How to Determine What Size Transformer Is Needed for a Service Based on Voltage?

Transformer sizing depends on the power requirements of the electrical load. Electrical loads require power in the form of watts and the transformer must support the required kilovolt-amperes, or "KVA," necessary to deliver watts to the load. KVA readings indicated the units of power delivered by the power company to the transformer. For this reason, transformers are sized in units of KVA.


    • 1

      Determine the input voltage, or "Vin" needed by the service or electrical load. As an example, assume Vin is 120-volts.

    • 2

      Find or measure the input impedance of the electrical load. Call that value "Z." As an example, assume Z = 30 ohms.

    • 3

      Calculate the current requirements, or "I," associated with the service using the formula I = V/Z. Continuing with the example:

      I = 120/30 = 4 amps

    • 4

      Calculate the kilowatts power required by the service using the formula KW = (V x I)/1000. Continuing with the example:

      KW = (120 x 4)/1000 = 480/1000 = 0.48 watts

    • 5

      Calculate the kilovolt-amperes, or "KVA," required to deliver the KW to the load using the formula KVA = KW/0.8. Continuing with the example:

      KVA = 0.48/0.8 = 0.6 KVA

    • 6

      Pick a transformer that can deliver the KVA from Step 4. For example, transformers come in standard KVA sizes such as 1 KVA, 5 KVA, 10 KVA, 15 KVA, and so on. In the example, a 1 KVA transformer will be adequate.

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