kVA to Amps Calculator — Free Online Calculator
Convert kilovolt-amps (kVA) to amps for transformer and generator sizing. Single and three-phase.
How to Use This Calculator
Enter kVA rating, voltage, and phase to convert to amps.
The Formula Explained
Single-phase: Amps = kVA × 1000 / Volts. Three-phase: Amps = kVA × 1000 / (Volts × √3).
kVA: The Forgotten Half of Power
Most electrical power in residential settings is treated as watts or kilowatts because resistive loads (heaters, incandescent bulbs) have power factor 1.0, meaning apparent power equals real power. But commercial and industrial installations are full of inductive loads (motors, transformers, fluorescent ballasts) with power factors as low as 0.6, meaning apparent power can exceed real power by 67%. Your transformer, wires, and switchgear must be sized for apparent power (kVA) because they handle the actual voltage and current, not just the useful work delivered.
This is why utility bills on commercial services include both a kWh charge (for energy used) and a demand charge based on peak kVA (for capacity required). A factory running mostly motors at 0.75 power factor pays for all the capacity they draw even though they only use 75% of it as real work. Power factor correction (adding capacitor banks) improves this by canceling the reactive component, bringing PF closer to 1.0 and reducing the kVA draw for the same kW output.
Worked Example: Sizing a 75 kVA Transformer Service
A small commercial building needs a 75 kVA service at 208Y/120V three-phase. Current calculation: I = kVA × 1000 / (V × 1.732) = 75,000 / (208 × 1.732) = 208 amps. Main breaker sized at 250A (next standard size above calculated 208A, per NEC 240.6).
Wire sizing for the service entrance: 250A copper feeder needs 250 kcmil per NEC Table 310.16 at 75°C. Conduit: 2.5-inch rigid metal. Transformer primary side (assuming 480V utility): 75,000 / (480 × 1.732) = 90 amps. Primary protection: 125A breaker (125% of continuous current per NEC 450.3).
The secondary-side calculation (208V) determines the building load distribution. 208 amps at 208V means you can run about 43 kW of resistive load, or about 30-35 kW of typical commercial mixed load (motors, lighting, HVAC, plug loads averaging 0.85 PF).
Worked Example: Generator Sizing for Emergency Power
A data center requires backup power for 40 kW of computing load and 20 kW of cooling load, 60 kW total. Computing loads have power factor near 1.0 (modern power supplies use PFC — power factor correction). Cooling (compressors) has power factor around 0.85. Total apparent power: 40 kVA + (20 / 0.85) = 40 + 23.5 = 63.5 kVA.
Generator sizing: standard generators come in steps like 60, 75, 100 kVA. Choose 75 kVA to give 18% margin for motor starting inrush (compressors draw 6-8x normal current for a few seconds during startup). At 480V three-phase, 75 kVA delivers 90 amps. Transfer switch rated 100A. Feeder from ATS to main distribution panel sized for 100A continuous plus derating.
Note: sizing the generator in kW alone would give 60 kW, and a 60 kW generator might be 75 kVA or might only be 60 kVA depending on the manufacturer's rating convention. Always verify both numbers and the rated power factor (usually 0.8) for the specific generator model.
Five kVA Calculation Pitfalls
1. Using kW instead of kVA for current calculations. A 100 kW motor load at 0.8 PF draws 125 kVA worth of current, not 100. Undersize conductors based on kW and they overheat on the reactive current.
2. Forgetting the square root of 3 for three-phase. The formula I = kVA × 1000 / (V × 1.732) catches people out. Miss the 1.732 and your amps are off by 73%.
3. Using line-to-neutral instead of line-to-line voltage. Three-phase formulas use line-to-line voltage (like 480V on a 480Y/277V system, or 208V on a 208Y/120V system). Using 277V or 120V gives nonsense results.
