45 kva transformer amps
The following is the calculation of the rated current and the selection analysis of a **45 kVA transformer**, distinguishing between
**three-phase** and **single-phase** scenarios:
I. Calculation of the Current of a Three-phase Transformer
**Formula**:
Common Voltage Levels and Currents
| Input/Output Voltage (V) | Calculation Formula | Rated Current (A) | Application Scenarios |
| 480V → 208Y/120V | 45,000 ÷ (1.732×480) | 54.1 A | Power distribution in commercial buildings (such as shopping malls and office buildings) |
| 600V → 480V | 45,000 ÷ (1.732×600) | 43.3 A | Power supply for industrial equipment |
| 10kV → 480V | 45,000 ÷ (1.732×10,000) | 2.6 A | High-voltage power transmission to low-voltage power distribution |
| 240V (Three-phase) | 45,000 ÷ (1.732×240) | 108.2 A | Three-phase motors in small industries |
II. Calculation of the Current of a Single-phase Transformer
**Formula**:
Common Voltage Levels and Currents
| Input/Output Voltage (V) | Current (A) | Application Scenarios |
| 240V → 120V | 187.5 A | Large single-phase loads (such as welding machines and heat pumps) |
| 480V → 240V | 93.75 A | Single-phase industrial equipment |
| 120V | 375 A | Rarely used (the current is too large and customization is required) |
III. Practical Selection Suggestions (Scenarios in the United States)
1. **Conductor Specifications (NEC Standards)**
- **Three-phase 480V 54.1A**: **8 AWG copper cable** is recommended (current-carrying capacity 55A, THHN insulation).
- **Single-phase 240V 187.5A**: **2/0 AWG copper cable** is required (current-carrying capacity 200A).
2. **Selection of Circuit Breakers**
- Three-phase: **60A circuit breaker** (1.1 times the rated current, meeting the protection requirements of NEC 450.3).
- Single-phase: **200A circuit breaker** (The starting current of the load needs to be confirmed).
3. **Recommendations for Transformer Types**
- **Indoor Scenarios**: Dry-type transformers (such as Square D 45 kVA, 480V→208Y/120V, $1,800~$3,000).
- **Outdoor Scenarios**: Oil-cooled transformers (such as Eaton 45 kVA, with NEMA 3R casing, $1,500~$2,500).
IV. Summary and Comparison Table
| Number of Phases | Voltage Level (V) | Rated Current (A) | Typical Applications | Conductor Specifications (Copper Cable) |
| Three-phase | 480 → 208/120 | 54.1 A | Commercial power distribution | 8 AWG |
| Three-phase | 240 (Three-phase) | 108.2 A | Small industries | 4 AWG |
| Single-phase | 240 → 120 | 187.5 A | Welding machines, large heaters | 2/0 AWG |
**Note**:
- If the transformer is of the **step-up/step-down type**, the currents on the primary side and the secondary side need to be
calculated separately.
- A **10%~20% margin** should be left for the actual current to avoid overloading (for example, the long-term load of a 45 kVA
transformer is recommended to be ≤ 40.5 kVA).
