Complete Guide to Transformer Sizing
The Transformer Size Estimator calculates minimum kVA using NEC 220 demand factors for motors, lighting, and receptacles. Our free tool ensures proper capacity and avoids overload.
What is Transformer Sizing?
kVA = Total Demand Load. NEC 450.3: Protect at 125%. Our Transformer Size Estimator applies Article 220.
Why Size Correctly?
Prevents:
- Overheating: Fire risk.
- Voltage Drop: Equipment issues.
- Utility Rejection: Undersized.
- Costly Upgrade: Future loads.
Use with Wire Size and Breaker Size.
NEC Demand Factors
| Load Type | Demand Factor |
|---|---|
| Lighting | 100% first 10kVA, 50% rest |
| Receptacles | 100% first 10kVA, 50% rest |
| Continuous | 125% |
| Motors | FLA × 1.25 + others |
Standard Transformer Sizes (kVA)
| Single-Phase | Three-Phase |
|---|---|
| 15, 25, 37.5, 50 | 15, 30, 45, 75 |
| 75, 100, 167 | 112.5, 150, 225 |
| 250, 333 | 300, 500, 750 |
Calculation Steps
- List Loads: kW, HP, kVA.
- Apply 125%: Continuous.
- Demand: Lighting, receptacles.
- Largest Motor: FLA × 1.25.
- Sum: Total VA.
- Next Size Up: Standard kVA.
Example: Commercial Building
50 HP motor, 20 kW lighting, 15 kVA receptacles, 30 kW HVAC (continuous).
- Motor: 65A × 480V × √3 / 1000 = 54 kVA → 125% = 67.5 kVA
- Lighting: 20 kVA → 10 + 50%×10 = 15 kVA
- Receptacles: 15 kVA → 10 + 50%×5 = 12.5 kVA
- HVAC: 30 × 1.25 = 37.5 kVA
- Total: 132.5 kVA → 150 kVA
Key Factors
- Future Growth: +20–30%.
- Power Factor: Motors ~0.8.
- Ambient Temp: Derate >40°C.
- Harmonics: K-rated transformer.
Practical Applications
For:
- Factories: Heavy motors.
- Offices: Lighting + HVAC.
- Retail: Receptacles.
- EV Charging: High demand.
Tips
- Round Up: Never down.
- Check Inrush: Motors.
- Primary Protection: 125%.
- Consult Utility: Available fault current.
Common Mistakes
- No 125%: Continuous loads.
- Ignore Demand: Over-size.
- Wrong Voltage: 208 vs 240.
- No Spare: Future expansion.
Advanced
- Load Profile: Diversity.
- Efficiency: 98% typical.
- Impedance: 5–6%.
- Sound Level: dB rating.
Conclusion
Accurate transformer sizing ensures reliability and efficiency. Our Transformer Size Estimator automates NEC compliance. Pair with Load Balance and Construction tools. Power up with 1000 Calculators.