Configurator + Calculator
To receive a quote, please fill in the calculator values and press 'Calculate'. Or generate a catalog code by entering below information.
Use the MGM transformer calculator to size any unit by kVA, full-load amps, and voltage — for single-phase or three-phase systems — convert between kVA, amps, and kW, then generate a catalog code and request a quote. Enter any two of kVA, volts, or amps and the tool solves the third, or build a complete configuration from phase, voltage, taps, temperature rise, K-factor, and enclosure.
1. CATALOG CODE
2. Get a Quote
Please enter your catalog code or create a code using the drop-down options below to display the configuration of your unit.
If your code is: HT112A3B2SH–HK0160LN03R
you would enter it as:
Your CATALOG CODE
Unit Configuration
How to Use the Transformer Calculator
- 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.
- Select phase. Toggle single-phase or three-phase — the formula and the √3 factor adjust automatically.
- Enter any two values (kVA, volts, amps) and the calculator returns the third, plus full-load current.
- Configure & 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↔amps does not need power factor; kW↔kVA does.
- 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 — sizing on kW alone leaves you undersized. Typical PF by load:
| Application | Typical PF |
|---|---|
| Data centers (AI/HPC) | 0.85 – 0.95 |
| Industrial / motor loads | 0.70 – 0.90 |
| Hospitals | 0.80 – 0.90 |
| Resistive / heating | ~1.0 |
How to Size a Transformer
- Identify the load. Note the load voltage and full-load current (amps), or the connected kW and power factor.
- Calculate required kVA with the formula above for your phase type.
- 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.
- Round up to a standard kVA rating (see the table below).
- 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 → 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 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)):
| kVA | 208V | 240V | 480V | 600V |
|---|---|---|---|---|
| 15 | 41.6 | 36.1 | 18.0 | 14.4 |
| 30 | 83.3 | 72.2 | 36.1 | 28.9 |
| 45 | 124.9 | 108.3 | 54.1 | 43.3 |
| 75 | 208.2 | 180.4 | 90.2 | 72.2 |
| 112.5 | 312.3 | 270.6 | 135.3 | 108.3 |
| 150 | 416.4 | 360.8 | 180.4 | 144.3 |
| 225 | 624.5 | 541.3 | 270.6 | 216.5 |
| 300 | 832.7 | 721.7 | 360.8 | 288.7 |
| 500 | 1387.9 | 1202.8 | 601.4 | 481.1 |
| 750 | 2081.8 | 1804.2 | 902.1 | 721.7 |
| 1000 | 2775.7 | 2405.6 | 1202.8 | 962.3 |
Single-phase full-load current (A = kVA × 1000 ÷ V):
| kVA | 120V | 240V | 480V |
|---|---|---|---|
| 5 | 41.7 | 20.8 | 10.4 |
| 15 | 125.0 | 62.5 | 31.2 |
| 25 | 208.3 | 104.2 | 52.1 |
| 50 | 416.7 | 208.3 | 104.2 |
| 75 | 625.0 | 312.5 | 156.2 |
| 100 | 833.3 | 416.7 | 208.3 |
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:
| Protection method | Primary | Secondary |
|---|---|---|
| Primary only (primary FLA ≥ 9 A) | ≤ 125% | — |
| Primary & secondary (secondary FLA ≥ 9 A) | ≤ 250% | ≤ 125% |
| Primary only (primary FLA 2–9 A) | ≤ 167% | — |
| Primary only (primary FLA < 2 A) | ≤ 300% | — |
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 5–20%, 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 — size your secondary breakers and conductors for that available fault current.
Configure & Order Your Transformer
Once the calculator gives you a rating, the Configurator turns your selections into an MGM catalog code — a real, buildable unit, not just a number. As a US manufacturer with stocked general-purpose units and full custom capability, we quote fast.
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.
Frequently Asked Questions
How do I calculate transformer size?
Convert load voltage and current to kVA (kVA = V × A × 1.732 ÷ 1000 for three-phase), then round up to the next standard rating with about 25% headroom.
What is the rule of thumb for transformer sizing?
Size for roughly 80% loading: divide your calculated minimum kVA by 0.8 and round up to a standard size.
How many amps is a 75 kVA transformer?
About 208 A at 208 V and 90 A at 480 V (three-phase). Use the calculator for any voltage.
How many amps is a 25 kVA transformer?
About 69 A at 208 V and 30 A at 480 V (three-phase).
How many kVA is 200 amps?
About 83 kVA at 240 V three-phase, or 48 kVA at 240 V single-phase.
What size transformer do I need for a 400 amp service?
A 400 A, 240 V three-phase load is ~166 kVA minimum — with headroom, a 225 kVA standard unit.
What is the difference between kVA and kW?
kVA is apparent power; kW is real power. kW = kVA × power factor. Transformers are rated in kVA.
Does the calculator support single-phase and three-phase?
Yes — toggle the phase before entering values and the formula switches automatically.
How accurate is the calculator?
It returns standard engineering values for planning. Confirm final sizing and overcurrent protection with a licensed electrical engineer and the NEC.
Why Use the MGM Transformer Calculator
MGM Transformers has built dry-type and liquid-filled transformers in the USA for over 50 years, with ISO 9001:2015-certified manufacturing and UL energy verification. This calculator is backed by real, buildable products — size it here, then get a quote from the people who make the unit.
Reviewed by MGM engineering. Always verify final transformer sizing and overcurrent protection with a licensed electrical engineer for your specific application.
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</script>Phase Calculator
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