Three Phase Applications
Transformer Specifications
Common: 208, 480, 4160, 13800V
Common: 208, 240, 480V
Apparent power rating (S = P / pf)
Typical: 0.85 (inductive), 1.0 (resistive)
Transformer Types
Indoor, safe, maintenance-free
High power, outdoor, cooling
Underground distribution
Sizing Results
Recommended Size
Primary
Secondary
Turns Ratio
Efficiency & Losses
Standard 3φ Sizes (kVA)
🟣 Recommended • 🟢 Acceptable • ⚪ Too small
Calculation Steps
1. Configuration: 3-phase transformer
2. Primary voltage: 480 V
3. Secondary voltage: 208 V
4. Power: 50 kVA (50000 VA)
5. Turns ratio: Np/Ns = Vp/Vs
6. Turns ratio: 480 / 208 = 2.3077:1
7. Step-down transformer (voltage reduced by 0.43×)
8. Efficiency: 95.00% (specified)
9. Primary current (3-phase): Ip = S / (√3 × Vp)
10. Ip = 50000 / (1.732 × 480) = 60.14 A
11. Secondary current (3-phase): Is = S / (√3 × Vs)
12. Is = 50000 / (1.732 × 208) = 138.79 A
13. Current ratio: Ip/Is = 0.4333 ≈ 0.4333 (Vs/Vp) ✓
14. Output apparent power: 50000 VA × 0.9500 = 47500.0 VA
15. Output real power: 47500.0 VA × 0.85 PF = 40375.0 W
16. Losses (heat): 50000 VA × (1 - 0.9500) = 2500.0 W
17. Required capacity (with 10% margin): 50 × 1.1 = 55.00 kVA
18. Standard transformer size: 75 kVA
19. At load power factor 0.85:
20. Real power: 42500.0 W
21. Reactive power: 26339.1 VAR
22. ℹ️ Large transformer - consider:
23. • Inrush current (5-10× rated current)
24. • Cooling requirements (ONAN, ONAF)
25. • Protection (fuses, thermal relays)