula can be used.
  • Make sure when selecting that all the items sealed VAs together also agrees with the minimum VA of the selected transformer.
  • The values in the table are finite. If your calculated value exceeds a listed valued selected the next largest.


Control Transformer Selection Tips

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If you are still not finding the right transformer to meet your needs visit our Ask An Expert page and submit your specifications. Our expert will respond to your inquiry within 24 hours. If we do not currently stock the transformer size you need you can request that we put it in stock by filling out the form on our Contact Us page.