Use this transformer sizing calculator to choose a transformer kVA rating for your load. Enter the load in kW, kVA or amps, add a margin for future growth, and the calculator selects the next standard size from IEC or ANSI/IEEE ratings, along with the secondary full-load current.
How to size a transformer
- Find the maximum demand. Add up the connected loads and apply demand or diversity factors, because not every load runs at full power at the same time. See our guide to demand and diversity factors.
- Convert to kVA. kVA = kW ÷ power factor. For three-phase loads given in amps, kVA = √3 × V × I ÷ 1000.
- Add a margin. 20–25% is typical for future expansion and to keep the transformer running at an efficient load.
- Pick the next standard size at or above the required kVA.
- Check the motor starting. Large direct-on-line motors can draw 6–8 times their rated current on starting; make sure the voltage dip is acceptable.
Worked example
A building has a maximum demand of 120 kW at a power factor of 0.85, supplied at 400 V three-phase. Load in kVA = 120 ÷ 0.85 = 141.2 kVA. With a 25% growth margin: 141.2 × 1.25 = 176.5 kVA. The next standard IEC size is 200 kVA (ANSI: 225 kVA). Full-load secondary current = 200,000 ÷ (1.732 × 400) = 288.7 A, and today’s loading is about 71%.
The same building in the US at 208Y/120 V with ANSI sizes: 176.5 kVA rounds up to 225 kVA, with a full-load secondary current of 225,000 ÷ (1.732 × 208) = 624.5 A.
Common US transformer arrangements
- 480 V delta to 208Y/120 V dry-type step-down transformers feed lighting and receptacle panels in commercial buildings with a 480Y/277 V service.
- Utility pad-mount or pole-mount transformers supply 208Y/120 V or 480Y/277 V services to commercial buildings.
- Single-phase 120/240 V pole or pad transformers (typically 25–167 kVA) serve houses, often several homes per transformer.
| Three-phase kVA | FLA at 208 V | FLA at 480 V |
|---|---|---|
| 45 | 124.9 A | 54.1 A |
| 75 | 208.2 A | 90.2 A |
| 112.5 | 312.3 A | 135.3 A |
| 150 | 416.4 A | 180.4 A |
| 225 | 624.5 A | 270.6 A |
| 300 | 832.7 A | 360.8 A |
| 500 | 1387.9 A | 601.4 A |
| 750 | 2081.8 A | 902.1 A |
| 1000 | 2775.7 A | 1202.8 A |
| Single-phase kVA | FLA at 240 V (120/240 V) |
|---|---|
| 25 | 104.2 A |
| 37.5 | 156.3 A |
| 50 | 208.3 A |
| 75 | 312.5 A |
| 100 | 416.7 A |
| 167 | 695.8 A |
Standard transformer kVA ratings
| Standard | Typical ratings (kVA) |
|---|---|
| IEC 60076 (distribution) | 25, 50, 100, 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000, 2500, 3150 |
| ANSI/IEEE three-phase | 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000, 1500, 2000, 2500 |
| ANSI/IEEE single-phase | 10, 15, 25, 37.5, 50, 75, 100, 167, 250, 333, 500 |
Oil-filled distribution transformers above about 500 kVA are usually fitted with a Buchholz relay for internal fault protection.
Frequently asked questions
How do I calculate the kVA rating of a transformer?
Divide the maximum demand in kW by the power factor to get kVA, add a margin of about 20–25% for growth, and choose the next standard size. For a 3-phase load in amps, kVA = 1.732 × line voltage × current ÷ 1000.
What is the ideal loading of a transformer?
Distribution transformers are most efficient at roughly 50–70% of rated load. Running continuously above 80–90% shortens insulation life, especially in hot climates.
Should I use connected load or maximum demand?
Use maximum demand, which is the connected load multiplied by demand and diversity factors. Sizing on full connected load usually results in an oversized, inefficient transformer.
What size transformer for a 400 A, 208 V three-phase load?
400 A at 208 V three-phase is 1.732 × 208 × 400 ÷ 1000 = 144.1 kVA. A 150 kVA transformer covers it with almost no spare capacity; with a 25% growth margin (180 kVA) the next ANSI size is 225 kVA.
What is the full-load current of a 1000 kVA transformer?
At 480 V three-phase, the full-load current is 1,000,000 ÷ (1.732 × 480) = 1203 A; at 208 V it is 2776 A. At 400 V it is 1443 A, and at 11 kV it is 52.5 A.
Related calculators and guides
These calculators give engineering estimates for planning and learning. Final designs must follow your local code (NEC, IEC/BS 7671, PEC or the authority having jurisdiction) and be checked by a qualified electrical engineer or licensed electrician.