 How to Calculate the Required Capacity kVA Rating or Amperage Capacity for Single and Three Phase Transformers

FA101694

17/02/2018

Issue:
Calculation of kVA capacity for a Single or Three Phase Transformer, based on Winding Voltage and Amperage information.

Environment:
Applies to all Single and Three Phase Transformers.

Cause:
kVA sizing must often be calculated from Primary or Secondary Winding Voltage and Amperage information.

Resolution:

To determine kVA you must have at least two pieces of information:

1. the load line-to-line voltage (V)
2. the maximum load phase current (I)

Single Phase Transformers: kVA = (V * I) /1000
Three Phase Transformers:  kVA = (V * I * 1.732) / 1000
where 1.732 is the simple numerical value for the square root of 3 (1.7320508...)
Then round up to the next standard 3ph kVA rating as found in Section 14 of SquareD/Schneider Electric Catalog, Digest 177.

Single Phase Transformer Example: V = 240, I = 175; Therefore: kVA = (240 x 175) / 1000 = 42 kVA
This calculates to 42 kVA, thus we round up to a standard Single Phase size 50kVA.  From the Digest, an EE50S3H will satisfactorily serve this load.

Three Phase Transformer Example:  V = 208, I = 175; Therefore: kVA = (208 x 175 x 1.732) / 1000 = 63.05kVA
This calculates to 63+ kVA, thus we round up to a standard Three Phase size 75kVA.  From the Digest, an EX75T3H will satisfactorily serve this load.

Note:  This is a sample kVA calculation and does not take into account possible special load requirements, such as found with motors or some medical equipment, or other specialty applications.

Amperage provided for a given KVA can be determined by similar techniques:

Single Phase Example:  Using a 50 KVA Single Phase Transformer as a starting point.  50KVA is equal to 50,000 VA. (K= 1,000)  The full value in VA, 50,000 is then divided by the Voltage 240V = 208 Amperes.  This is a "Two Step Division", technique:  VA / Voltage = Amperage

Three Phase Example:  Using a 75 KVA Three Phase Transformer as a starting point.  75 KVA is equal to 75,000 VA. (K= 1,000)  The full value in VA, 75,000 divided by 1.732 = 43,302, which is then divided by the Voltage 208V = 208.2 Amperes.  This is a "Three Step Division", technique:  VA / 1.732 / Voltage = Amperage

As an alternative to these calculations, you may prefer to use the charts below.  Standard KVA sizes are shown on the left margin, Standard Line-to-Line Voltages are shown along the top margin.

Chart Example:  Using the numbers from the earlier Three Phase Transformer Example V=208, I=175.  Use the chart titled "Three Phase Low Voltage Dry Type Transformers".  Following the 208V top margin entry, and reading down vertically, it is seen that the first entry in that 208V vertical column that will cover 175A, and then some, is 208A, which indicates a 75 KVA Transformer, shown in the next over vertical column to the left.

See attached document 7400HO9501 "Looking for Low Voltage Transformer Solutions" for more information.  