Transformer FLC Calculator
Calculate the primary and secondary full-load current (FLA) of power transformers. Supports standard kVA capacities and voltage ratings.
Transformer FLC Calculator
Results
Ip = (kVA × 1000) / (√3 × Vp)Is = (kVA × 1000) / (√3 × Vs)Transformer Sizing & Calculation
Transformers transfer electrical energy between winding circuits using electromagnetic induction. Sizing the primary and secondary currents is essential for selecting overcurrent protection devices (circuit breakers or fuses) and feed conductors.
Winding Formulas
Winding currents are inversely proportional to winding voltages. The turns ratio ($a$) relates voltage and current ratios:
Single-Phase Transformers:
Three-Phase Transformers:
Step-by-Step Worked Example
Problem: A three-phase transformer has a rating of 45 kVA. The primary voltage is 480V, and the secondary distribution voltage is 208V. Find the primary and secondary full-load currents.
- Identify Knowns: kVA = 45, Primary Voltage ($V_p$) = 480 V, Secondary Voltage ($V_s$) = 208 V. Multiplier = $\sqrt3 \approx 1.732$ (Three Phase).
- Calculate Primary Current ($I_p$):
Ip = (45 × 1000) / (1.732 × 480V)Ip = 45,000 / 831.36 = 54.12 Amperes. - Calculate Secondary Current ($I_s$):
Is = (45 × 1000) / (1.732 × 208V)Is = 45,000 / 360.26 = 124.91 Amperes. - Turns Ratio:
Ratio = 480V / 208V = 2.31(Primary turns are 2.31 times secondary turns).
Frequently Asked Questions (FAQs)
Why are transformers rated in kVA instead of kW?
Transformers are rated based on their thermal limitations (current and voltage capacity). The power factor of the load determines the kW output, which is out of the transformer's control. Rating in Apparent Power (kVA) ensures winding current bounds are not exceeded regardless of the load's power factor.
What is the 125% rating rule for transformer overcurrent protection?
National Electrical Codes (such as NEC 450.3) mandate that transformer primary protection devices be sized up to 125% of the primary FLC to prevent nuisance tripping during initial transformer magnetization (inrush current), which occurs when the iron core is energized.
Does winding connection type (Delta/Wye) change FLC calculations?
No. The total three-phase transformer full-load current calculation remains identical whether the coils are wired in Delta or Wye configurations. However, Delta/Wye configurations affect internal phase voltages and currents inside the transformer housing.