About the kVA and Transformer Size Calculator
This kVA calculator converts apparent power in kilovolt-amperes to line current in amps, or current back to kVA, for single-phase and three-phase systems. It also shows real power in kW at the power factor you enter and recommends the next standard transformer rating after adding a growth margin.
Electricians, electrical engineers, facility managers and generator or UPS buyers use it to size feeders, breakers and transformers, and to translate nameplate kVA into the current a circuit must carry. The table lists the current for the same kVA at other common voltages, which helps when comparing 208 V, 480 V or 400 V services.
Transformers are rated in kVA rather than kW because their heating depends on current, not on the power factor of the load. Standard sizes listed are common North American liquid-filled and dry-type ratings; check manufacturer catalogs and code requirements (such as NEC Article 450 overcurrent protection) for the final selection.
With the default inputs, the current is 90.2 A. Change any value above to recalculate instantly.
How to use the kva and transformer size calculator
- 1Choose whether to convert kVA to amps or amps to kVA.
- 2Select single-phase or three-phase.
- 3Enter the kVA (or current) and the system voltage.
- 4Set the power factor to see kW, and a growth margin for transformer sizing.
- 5Read the current, kW and the recommended standard transformer size.
Formula and method
Apparent power (kVA) is voltage multiplied by current. On a single-phase circuit that is simply V × I ÷ 1000. On a balanced three-phase circuit with line-to-line voltage V, total apparent power is √3 × V × I ÷ 1000, so current is kVA × 1000 ÷ (1.732 × V). Rearranging gives kVA from a measured current.
Real power in kW equals kVA × power factor, and reactive power in kVAR is kVA × √(1 − PF²). For transformer sizing the load kVA is increased by the growth margin and rounded up to the next standard rating. Transformers are sized on kVA because their losses depend on current regardless of power factor.
- I
- Line current in amps
- V
- Voltage (line-to-line for three-phase)
- kVA
- Apparent power
- PF
- Power factor (0–1)
Worked examples
75 kVA three-phase at 480 V
Current = 75,000 ÷ (1.732 × 480) ≈ 90.2 A. At a 0.85 power factor the load does 63.75 kW of real work. Adding 20% growth gives 90 kVA, so the next standard size is 112.5 kVA.
200 A single-phase service at 240 V
Single-phase kVA = 240 × 200 ÷ 1000 = 48 kVA. With a unity power factor that is also 48 kW. A 20% margin raises it to 57.6 kVA, above the 50 kVA size, so a 75 kVA transformer is the next standard rating.
500 kVA at 400 V (IEC system)
Current = 500,000 ÷ (1.732 × 400) ≈ 721.7 A per phase. With a 0.9 power factor the real power is 450 kW and the reactive power is 500 × √(1 − 0.81) ≈ 217.9 kVAR.
Frequently asked questions
How do I convert kVA to amps?+
For three-phase, divide kVA × 1000 by (1.732 × line voltage). For single-phase, divide kVA × 1000 by the voltage. For example, 75 kVA at 480 V three-phase is about 90 A.
What is the difference between kVA and kW?+
kVA is apparent power (volts × amps); kW is real power that does useful work. kW = kVA × power factor, so at a 0.8 power factor a 100 kVA load delivers 80 kW.
Why are transformers rated in kVA?+
Transformer heating comes from current flowing in the windings and from core losses, neither of which depends on the load power factor. Rating in kVA describes the current capacity at the rated voltage regardless of the load type.
How much spare capacity should a transformer have?+
Designers often add 20–25% for future growth, and avoid running a transformer continuously above about 80% of its rating. Motor starting, harmonics and ambient temperature can call for more headroom.
How many amps is a 1000 kVA transformer?+
At 480 V three-phase, a 1,000 kVA transformer has a full-load secondary current of about 1,203 A. At 208 V it is about 2,776 A, and at 400 V about 1,443 A.