Transformer KVA Calculator
Size single and three-phase transformers based on load. Enter voltage and current, or load watts, and get a standard transformer recommendation.
| Parameter | Value |
|---|
Note: NEC recommends loading continuous loads at no more than 80% of transformer rating.
How to Size a Transformer
Determine the right transformer for your electrical load.
Input Load Values
Enter voltage and current, or load in watts with power factor. Add multiple loads with demand factors for accurate sizing.
Select Phase
Choose single-phase or three-phase. Three-phase uses the 1.732 (sqrt of 3) multiplier for line-to-line voltage calculations.
Get Recommendation
See calculated KVA, the next standard transformer size, primary and secondary currents, and percent loading.
Standard Transformer Sizes (kVA)
Common single-phase and three-phase transformer ratings per NEMA standards.
| Rating | Single-Phase | Three-Phase |
|---|---|---|
| 1 kVA | 4.17A @ 240V | 2.41A @ 240V |
| 3 kVA | 12.5A @ 240V | 7.22A @ 240V |
| 5 kVA | 20.8A @ 240V | 12.0A @ 240V |
| 10 kVA | 41.7A @ 240V | 24.1A @ 240V |
| 25 kVA | 104A @ 240V | 60.1A @ 240V |
| 50 kVA | 208A @ 240V | 120A @ 240V |
| 75 kVA | 312A @ 240V | 180A @ 240V |
| 100 kVA | 417A @ 240V | 241A @ 240V |
| 167 kVA | 696A @ 240V | 402A @ 240V |
| 250 kVA | 1042A @ 240V | 602A @ 240V |
| 500 kVA | 2083A @ 240V | 1203A @ 240V |
Frequently Asked Questions
Common questions about transformer sizing and selection.
Add up all connected loads, enter voltage and power factor, and select the next standard size that exceeds your calculated KVA. For continuous loads lasting 3+ hours, aim for 80% loading or less per NEC Article 220.
Single-phase KVA = V x I / 1000. Three-phase KVA = V x I x 1.732 / 1000. Three-phase transformers deliver more power in a smaller footprint and are standard for commercial and industrial applications.
NEC requires continuous loads to not exceed 80% of transformer rating to prevent overheating and allow for future load growth. Overloading a transformer reduces efficiency and lifespan.