CALC 07 · POWERFREE

Transformer Sizing

TRANSFORMER SPECIFICATIONS
V

Common: 208, 480, 4160, 13800V

V

Common: 208, 240, 480V

kVA

Apparent output power used for the transformer kVA rating

Nameplate or test-report value at this load, as a fraction (0.95 = 95%). The pre-filled number is an example — this tool does not supply typical values by construction type.

Design value from the actual load. The pre-filled number is an example, not an assumed load.

QUICK PRESETS

Three Phase Applications

REFERENCE — EFFICIENCY DATA

Efficiency depends on construction (dry-type, liquid-filled, pad-mount), rating, and load point. This tool does not carry typical values for any of them — use the nameplate, the manufacturer test report, or the applicable efficiency-standard table for the unit in hand.

CALCULATION SHEET
Screening Size
75 kVA
Next listed rating ≥ 1.1 × rated output (H-015 screening policy, not a manufacturer table)
Primary Line Current
60.14 A
@ 480 V L-L
Secondary Line Current
138.79 A
@ 208 V L-L
Turns Ratio
2.308 : 1
N₁/N₂ = V₁/V₂ = 480/208

Efficiency & Losses

Efficiency:95.00%
Power Loss:2.24 kW
Rated Apparent Output:50.00 kVA
Load Real Output:42.50 kW
Estimated Real Input:44.74 kW
Energy Flow
95.0%
Loss

Screening Size List (kVA)

Engine screening list (H-015), the same for 1φ and 3φ — not a manufacturer or standards table. Classification uses the engine's selected size, so it is unit-safe for kVA and VA input.

0.05
0.1
0.15
0.25
0.5
0.75
1
1.5
2
3
5
7.5
10
15
25
30
37.5
45
50
75
100
112.5
150
225
300
500
750
1000
1500
2000
2500
3000
5000
7500
10000

SelectedLarger listed ratingBelow required capacity

Derivation
  1. 01Configuration: 3-phase transformer
  2. 02Primary voltage: 480 V (line-to-line)
  3. 03Secondary voltage: 208 V (line-to-line)
  4. 04Power: 50 kVA (50000 VA)
  5. 05Turns ratio: a = Np/Ns = Vp/Vs
  6. 06Turns ratio: 480 / 208 = 2.3077:1
  7. 07Step-down transformer: Vs = Vp / 2.31 (× 0.4333)
  8. 08Efficiency: 95.00% (specified)
  9. 09Primary line current (3φ, Vp line-to-line): Ip = S / (√3 × Vp)
  10. 10Ip = 50000 / (1.732 × 480) = 60.14 A
  11. 11Secondary line current (3φ, Vs line-to-line): Is = S / (√3 × Vs)
  12. 12Is = 50000 / (1.732 × 208) = 138.79 A
  13. 13Line currents only: winding current equals line current for a wye winding; a delta winding carries I_line / √3. Winding connection is not an input here.
  14. 14Current ratio: Ip/Is = 0.4333 ≈ 0.4333 (Vs/Vp) ✓
  15. 15Rated apparent output: 50000.0 VA (kVA rating is not efficiency-derated)
  16. 16Output real power: 50000.0 VA × 0.85 PF = 42500.0 W
  17. 17Input real power: 42500.0 W ÷ 0.9500 = 44736.8 W
  18. 18Losses (heat): 44736.8 - 42500.0 = 2236.8 W (single-point efficiency at this load; no-load/load-loss split not modelled)
  19. 19Required capacity (with 10% margin): 50 × 1.1 = 55.00 kVA
  20. 20Screening-list size (next listed rating ≥ 55.00 kVA): 75 kVA
  21. 21At load power factor 0.85:
  22. 22 Real power: 42500.0 W
  23. 23 Reactive power: 26339.1 VAR
  24. 24ℹ️ Large transformer - consider:
  25. 25 • Inrush current (5-10× rated current)
  26. 26 • Cooling requirements (ONAN, ONAF)
  27. 27 • Protection (fuses, thermal relays)
Formula
Vp/Vs = Np/Ns = Is/Ip, S = √3 × V × I
REFERENCE — WHAT THIS SHEET DOES NOT DECIDE
•Spare capacity beyond the 1.1× screening margin is a project decision — take it from the specification, not from this sheet.
•Voltage regulation needs the nameplate impedance (R and X) and the load power factor — not computed here.
•Ventilation, clearances, and cooling follow the manufacturer's installation instructions and the applicable code edition.
•Primary and secondary overcurrent protection is a separate selection under the applicable code edition — not sized here.
•Grounding and bonding of the separately derived system follow the applicable code edition — not evaluated here.
•Currents shown are line currents on a line-to-line voltage basis; winding connection (delta/wye) is not an input.

Related Calculators

Three-Phase Power

3-phase power calculations

free
Cable Sizing

NEC-referenced cable selection

preview
Power Factor Correction

Capacitor bank sizing

preview