TRANSFORMER SPECIFICATIONS
V
Common: 208, 480, 4160, 13800V
V
Common: 208, 240, 480V
kVA
Apparent power rating (S = P / pf)
Typical: 0.85 (inductive), 1.0 (resistive)
QUICK PRESETS
Three Phase Applications
TRANSFORMER TYPES
Dry Type95% eff
Indoor, safe, maintenance-free
Oil Filled98% eff
High power, outdoor, cooling
Pad Mount97% eff
Underground distribution
CALCULATION SHEET
Recommended Size
75 kVA
Standard 3φ transformer
Primary
60.14 A
@ 480V
Secondary
138.79 A
@ 208V
Turns Ratio
2.308 : 1
N₁/N₂ = V₁/V₂ = 480/208
Efficiency & Losses
Efficiency:95.00%
Power Loss:2.50 kW
Input Power:50.00 kVA
Output Power:40.38 kW
Energy Flow
95.0%
Loss
Standard 3φ Sizes (kVA)
3
6
9
15
30
45
75
112.5
150
225
300
500
750
1000
1500
2000
RecommendedAcceptableToo small
Derivation
- 01Configuration: 3-phase transformer
- 02Primary voltage: 480 V
- 03Secondary voltage: 208 V
- 04Power: 50 kVA (50000 VA)
- 05Turns ratio: Np/Ns = Vp/Vs
- 06Turns ratio: 480 / 208 = 2.3077:1
- 07Step-down transformer (voltage reduced by 0.43×)
- 08Efficiency: 95.00% (specified)
- 09Primary current (3-phase): Ip = S / (√3 × Vp)
- 10Ip = 50000 / (1.732 × 480) = 60.14 A
- 11Secondary current (3-phase): Is = S / (√3 × Vs)
- 12Is = 50000 / (1.732 × 208) = 138.79 A
- 13Current ratio: Ip/Is = 0.4333 ≈ 0.4333 (Vs/Vp) ✓
- 14Output apparent power: 50000 VA × 0.9500 = 47500.0 VA
- 15Output real power: 47500.0 VA × 0.85 PF = 40375.0 W
- 16Losses (heat): 50000 VA × (1 - 0.9500) = 2500.0 W
- 17Required capacity (with 10% margin): 50 × 1.1 = 55.00 kVA
- 18Standard transformer size: 75 kVA
- 19At load power factor 0.85:
- 20 Real power: 42500.0 W
- 21 Reactive power: 26339.1 VAR
- 22ℹ️ Large transformer - consider:
- 23 • Inrush current (5-10× rated current)
- 24 • Cooling requirements (ONAN, ONAF)
- 25 • Protection (fuses, thermal relays)
Formula
Vp/Vs = Np/Ns = Is/Ip, S = √3 × V × I
REFERENCE — INSTALLATION NOTES
•Allow 25% spare capacity for future expansion
•Check voltage regulation (typically 2-5% at full load)
•Ensure adequate ventilation (dry-type: 12\" clearance)
•Install primary and secondary overcurrent protection
•Ground per NEC 250 (case + secondary neutral)
•Label voltage, kVA rating, and connection diagram