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:
- 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.
- 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
