Transformer Calculator: Sizing, kVA, Buck and Boost
A free transformer calculator for sizing distribution transformers, picking a buck and boost transformer to correct line voltage, computing primary and secondary current from a kVA rating, and calculating available fault current. Built around the standard transformer formulas and NEC 450 protection sizing rules.
Transformer Sizing from Load
Calculate required transformer size based on load requirements with NEC continuous load multipliers.
Operates ≥3 hours continuously
Operates <3 hours
Typical: 0.80-0.90
For future growth
NEC Requirements
- NEC 215.2(A)(1): Continuous loads must be calculated at 125% of actual load
- Definition: Continuous load operates for 3 hours or more
- Power Factor: Lower power factor requires larger transformer for same kW load
- Safety Factor: 25-30% typical for moderate future growth
- Loading: 50-80% utilization is optimal for efficiency and lifespan
Need this validated by a licensed PE?
A free calculator gets you in the ballpark. For permit-stamped, defensible work, ClarkTE delivers a PE-stamped transformer sizing report from a working engineer — typically within 48 hours of receiving your one-line and load data.
How does a transformer work?
A transformer transfers electrical energy between two circuits using magnetic coupling. Alternating current in the primary winding creates a changing magnetic flux in the iron core. That flux induces a voltage in the secondary winding. The voltage ratio between primary and secondary is set by the turns ratio — more turns on a winding means more voltage on that side.
Power on the secondary equals power on the primary (minus a small loss to core heating, copper resistance, and stray flux). So when voltage steps down, current steps up by the same ratio. A 10:1 step-down transformer carrying 10 A on the primary delivers roughly 100 A on the secondary at 1/10 the voltage. The "Sizing from Load" and "Current Calculator" tabs above apply that relationship along with NEC 215.2 and NEC 450.3 protection rules.
Three things drive transformer selection: the kVA rating (how much apparent power it can carry), the voltage ratio (what it converts from and to), and the percent impedance (%Z, which sets fault current and voltage regulation). The fault current tab uses %Z to estimate worst-case bolted fault on the secondary — that number drives interrupting ratings for downstream breakers and is the starting point for IEEE 1584 arc flash analysis.
Buck and boost transformer sizing
A buck and boost transformer is a small autotransformer wired to add (boost) or subtract (buck) a fixed percentage of the input voltage. Common applications include correcting a 208 V supply up to 230 V to match motor or HVAC nameplate, or knocking 240 V down to 208 V for sensitive equipment. The unit is the same — wiring determines whether it bucks or boosts.
To size a buck and boost transformer, you need three numbers: the source voltage, the load voltage, and the load current at the corrected voltage. The kVA rating you select is much smaller than the full load kVA, because only the difference between input and output passes through the windings. A 1 kVA buck-boost can typically handle a 10 kVA load when the voltage shift is small (under 10%).
Use the "kVA Selection" tab above to find the full-load primary and secondary current, then size the buck-boost using the manufacturer's connection diagram. ClarkTE's engineering team has sized these for everything from single hot tubs to 480 V industrial drives — happy to confirm a configuration if you want a second pair of eyes.
Common transformer sizing questions
What kVA transformer do I need?
Add up your continuous load (operates 3+ hours) and multiply by 1.25, then add non-continuous load. Divide by your power factor to get kVA. Add 25% headroom for future growth and round up to the next standard ANSI/IEEE size (15, 25, 37.5, 50, 75, 112.5, 150, 225, 300, 500, 750, 1000 kVA, etc.). The "Sizing from Load" tab automates all of that.
Why is %Z important?
Percent impedance sets two things: how much the secondary voltage sags under load, and how much fault current flows on a short circuit. Lower %Z = stiffer transformer = higher fault current. Distribution transformers are typically 2–6% Z. A 1000 kVA, 480 V, 5.75% Z transformer delivers about 21,000 A of bolted fault current — every downstream breaker has to be rated for that.
Single-phase vs three-phase formulas?
Single phase: I = (kVA × 1000) / V. Three phase: I = (kVA × 1000) / (√3 × V). The √3 factor (≈1.732) comes from the 120° phase relationship between line voltages. Both formulas are baked into every tab of this transformer calculator.
Need a real transformer evaluated, not just sized?
Sizing is one thing. Knowing whether a transformer in service is healthy is another. ClarkTE's transformer services team handles power factor (Doble) testing, sweep frequency response analysis, dissolved gas analysis, and PE-stamped condition reports for distribution and substation transformers from 75 kVA to 230 kV.
Transformer FAQ
What size transformer do I need for a 208/240 V buck-boost application?
Size the buck-boost on the kVA difference between input and output, not the full load kVA. A 1 kVA buck-boost can typically carry a 10 kVA load when the voltage shift is small (under 10%). The kVA Selection tab returns full-load primary and secondary current; pair that with the manufacturer's connection diagram for the right unit.
How does a transformer work, in one paragraph?
Alternating current in the primary winding creates a changing magnetic flux in the iron core. That flux induces a voltage in the secondary winding. Voltage ratio is set by the turns ratio. Power on each side is roughly equal, so when voltage steps down, current steps up by the same ratio. Real-world transformers also have impedance, which sets fault current and voltage regulation.
What is %Z and why does it matter for fault current?
Percent impedance (%Z) sets two things: how much the secondary voltage sags under load, and how much fault current flows on a short. Lower %Z = stiffer transformer = higher fault current. Distribution transformers typically run 2-6% Z. A 1000 kVA, 480 V, 5.75% Z unit delivers about 21,000 A of bolted fault current — every downstream breaker has to be rated for that.
Single-phase vs three-phase formulas?
Single phase: I = (kVA × 1000) / V. Three phase: I = (kVA × 1000) / (√3 × V). The √3 factor (≈1.732) comes from the 120° phase relationship between line voltages. Both formulas are baked into every tab.