The key formula for calculating rated current in three-phase power systems (the standard for distribution transformers globally) is:
Apparent Power (kVA) = Line Voltage (kV) × Line Current (A) × √3 (1.732)
Here, the factor 1.732 represents the square root of 3, a mathematical constant accounting for the phase difference in three-phase electrical systems.
Let’s use the industry-common voltage ratio of 10 kV (HV side) to 0.4 kV (LV side) to illustrate the calculation:
By rearranging the formula to solve for current:
Line Current (A) = Apparent Power (kVA) ÷ (Line Voltage (kV) × 1.732)
Substituting the values:
Current = 100 kVA ÷ (10 kV × 1.732) ≈ 5.77 A
This low current value is intentional—higher HV reduces current flow, minimizing energy loss through conductor resistance and enabling efficient long-distance power transmission.
Using the same formula for the 0.4 kV LV side:
Current = 100 kVA ÷ (0.4 kV × 1.732) ≈ 144.34 A
The significantly higher current on the LV side is necessary to deliver the same 100 kVA of apparent power to end-user equipment (e.g., industrial machinery, commercial buildings) that operates at lower voltages. This is why LV circuits typically use thicker conductors to handle the increased current safely.
The question of how many kilowatt (kW) loads a 100 kVA transformer can carry requires moving beyond apparent power (kVA) to active power (kW)—the actual power consumed by electrical devices to perform work (e.g., lighting, motor operation, heating). The conversion between kVA and kW depends on two key factors: power factor (PF) and sustained load capacity.
Power factor measures the efficiency of power conversion, ranging from 0 to 1. Most industrial and commercial loads (e.g., induction motors, pumps, HVAC systems) have a typical power factor of 0.8 under normal operation—this is a conservative, industry-accepted baseline for load calculations. A lower power factor (e.g., 0.7) indicates more “reactive power” (unused power circulating in the system), reducing the available active power.
The relationship between kVA and kW is:
Active Power (kW) = Apparent Power (kVA) × Power Factor (PF)At full load (100% of nominal capacity), the maximum active power would be:
kW = 100 kVA × 0.8 = 80 kW
However, long-term full-load operation is not recommended. Transformers generate heat during operation, and sustained 100% load can lead to excessive thermal stress, accelerate insulation aging, and shorten the unit’s service life. Industry standards (e.g., IEEE, IEC) and safety guidelines mandate that transformers operate at no more than 80% of their nominal capacity for extended periods to ensure reliability, thermal stability, and compliance.
Accounting for the 80% sustained load limit, the safe active power capacity under normal conditions is:
kW = 100 kVA × 0.8 (power factor) × 0.8 (sustained load ratio) = 64 kW
Key Notes on Real-World Application
- If the connected loads have a higher power factor (e.g., 0.9, common with energy-efficient equipment or power factor correction devices), the available kW increases. For example: 100 kVA × 0.9 × 0.8 = 72 kW.
- Load types matter: Peak loads (temporary surges) can exceed 64 kW briefly, but continuous operation above this threshold risks overheating and equipment damage.
- Proper transformer sizing should also consider future load growth, ambient temperature, and installation conditions (e.g., indoor vs. outdoor) to ensure optimal performance.
In summary, a 100 kVA distribution transformer has a rated current of ~5.77 A on the 10 kV side and ~144.34 A on the 0.4 kV side. Under normal operating conditions (with a power factor of 0.8 and adhering to 80% sustained load limits), it can safely support approximately 64 kW of active load.
