How it's sized
Load kVA = V × I ÷ 1,000 (single-phase)
Load kVA = 1.732 × V × I ÷ 1,000 (three-phase)
Design kVA = Load kVA × (1 + spare %)
FLA = kVA × 1,000 ÷ V (÷ 1.732 for three-phase)
The calculator then rounds up to the next standard rating.
Standard dry-type ratings
| Single-phase kVA | 10 · 15 · 25 · 37.5 · 50 · 75 · 100 · 167 · 250 · 333 · 500 |
|---|---|
| Three-phase kVA | 15 · 30 · 45 · 75 · 112.5 · 150 · 225 · 300 · 500 · 750 · 1,000 · 1,500 · 2,000 · 2,500 |
Worked example
A 208 V three-phase panel carrying 120 A: 1.732 × 208 × 120 ÷ 1,000 = 43.2 kVA. With 25% spare that's 54 kVA, so a 75 kVA unit. Its full-load current is 90 A on the 480 V primary and 208 A on the secondary.
Things to check after sizing
- Overcurrent protection per NEC 450.3(B): typically 125% of primary FLA with primary-only protection.
- Secondary conductor rules and tap lengths in 240.21(C).
- K-rated or harmonic-mitigating units for heavy nonlinear loads such as VFDs and IT equipment.
- Inrush on energizing, which can trip undersized primary breakers.
Frequently asked questions
- What size transformer do I need for a 200 amp 208 V panel?
- If the panel will actually carry 200 A: 1.732 × 208 × 200 ÷ 1,000 = 72 kVA, so a 75 kVA unit with no spare, or 112.5 kVA with 25% spare. Size from the calculated load, not just the main breaker rating, when the load is known.
- Why add spare capacity?
- Transformers run hotter and less efficiently near full load, and loads tend to grow. 20–25% headroom is a common design practice.
Results are estimates for planning and education. Verify against the current code edition your jurisdiction has adopted, manufacturer data and a licensed professional before you build or install.