Transformer kVA Calculator
Calculate required kVA from load amps and voltage, or find secondary amps from a known kVA rating. Single-phase and three-phase. Primary OCPD sizing per NEC 450.3.
Required kVA
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kVA
Next std size
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kVA
Primary FLA
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Max primary OCPD
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NEC 450.3
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Standard dry-type transformer sizes
| kVA rating | 3Ø 480V sec. FLA | 3Ø 208V sec. FLA | 1Ø 240V sec. FLA |
|---|---|---|---|
| 3 | 3.6 | 8.3 | 12.5 |
| 5 | 6.0 | 13.9 | 20.8 |
| 7.5 | 9.0 | 20.8 | 31.3 |
| 10 | 12.0 | 27.8 | 41.7 |
| 15 | 18.0 | 41.6 | 62.5 |
| 25 | 30.1 | 69.4 | 104.2 |
| 37.5 | 45.1 | 104.0 | 156.3 |
| 50 | 60.2 | 138.9 | 208.3 |
| 75 | 90.2 | 208.2 | 312.5 |
| 100 | 120.3 | 277.6 | 416.7 |
| 150 | 180.4 | 416.4 | 625.0 |
| 225 | 270.6 | 624.6 | 937.5 |
| 300 | 360.8 | 832.8 | 1250.0 |
| 500 | 601.4 | 1387.9 | 2083.3 |
| 750 | 902.1 | 2082.0 | 3125.0 |
| 1000 | 1202.8 | 2775.9 | 4166.7 |
Transformer sizing explained
Transformers are sized in kilovolt-amperes (kVA), not kilowatts (kW). kVA is the apparent power — it's what the transformer actually has to handle regardless of power factor. The formulas:
Single-phase: kVA = (V × I) / 1000
Three-phase: kVA = (V × I × 1.732) / 1000
Primary FLA = kVA × 1000 / (V_primary × 1.732) [3-phase]
Secondary FLA = kVA × 1000 / (V_secondary × 1.732) [3-phase]
Sizing rules of thumb
- Never size to exactly 100% of calculated demand — always go to the next standard size up
- Add 20–25% for load growth before selecting the transformer size
- For motor loads, add motor inrush (typically 6–8× FLA for 6–8 cycles) to the demand calculation if motors start frequently
- Transformers derate at high ambient temperatures — check the nameplate rating for your installation location
NEC 450.3 — Overcurrent protection
Transformers 1000V and below (the common dry-type distribution transformer):
- Primary OCPD only (no secondary protection): Maximum 125% of primary FLA. Round up to next standard size.
- Primary OCPD with secondary protection: Primary OCPD can be up to 250% of primary FLA. Secondary OCPD must be 125% or less of secondary FLA.
- Transformers rated 9 kVA or less: Primary OCPD can be up to 300% of primary FLA.
Frequently asked questions
kVA vs kW — what's the difference?
kW is real power (what does actual work). kVA is apparent power (what the wiring and transformer have to handle). kW = kVA × power factor. A 100 kVA transformer at 0.8 PF delivers 80 kW of real power. Transformers are rated in kVA because they handle current regardless of power factor.
Can I parallel two transformers?
Yes, but they must have the same kVA rating, same impedance (within 10%), same voltage ratio, and same phase relationship (same winding configuration — delta-wye must match delta-wye). Mismatched impedances cause one transformer to take more than its share of load.
How do I calculate transformer inrush?
Transformer inrush (magnetizing inrush) occurs at energization and can reach 8–12× the transformer's full load current for several cycles. OCPD must be selected to ride through inrush — that's why NEC 450.3 allows 125–250% of FLA rather than the standard 100%.
What does transformer impedance affect?
Impedance (Z%) determines maximum available fault current at the secondary. Fault current ≈ FLA / (Z%/100). A 75 kVA, 480V, 3-phase transformer (90A FLA) at 5% impedance delivers: 90 / 0.05 = 1800A maximum fault current. This number affects downstream bus ratings and OCPD interrupt ratings.