Transformer Sizing Guide: kVA Calculations & NEC Requirements
Proper transformer sizing ensures reliable power distribution. Learn kVA calculations and NEC Article 450 protection requirements.
Understanding kVA Ratings
Transformers are rated in kilovolt-amperes (kVA), representing their apparent power capacity. Unlike resistive loads measured in kW, kVA accounts for both real and reactive power.
kVA = (Volts × Amps) / 1000
Single-phase: kVA = (V × I) / 1000
Three-phase: kVA = (V × I × 1.732) / 1000
Sizing Steps
- Calculate total connected load — Sum all loads on the secondary
- Apply demand factors — Not all loads operate simultaneously
- Add growth allowance — Typically 20-25% for future expansion
- Select standard size — Choose next larger standard kVA rating
Standard Transformer Sizes
Common dry-type transformer ratings (kVA):
Primary & Secondary Current
Once you know the kVA rating, calculate the full-load currents for conductor and protection sizing:
Single-Phase
I = kVA × 1000 / V
Three-Phase
I = kVA × 1000 / (V × 1.732)
Example: 75 kVA, 480V to 208V, 3-Phase
Primary current: 75,000 / (480 × 1.732) = 90.2A
Secondary current: 75,000 / (208 × 1.732) = 208.2A
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Overcurrent Protection (NEC 450.3)
NEC Article 450 provides specific requirements for transformer overcurrent protection. Rules differ based on voltage and transformer type.
Transformers 1000V or Less (450.3(B))
| Protection Method | Primary Only | Primary + Secondary |
|---|---|---|
| Primary OCPD (9A or more) | 125% | 250% |
| Primary OCPD (less than 9A) | 167% | 300% |
| Secondary OCPD | — | 125% |
Conductor Sizing
Size conductors based on the full-load current at each voltage level:
- Primary conductors: Sized for primary full-load current
- Secondary conductors: Sized for secondary full-load current
- Apply ampacity correction factors for temperature and conduit fill
Example Installation
Size a transformer for 150A of 208V, 3-phase load
1. Calculate required kVA:
kVA = (208 × 150 × 1.732) / 1000 = 54 kVA
2. Add 25% growth: 54 × 1.25 = 67.5 kVA
3. Select standard size: 75 kVA transformer
4. Primary current (480V): 90.2A
5. Primary OCPD (125%): 90.2 × 1.25 = 112.75A → 110A breaker
6. Primary conductors: #2 AWG copper (115A @ 75°C)
Installation Considerations
- Ventilation: Dry-type transformers require adequate airflow
- Clearances: Maintain manufacturer-specified clearances
- Grounding: Secondary must be grounded per NEC 250
- Accessibility: Ensure access for maintenance and inspection
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