To determine the appropriate size and capacity of a distribution transformer for a given application, it is important to calculate the transformer’s full load current. This allows engineers to select a transformer that can safely and efficiently handle the expected current demand without overloading. This article provides the formula and steps to calculate the full load current of a distribution transformer.

Key Parameters

Before calculating the full load current, you need to know two key parameters of the transformer:

1. Transformer rated capacity (kVA): This is the apparent power rating of the transformer, expressed in kilovolt-amperes (kVA). It represents the maximum load the transformer can handle without exceeding its thermal limits. Common distribution transformer sizes range from 50 kVA to 2500 kVA.
2. Transformer secondary voltage (V): This is the voltage at the secondary winding of the transformer, which supplies power to the load. Distribution transformers typically have secondary voltages of 120/240V, 240/480V, or 277/480V for three-phase systems and 120/240V for single-phase systems.

Full Load Current Formula

The formula to calculate the full load current of a distribution transformer depends on whether it is a single-phase or three-phase transformer:

Single-Phase Transformer:

Full Load Current (A) = Transformer Rated Capacity (kVA) × 1000 / Transformer Secondary Voltage (V)

Three-Phase Transformer:

Full Load Current (A) = Transformer Rated Capacity (kVA) × 1000 / (√3 × Transformer Secondary Line-to-Line Voltage (V))

In the three-phase formula, the secondary voltage is the line-to-line voltage, and the √3 factor accounts for the three-phase system.

Step-by-Step Calculation

Single-Phase Transformer Example:
Given:

• Transformer Rated Capacity = 100 kVA
• Transformer Secondary Voltage = 240 V

Step 1: Convert kVA to VA
100 kVA × 1000 = 100,000 VA

Step 2: Calculate the Full Load Current
Full Load Current = 100,000 VA / 240 V = 416.67 A

Three-Phase Transformer Example:
Given:

• Transformer Rated Capacity = 500 kVA
• Transformer Secondary Line-to-Line Voltage = 480 V

Step 1: Convert kVA to VA
500 kVA × 1000 = 500,000 VA

Step 2: Calculate the Full Load Current
Full Load Current = 500,000 VA / (√3 × 480 V) = 601.39 A