4. Ignoring power factor on motor circuits. Motors nameplate in HP or kW, but you need kVA to size wire, breakers, and overload relays. The motor nameplate gives a full-load amps (FLA) value that you should use directly when available.
5. Mixing up generator nameplate modes. Standby-rated, prime-rated, and continuous-rated generators have different kVA capacities for the same physical machine. Standby ratings allow higher kVA for short-duration backup use. Prime ratings are lower for continuous operation. Match the rating class to your application.
Common Transformer and Generator Sizes
Standard transformer sizes (kVA): 15, 25, 37.5, 50, 75, 100, 112.5, 150, 167, 225, 300, 500, 750, 1000, 1500, 2000, 2500, 3000, 5000. Residential pole-mount transformers are typically 15-50 kVA. Small commercial pad-mount: 75-500 kVA. Industrial: 750 kVA and up.
Generator sizes run from portable 5 kVA through 2000 kVA for data centers and hospitals. Common intermediate sizes: 20, 30, 45, 60, 80, 100, 150, 200, 250 kVA. Diesel generators dominate above 100 kVA; natural gas and propane are common for smaller sizes. Gensets over 500 kVA are typically engineered installations with custom switchgear.
Code References for kVA Calculations
NEC 450 covers transformers: 450.3 for overcurrent protection, 450.4 for autotransformers, 450.9 for ventilation. NEC 220 covers load calculations for feeders and services, with specific methods for different occupancy types. NEC 695 covers fire pump circuits with specific kVA and voltage drop requirements.
IEEE Std 141 (Red Book) is the industrial power distribution reference including transformer sizing methodology. ANSI C57 standards govern transformer ratings, testing, and installation. NEMA MG 1 standards govern motor efficiency and power factor, which feeds back into the kVA calculations for motor loads.
kVA to amps: transformer and generator current sizing
Transformers and generators are rated in kVA (apparent power) because their heating is driven by current, regardless of what the load does with the energy. A 25 kVA transformer can deliver 25 kW only if the load is purely resistive; on a PF 0.8 load it delivers 20 kW before hitting its kVA limit.
The formula and what it does
For 480 V three-phase: I = kVA x 1.203. So 75 kVA at 480 V draws 90 A line current. For 208 V three-phase: I = kVA x 2.78. Same 75 kVA at 208 V draws 208 A.
Worked example
Scenario: 75 kVA generator with 480 V three-phase output. Find max load current and conductor.
I = 75,000 / (1.732 x 480) = 90.2 A. NEC 445.13 requires generator conductors at 115 percent of rated current: 103.7 A. From NEC 310.16, 2 AWG copper at 75 C handles 115 A. OCPD per 240.21(B): up to 115 percent of generator rating = 103 A. Round to 100 A breaker; tap-rule applies for longer runs to distribution panel.
Common mistakes to avoid
undefinedFrequently asked questions
Why is the transformer rated kVA not kW?
Transformer heat loss depends on copper losses (I squared R) and iron losses (constant). Both relate to current, not real power. A transformer can pass any PF up to its kVA limit.
Can I run a kVA-rated genset above its kW rating?
No. The kW rating is the engine power limit. The kVA rating is the alternator current limit. You can hit either first; the lower one applies.
How do I size a transformer for a motor load?
Most motors run at PF 0.85-0.95 at full load. Multiply load kW by 1.15 to get kVA. NEC 450.3 sets transformer OCPD; primary at 125-300 percent depending on type.
Does NEC 450 apply to my generator?
No. Generators are NEC 445. Transformers are NEC 450. Different sizing rules though similar in concept.
What happens if I exceed kVA on a transformer?
It overheats. Insulation life halves for every 8 C above rated temperature (Arrhenius rule for transformer oil). Short overloads are tolerated (load cycle); sustained overload destroys the unit.
How much overload can a transformer tolerate?
ANSI C57.91 allows short emergency overloads to about 200 percent of nameplate for 30 minutes, decreasing with time. Continuous operation must stay below nameplate kVA.