What is a transformer?
A transformer is an electrical device that is used in AC power distribution systems to safely “step up” or “step down” voltages to meet the incoming power requirements of connected equipment. Transformers achieve this goal without modifying the frequency or amount of electrical power to help ensure connected devices can operate reliably.
Transformers are static devices, meaning they contain no continuously moving mechanical parts. This contributes to their long lifecycle and minimal maintenance requirements. They can range in size from enormous devices that weigh several tons, commonly used in power substations, to very small devices that weigh only a few ounces and are commonly found in compact electrical equipment.
Distribution transformers are used in nearly every power distribution system application including commercial buildings, schools, healthcare & institutions, water/wastewater, industrial, oil & gas, and data centers. In this article, we will discuss How to calculate the KVA rating of the transformer.
How do transformers work?
The typical transformer has several secondary windings or coils of insulated wire conductors wrapped around a laminated steel core. When voltage is introduced to one coil, called the primary, it magnetizes the iron core. A voltage is then induced in the other coil, called the secondary or output coil. The change of voltage (or voltage ratio) between the primary and secondary depends on the ratio of the turns in the two coils.
There are two primary types of transformers that are defined by the insulation used within the enclosure: dry-type and liquid-filled.
How to calculate the KVA rating of the transformer
in order to know How to calculate the KVA rating of the transformer let’s see step by step process:
To determine kVA you must have at least two pieces of information:
- The load line-to-line voltage (V)
- 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 (1.732)
Then round up to the next standard 3ph kVA rating of transformer available.
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 of 50kVA.
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 of 75kVA.
Amperage calculation for given KVA
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.
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