Construction
- Three-legged core: one core with a winding set on each leg. Compact and the most common design.
- Bank of three single-phase units: easier to replace one failed unit; used in some utility and large industrial installations.
- Dry-type: air-cooled, typical indoors in commercial buildings at low voltage.
- Liquid-filled: oil or less-flammable fluid. Typical for pad-mounted utility and larger units, with higher overload tolerance.
Winding configurations
| Connection | Typical use | Notes |
|---|---|---|
| Δ–Y (delta–wye) | Building service and step-down, e.g., 480Δ to 208Y/120 | Grounded neutral on secondary; 30° shift |
| Y–Δ | Step-down at generating stations; some industrial | No secondary neutral |
| Y–Y | Some utility distribution | Needs careful grounding; tertiary winding sometimes added |
| Δ–Δ | Older industrial systems | Can run open-delta at 57.7% capacity if one unit fails |
Deeper comparison: Wye vs. Delta Connections.
Sizing by kVA
kVA = √3 × V × I ÷ 1,000. A 75 kVA transformer at 208 V delivers about 208 A full-load secondary current. Size for calculated demand plus growth, and consider non-linear loads, which may call for a K-rated or harmonic-mitigating transformer. The Power Calculator handles the arithmetic.
| kVA | Full-load A @ 208 V | Full-load A @ 480 V |
|---|---|---|
| 30 | 83 | 36 |
| 45 | 125 | 54 |
| 75 | 208 | 90 |
| 112.5 | 312 | 135 |
| 150 | 416 | 180 |
| 225 | 625 | 271 |
| 300 | 833 | 361 |
| 500 | 1,388 | 601 |
Impedance and fault current
Percent impedance (%Z) limits the fault current a transformer can deliver. A rough upper bound is full-load amps ÷ (%Z ÷ 100), so a lower %Z means higher available fault current downstream. That affects the interrupting ratings of panels and breakers. NEC 110.24 requires available fault current to be marked at many service locations.
Installation considerations
- Overcurrent protection on primary and secondary per NEC 450.3
- Grounding and bonding of the secondary as a separately derived system per NEC 250.30
- Ventilation, clearances and sound levels for dry-type units
- DOE efficiency requirements for distribution transformers