What is the difference between kVA and kW?

kVA (kilovolt-amperes) is apparent power — the total power the circuit must carry, including both useful and reactive components. kW (kilowatts) is real power — the portion that actually does work (runs motors, generates heat, powers electronics). The relationship is kW = kVA x power factor. A 100 kVA transformer at 0.85 power factor delivers 85 kW of usable power. The remaining 15 kVA is reactive power that flows back and forth without doing work but still requires wire capacity to carry.

Why does three-phase power draw less current than single-phase?

Three-phase distributes the power across three conductors instead of two, and the phases are offset by 120 degrees. This means current flows more evenly and each conductor carries less. The math includes a factor of 1.732 (square root of 3) in the denominator: Amps = kVA x 1000 / (V x 1.732). A 50 kVA load at 240V single-phase draws 208A, but the same 50 kVA at 208V three-phase draws only 139A per line — smaller wire, smaller conduit, and lower installation cost.

What power factor should I use for my calculation?

For purely resistive loads (heaters, incandescent lights, electric ranges), use 1.0. For motor-heavy loads (HVAC, pumps, compressors), use 0.80-0.85. For mixed commercial loads (offices, retail), use 0.85-0.90. For data centers and electronic loads with power factor correction, use 0.95-0.99. When in doubt, 0.85 is a safe general-purpose assumption for most commercial and industrial facilities. Check the utility bill — many providers report the facility's average power factor.

How do I convert kVA to amps for a three-phase system?

The formula is: Amps = (kVA x 1,000) / (Voltage x 1.732). The 1.732 is the square root of 3, which accounts for the three-phase power relationship. For example, a 25 kVA transformer at 480V three-phase: (25 x 1,000) / (480 x 1.732) = 25,000 / 831.4 = 30.1 amps per line. This is the line current each conductor must carry. Size wire and breakers for this value, applying any applicable NEC multipliers for continuous loads.

kVA to Amps Calculator — Free Online

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Convert kVA to amps for any single-phase or three-phase system. Enter the transformer or equipment kVA rating, voltage, and power factor to get the full load current, true power in kW, and reactive power in kVAR — everything you need for wire sizing and breaker selection.

kVA to amps conversion dividing apparent power by system voltage for current.

kVA vs kW: Apparent Power and True Power Explained

A transformer rated at 10 kVA does not necessarily deliver 10 kW of usable power. The "kVA" rating is apparent power — the total power flowing through the circuit, including both the useful component (real power, measured in kW) and the wasted back-and-forth component (reactive power, measured in kVAR).

The relationship is straightforward: kW = kVA x Power Factor. A 10 kVA transformer serving a load with 0.85 power factor delivers 8.5 kW of real power. The remaining 1.5 kVA is reactive power — it oscillates between the source and the inductive load (usually motors or fluorescent ballasts) without doing useful work, but it still creates current in the wires.

This distinction matters because wires and breakers carry amps, and amps come from kVA — not kW. A 10 kVA load at 240V single-phase draws 41.7A regardless of the power factor. The power factor determines how much of that current does useful work, but all of it flows through your conductors and generates heat. Size your wire and breaker for the full kVA-based current, not the reduced kW figure.

Purely resistive loads like space heaters and incandescent lights have a power factor of 1.0, so kVA equals kW. But most real-world loads include motors, transformers, or electronic power supplies that pull the power factor below 1.0. Our amps draw calculator handles the simpler watts-to-amps conversion for resistive loads where power factor is not a concern.

kVA-to-amps formula for single-phase and three-phase with the square-root-3 factor shown. — For three-phase systems, divide by V × √3 (1.732) instead of just V — a 10 kVA 3-phase load at 480V draws only 12A versus 41.7A on single-phase 240V.

Using kVA-to-Amps Results for Wire Sizing

Get the full load amps from this calculator. For single-phase: Amps = kVA x 1000 / Voltage. For three-phase: Amps = kVA x 1000 / (Voltage x 1.732). Enter your values above and read the current result directly.
Apply the NEC continuous load multiplier if applicable. Loads expected to run for 3+ hours continuously (lighting, HVAC, commercial kitchen equipment) must have conductors rated at 125% of the calculated current per NEC 210.20. A 41.7A continuous load requires conductors rated for 52.1A.
Select the wire gauge from NEC ampacity tables. Use NEC Table 310.16 for conductors in raceway or cable. The gauge must handle the multiplied current at the applicable temperature rating (60°C, 75°C, or 90°C depending on the conductor insulation and terminal ratings).
Check voltage drop on the run. Long feeder runs from transformer to panel can accumulate noticeable voltage drop. After selecting the gauge for ampacity, verify the drop stays under 3% using our wire distance calculator. Upgrade the gauge if needed.
Size the overcurrent protection. Transformer primary and secondary protection follows NEC Article 450. The fuse or breaker size depends on the transformer kVA, voltage, and whether you are protecting the primary or secondary side. Our transformer fusing calculator handles this directly.

