Transformers, generators and uninterruptible power supplies are rated in kVA — thousands of volt-amperes — because their limits are set by current and voltage, regardless of how much of that current does useful work. This calculator converts between kVA and amps for single-phase and three-phase systems, shows the real power in kW at a chosen power factor, and suggests the next standard transformer size.
How to use the kVA calculator
- Choose kVA to amps or amps to kVA.
- Choose single-phase or three-phase.
- Enter the kVA or the current, and the voltage — line-to-line for three-phase systems (208, 240, 480 V and so on).
- Optionally enter a power factor to see real power in kW and reactive power in kVAR. Generators are typically rated at 0.8.
- Read the result, the next standard transformer size, and the current at other common voltages.
kVA formulas
Worked examples
75 kVA, three-phase, 208 V (the default)
I = 75,000 ÷ (1.732 × 208) = 208.2 A per line.
At a power factor of 0.8 the transformer can deliver 60 kW of real power, with 45 kVAR of reactive power. The same 75 kVA at 480 V carries only 90.21 A.
A 200 A single-phase service at 240 V. kVA = 240 × 200 ÷ 1,000 = 48 kVA. The next standard single-phase transformer rating is 50 kVA.
Full-load current of standard transformers
| kVA (three-phase) | 208 V | 480 V |
|---|---|---|
| 30 | 83.3 A | 36.1 A |
| 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 |
| 300 | 832.7 A | 360.8 A |
Common three-phase dry-type and pad-mount ratings run 15, 30, 45, 75, 112.5, 150, 225, 300 and 500 kVA; single-phase pole and pad-mount units run 10, 15, 25, 37.5, 50, 75, 100 and 167 kVA, following the ANSI/IEEE C57.12 series.
Sizing considerations
The full-load current is the most the unit can supply continuously at its rated temperature rise, so real loads should leave margin. Motors draw several times their running current when starting, which can cause a voltage dip on a small transformer or stall a small generator; generator manufacturers publish motor-starting kVA figures for that reason. Unbalanced single-phase loads on a three-phase transformer also limit capacity, because the most heavily loaded phase reaches its limit first.
Power factor matters for generators in particular. A generator rated 100 kVA at 0.8 PF can deliver 80 kW; a load with a lower power factor reaches the current limit before using all the engine’s power. The power factor calculator shows how capacitors reduce kVA for the same kW. For a full breakdown of three-phase power, use the three-phase power calculator, and for individual loads, the watts to amps calculator.
Calculations are for estimation only. Transformer and generator installations must follow your local electrical code and should be designed and installed by a licensed electrician or engineer.
Frequently asked questions
How do I convert kVA to amps?
Single-phase: I = kVA × 1,000 ÷ V. Three-phase: I = kVA × 1,000 ÷ (√3 × V), using the line-to-line voltage. A 75 kVA three-phase transformer at 208 V delivers up to 75,000 ÷ (1.732 × 208) ≈ 208 A.
What is the difference between kVA and kW?
kVA is apparent power, volts times amps, and it sets how much current a transformer or generator must carry. kW is real power, the part that does work. They are linked by the power factor: kW = kVA × PF, so a 100 kVA generator rated at PF 0.8 supplies 80 kW.
Why are transformers rated in kVA instead of kW?
A transformer's heating depends on the current through its windings and the voltage across its core, not on how much of that current does useful work. The manufacturer cannot know the power factor of the loads you will connect, so the rating is given in kVA.
How do I size a transformer?
Add up the load in kVA, allow for future growth and motor starting, and choose the next standard rating at or above the total. The calculator lists the next standard size, such as 15, 30, 45, 75, 112.5 or 150 kVA for three-phase units. Final sizing should be done by an engineer or licensed electrician.