Transformer Calculator & Configurator

1. Pick your mode. Use the Configurator to build a unit and generate a catalog code, or the Phase Calculator to solve kVA, volts, or amps.
2. Select phase. Toggle single-phase or three-phase, and the formula and the √3 factor adjust automatically.
3. Enter any two values (kVA, volts, amps) and the Phase Calculator returns the third, plus full-load current.
4. Configure and quote. Add voltage taps, temperature rise, K-factor, and enclosure, then send the catalog code to our team for a quote.

Transformer Sizing Formulas (kVA, Amps & Voltage)

The calculator uses the standard relationships between power (kVA), voltage (V), and full-load current (amps):

Single-Phase

• kVA = (V × A) ÷ 1000
• Amps = (kVA × 1000) ÷ V

Three-Phase

• kVA = (V × A × 1.732) ÷ 1000
• Amps = (kVA × 1000) ÷ (V × 1.732)

Turns ratio = primary voltage ÷ secondary voltage = N₁/N₂. The same equations give primary and secondary full-load amps (FLA).

Why √3 (1.732)? In a balanced three-phase system the three voltages are 120° apart, and the line-to-line voltage equals √3 × the line-to-neutral voltage. That is why three-phase power uses the 1.732 factor: use √3 with line-to-line voltage, or 3 with line-to-neutral voltage.

Worked example. A 75 kVA three-phase unit at 480 V: Amps = 75 × 1000 ÷ (480 × 1.732) = 90.2 A. At 208 V the same 75 kVA draws 208 A.

kVA to Amps, Amps to kVA & kW Conversions

Convert between apparent power (kVA), current (amps), and real power (kW). kVA and amps do not need power factor; kW and kVA do.

• kVA → Amps (1φ): A = kVA × 1000 ÷ V
• kVA → Amps (3φ): A = kVA × 1000 ÷ (1.732 × V)
• Amps → kVA (1φ): kVA = A × V ÷ 1000
• Amps → kVA (3φ): kVA = 1.732 × A × V ÷ 1000
• kW → kVA: kVA = kW ÷ PF.   kVA → kW: kW = kVA × PF.

Examples. 200 A at 240 V three-phase = 83 kVA. 70 kVA at 480 V three-phase = 84 A. 80 kW at 0.8 power factor = 100 kVA.

Power factor (PF) is real power ÷ apparent power. Transformers are rated in kVA, not kW, so sizing on kW alone leaves you undersized. Typical PF by load:

How to Size a Transformer

1. Identify the load. Note the load voltage and full-load current (amps), or the connected kW and power factor.
2. Calculate required kVA with the formula above for your phase type.
3. Add headroom. Size so the unit runs near 80% of capacity: divide the minimum kVA by 0.8 (about +25%) for continuous loading and future growth.
4. Round up to a standard kVA rating (see the table below).
5. Account for the environment. Choose temperature rise, K-factor (for non-linear / harmonic loads), and enclosure (NEMA) rating for the install.

Rule of thumb & example. A 400 A, 240 V three-phase service needs about 400 × 240 × 1.732 ÷ 1000 = 166 kVA minimum, so with headroom, a 225 kVA standard unit. Always confirm final sizing with a licensed engineer and the NEC.

Standard Transformer kVA Ratings

Transformers are built to standard kVA sizes. Round your calculated load up to the next size:

• Single-phase: 1, 1.5, 3, 5, 7.5, 10, 15, 25, 37.5, 50, 75, 100, 167, 250, 333, 500 kVA
• Three-phase: 3, 6, 9, 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000, 1500, 2000, 2500 kVA

MGM Transformers builds general-purpose, dry-type, K-factor, and custom units across this range, many in stock.

Full-Load Amps (FLA) Charts

Three-phase full-load current (A = kVA × 1000 ÷ (V × 1.732)):

Single-phase full-load current (A = kVA × 1000 ÷ V):

Need the full 600V-class and medium-voltage charts? Download our reference documents.

Transformer Overcurrent Protection (NEC 450.3)

For transformers rated 1000 V or less, NEC Table 450.3(B) sets the maximum overcurrent device rating as a percentage of the transformer’s rated full-load current. Simplified guidance:

Where 125% does not match a standard device size, the next higher standard rating is permitted. This is a simplified summary of NEC 450.3(B); always size protection to the current NEC edition and your AHJ.

Single-Phase vs Three-Phase Transformers

Use the single-phase transformer calculator mode for residential, light-commercial, and control-power loads; use the three-phase transformer calculator mode for most industrial and commercial distribution. At the same kVA and voltage, a three-phase unit draws lower current per line because of the √3 factor, which is why three-phase distribution is more efficient for larger loads.

Buck-Boost, Step-Up & Step-Down Calculations

A step-up transformer raises voltage (e.g., 240 V to 480 V); a step-down transformer lowers it (e.g., 480 V to 208 V). Buck-boost transformers make small corrections (typically 5% to 20%, such as 208 V to 240 V) and are sized by the load amps and the voltage change, not by full kVA, so they are far smaller and cheaper than an isolation transformer for minor adjustments.

Common buck-boost corrections: 208→240 V, 240→208 V, 480→600 V. Need one? See our buck-boost transformers.

Fault Current & Impedance (%Z)

A transformer’s nameplate impedance (%Z) sets the available short-circuit current on the secondary. Approximate available fault current = secondary full-load amps ÷ (%Z ÷ 100). Example: a 500 kVA, 480 V unit (FLA 601 A) at 5% impedance can deliver roughly 601 ÷ 0.05 = 12,000 A, so size your secondary breakers and conductors for that available fault current.

Transformer Terms & Definitions

• kVA: apparent power (kilovolt-amperes); the transformer’s capacity rating.
• kW vs kVAR vs kVA: real power (kW), reactive power (kVAR), and apparent power (kVA); kVA = √(kW² + kVAR²).
• Power factor (PF): kW ÷ kVA; how much current actually does work.
• Full-load amps (FLA): current drawn at the rated kVA.
• Impedance (%Z): nameplate value that sets available secondary fault current.
• Taps: winding connections that fine-tune output voltage (e.g., ±2.5%).
• Temperature rise: allowable winding temperature above ambient (e.g., 115°C, 150°C).
• K-factor: rating for handling harmonic (non-linear) loads such as data-center and VFD loads.

More terms in our transformer glossary.