Common Transformer Sizes and Full Load Amps

kVA Rating	120V 1Ø Amps	240V 1Ø Amps	208V 3Ø Amps	480V 3Ø Amps	Typical Use
5	41.7	20.8	13.9	6.0	Small commercial space, branch panel
10	83.3	41.7	27.8	12.0	Residential service, small shop
15	125.0	62.5	41.6	18.0	Multi-unit residential, office suite
25	208.3	104.2	69.4	30.1	Small commercial building
50	—	208.3	138.8	60.1	Medium commercial, light industrial
75	—	312.5	208.2	90.2	Large commercial panel
100	—	416.7	277.6	120.3	Industrial facility section
150	—	625.0	416.4	180.4	Large industrial, data center
300	—	—	832.8	360.8	Heavy industrial main service
500	—	—	1,388	601.4	Utility substation, campus main

These are maximum full load amps at unity power factor (PF = 1.0). Actual current at the typical 0.80-0.90 power factor of most facilities will be the same — kVA determines amps regardless of PF. The real power (kW) delivered to the load will be less. Dashes indicate combinations that are impractical or not standard for that transformer size.

Worked Examples

Sizing Wiring for a 10 kVA Single-Phase Transformer

Context

A 10 kVA single-phase transformer supplies a small commercial space at 240V. The load mix includes lighting, HVAC, and a few small motors with an estimated power factor of 0.85. You need to size the secondary conductors and breaker.

Calculation

Full load amps: (10 kVA x 1,000) / 240 V = 41.7 A

True power: 10 kVA x 0.85 PF = 8.5 kW

Reactive power: √(10² - 8.5²) = √(100 - 72.25) = √27.75 = 5.27 kVAR

For continuous loads, NEC requires conductors rated at 125%: 41.7 x 1.25 = 52.1 A minimum

Interpretation

The secondary conductors must handle at least 52.1A (continuous load rating). That puts you in 6 AWG copper territory per NEC Table 310.16 at 75°C. A 60A breaker on the secondary provides proper overcurrent protection with room for the load to grow slightly.

Takeaway

The 41.7A flowing through the wires exists regardless of the 0.85 power factor — that PF only reduces the real power (kW) the load uses, not the current the wires carry. For the wire run from transformer to panel, use our voltage drop calculator to verify the conductors are heavy enough for the distance.

Three-Phase Motor Starter Sizing

Context

A 25 kVA three-phase motor load runs at 480V with a measured power factor of 0.80. You need to determine the line current for wire sizing and starter selection.

Calculation

Full load amps: (25 kVA x 1,000) / (480 V x 1.732) = 25,000 / 831.4 = 30.1 A per line

True power: 25 kVA x 0.80 PF = 20.0 kW

Reactive power: √(25² - 20²) = √(625 - 400) = √225 = 15.0 kVAR

Interpretation

Each line conductor carries 30.1A. At 480V three-phase, this is modest current — 10 AWG copper is adequate for ampacity. The 15 kVAR of reactive power is substantial (60% of the real power), indicating the motor is operating at a relatively poor power factor. Power factor correction capacitors could reduce the line current.

Takeaway

A NEMA Size 1 starter handles up to 27A at 480V, so this motor needs a NEMA Size 2 (up to 45A). The motor FLA from the nameplate may differ from this kVA-based calculation — always verify with nameplate data. Our motor FLA calculator computes FLA directly from the motor's HP rating for a more precise result.

Frequently Asked Questions

Glossary

Apparent Power (kVA)

The total power flowing in an AC circuit, combining both real power (kW) and reactive power (kVAR). Measured in kilovolt-amperes. This is what transformers, conductors, and breakers must be rated for because it determines the actual current flowing through the circuit regardless of power factor.

True Power (kW)

The portion of apparent power that performs useful work — running motors, generating heat, powering electronics. Measured in kilowatts. True power equals apparent power multiplied by the power factor: kW = kVA x PF. This is what your electricity meter measures and what you pay for.

Reactive Power (kVAR)

The portion of apparent power that oscillates between the source and inductive or capacitive loads without doing useful work. Measured in kilovolt-amperes reactive. Motors, transformers, and fluorescent ballasts create reactive power. High reactive power means low power factor, which wastes conductor capacity and can incur utility surcharges.

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Transformer kVA ratings exist because transformers do not care about power factor — they must handle the full apparent power regardless of how much real work the load does. A transformer supplying a motor with 0.70 power factor carries 43% more current than one supplying a resistive heater of the same kW. If the load includes battery charging for a solar battery bank, size the transformer for the charger's full kVA rating including any power factor correction. Both scenarios are within the transformer's design, but the low-PF load requires heavier wiring and larger breakers. When sizing electrical infrastructure, always work from kVA and amps — not kW — to ensure your conductors and protection can handle the actual current flowing through them. For a deeper explanation of the power triangle and when this distinction matters, read our kVA vs kW guide.

Last updated: March 28, 2026

Written and maintained by Dan Dadovic, Commercial Director at Ezoic Inc. & PhD Candidate in Information Sciences. He works professionally as Commercial Director at Ezoic Inc., leading revenue strategy across digital publishing.

Disclaimer: Calculator results are estimates based on theoretical formulas. Actual performance varies with temperature, battery age, load patterns, and equipment condition. For critical electrical work, consult a licensed electrician.