Selecting Transformer

Selecting a Transformer

Single Phase

A single phase transformer is designed to transform single phase or three phase input (source) voltage to the single phase output (load) voltage required by your equipment. To select the correct single phase transformer, you must first determine:

  1. The equipment being installed operates on a single phase supply (see your equipment nameplate or installation manual).
  2. The Primary Voltage of the transformer. This is the same as the line input (or source) voltage, typically 480 or 600 volts AC.
  3. The Secondary Voltage of the transformer. The equipment being installed will have a specified supply voltage (see equipment nameplate or installation manual). The transformer you select must have a secondary voltage equal to the required supply voltage of the equipment, typically 120/240 VAC.
  4. The Frequency in hertz (cycles per second) of the input (source) voltage must be the same as the operating frequency of the equipment being supplied. The transformer selected must operate at the same frequency. Typical operating frequency is 60 Hz.
  5. The Total VA of the load is determined by the product of the voltage supplied across the load and the current passing through it. This is normally expressed in VA (Volt-Amperes) or kVA (kilo Volt-Amperes) on the equipment nameplate. The total load is often a combination of various loads (i.e. lights, heaters, motors). You must calculate these individual loads and add them together to obtain the total load of the transformer. The transformer you select must have a kVA rating equal to or greater than the load on the transformer.

How to use the full load chart to find kVA:


A) Determine the secondary voltage of your transformer.
B) Sum up the total amperes required by the load.
C) From the full load current table below, select a transformer under the corresponding secondary voltage, with a standard kVA capacity and amperage equal to or higher than the sum required by the load.

Calculating kVA


To calculate kVA when volts and amperes are known:

Single Phase Equation


Full Load Current Table - Single Phase Transformer


Single Phase AC Motor Full Load Running Currents in Amperes and Recommended Transformer Ratings





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Three Phase

A three phase transformer is designed to transform a three phase input (source) voltage to the single phase and three phase output (load) voltages required by your equipment.

In order to select the correct three phase transformer you must first determine:

  1. The equipment being installed operates on a three phase supply. Note: If both single phase and three phase equipment makes up the load, the single phase and three phase equipment makes up the load, the single phase equipment is connected to only one phase of the transformer.
  2. The Primary Voltage of the transformer. This is the same as the line input (or source) voltage, typically 480 or 600 volts AC.
  3. The Secondary Voltage of the transformer. This is the transformer's output voltage and must bt the same as the voltage required by the equipment being installed (see equipment nameplate, typically 208Y/120 volts).
  4. The Frequency in hertz (cycles per second) of the input (source) voltage must be the same as the operating frequency of the equipment being supplied. The transformer selected must operate at the same frequency. Typical operating frequency is 60 Hz.
  5. The Total VA of the load is determined by the product of the voltage supplied across the load and the current passing through it. This is normally expressed in VA (Volt-Amperes) or kVA (kil Volt-Amperes) on the equipment nameplate.

The total load is often as combination of various loads (i.e. lights, heaters, motors). You must calculate these individual loads and add them together to obtain the total load of the transformer.

The transformer you select must have a kVA rating equal to or greater than the load requirement. Note: The three phase transformer must be selected so that any one phase is not overloaded. If you are connecting a single phase load to one phase of the three phase transformer, you must calculate the load as if it were loading all three phases.

Calculating kVA


To calculate kVA when volts and amperes are known:

Three phase equation?


Full Load Current Table - Three Phase Transformer


Three Phase AC Motor Full Load Running Currents in Amperes and Recommended Transformer Ratings





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