Transformer Sizing Calculator
Professional transformer sizing for single and three-phase systems (400V/11kV)
Transformer Sizing Calculator
Standard 400V/11kV Transformer - Compliant with IEC 60076 & IEEE C57 Standards
Transformer Specifications:
Transformer KVA: 0.00
Your System kW: 0.00
Your System Current (A): 0.00 A
Your System Power Factor: 0.00
Turn Ratio: -
LT Rated Current: 0.00 A
HT Current: 0.00 A
LT Voltage: 0 V
HT Voltage: 0 V
Connection Type: -
How to Use the Transformer Sizing Calculator
Single Phase System
- Select "Single Phase" system type
- Choose load input type (kW or Amps)
- Enter load value and system voltage
- Set power factor (default 0.8)
- Optionally add safety factor (0-50%)
- Click Calculate for transformer specifications
Three Phase System (Default)
- System defaults to "Three Phase"
- Choose load input type (kW or Amps)
- Enter total load (default voltage 400V)
- Adjust power factor for your load type
- Add safety factor if needed (optional)
- Review calculated transformer specifications
Calculation Process
- For kW input: First converts kW to Amps
- Then converts Amps to kVA
- For Amps input: Directly calculates kVA
- Applies safety factor if specified
- Selects next standard transformer size
- Calculates all transformer specifications
How to Calculate Transformer Sizing
Step 1: Convert kW to Amps
Calculate current from power:
- Single phase: I = (kW × 1000) / (V × PF)
- Three phase: I = (kW × 1000) / (√3 × V × PF)
- PF = Power Factor (typically 0.8)
- V = System voltage
Step 2: Calculate KVA from Amps
- Single phase: kVA = (V × I) / 1000
- Three phase: kVA = (√3 × V × I) / 1000
- Apply safety factor: kVA × (1 + SF/100)
- Select next standard KVA rating
Step 3: Determine Specifications
- Calculate LT and HT currents
- Determine connection type
- Calculate turn ratio (HT:LT)
- Verify against standard ratings
Real-Life Example
Three Phase System Calculation:
Example - kW Input:
• Load: 400 kW
• System Voltage: 400V
• Power Factor: 0.8
• Safety Factor: 15%
Calculation:
• Step 1: I = (400 × 1000) / (√3 × 400 × 0.8) = 721.7 A
• Step 2: kVA = (√3 × 400 × 721.7) / 1000 = 500 kVA
• With Safety Factor = 500 × 1.15 = 575 kVA
• Standard Rating = 750 kVA
• LT Current = 750,000 / (√3 × 400) = 1083 A
• HT Current = 750,000 / (√3 × 11000) = 39.3 A
Calculation Formulas
kW to Amps (3-Phase):
I = (kW × 1000) / (√3 × V × PF)
kW to Amps (1-Phase):
I = (kW × 1000) / (V × PF)
Amps to kVA (3-Phase):
kVA = (√3 × V × I) / 1000
Amps to kVA (1-Phase):
kVA = (V × I) / 1000
With Safety Factor:
kVA = kVA × (1 + SF/100)
Three Phase Current:
I = (kVA × 1000) / (√3 × V)
Turn Ratio:
TR = HT Voltage / LT Voltage
Where:
• PF = Power Factor
• SF = Safety Factor (percentage)
• V = Line-to-Line Voltage
• I = Line Current
• TR = Turn Ratio (27.5:1)
• kVA = Apparent Power Rating
• kW = Real Power
Frequently Asked Questions
Why convert kW to Amps first?
Converting kW to Amps first and then to kVA ensures that the power factor is properly accounted for in the calculation. This two-step process follows electrical engineering best practices for transformer sizing.
Why is power factor important for transformer sizing?
Power factor determines how much of the apparent power (kVA) is converted to useful work (kW). A lower power factor means more reactive power and requires a larger transformer for the same kW load.
What power factor should I use for different loads?
Use 0.7-0.85 for motor loads, 0.8-0.9 for mixed commercial/industrial loads, 0.9-0.95 for modern lighting systems, and 1.0 for purely resistive loads like heaters.
When should I use a safety factor?
Safety factor is optional and should be used when you expect load growth, have varying loads, or want additional capacity margin. For exact sizing to known loads, you can leave it blank.
How is the turn ratio calculated?
Turn ratio = HT Voltage ÷ LT Voltage = 11000V ÷ 400V = 27.5:1. This determines the voltage transformation and current relationship between primary and secondary windings.
Current to kVA Conversion Chart (400V)
| Current (A) | Single Phase kVA | Three Phase kVA |
|---|---|---|
| 50 A | 20 kVA | 35 kVA |
| 100 A | 40 kVA | 69 kVA |
| 200 A | 80 kVA | 139 kVA |
| 300 A | 120 kVA | 208 kVA |
| 400 A | 160 kVA | 277 kVA |
| 500 A | 200 kVA | 346 kVA |
| 750 A | 300 kVA | 520 kVA |
| 1000 A | 400 kVA | 693 kVA |
| 1500 A | 600 kVA | 1039 kVA |
This table shows kVA values for different currents at 400V.