Transformer Sizing Calculator

Frequently Asked Questions

How do I size a transformer?

Total the connected load in kVA (or volts × amps), add headroom for growth and inrush (typically 20–25%), and choose the next standard kVA rating above that.

How do I find full-load amps?

Single-phase: I = kVA × 1000 ÷ V. Three-phase: I = kVA × 1000 ÷ (√3 × V). Compute both primary and secondary to size conductors and protection.

Why add headroom?

Continuous loading at 100% shortens insulation life and leaves no margin for motor starting or future load. Most designs target 70–80% steady loading.

Is kVA or kW used for sizing?

Transformers are rated in kVA because they are limited by current (heating), independent of power factor. Convert kW to kVA with the load power factor before sizing. This is an estimate - confirm with code and a qualified engineer.

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Estimates for informational purposes only.

Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator Works

A transformer is rated in kVA (apparent power) because its conductors and core are limited by current and voltage, not the load's power factor. Sizing has three steps. First, determine the connected load in kVA - entered directly, or computed from voltage and current: kVA = V×I÷1000 for single phase and kVA = √3×V×I÷1000 for three phase. Second, add headroom (a spare-capacity / growth allowance, commonly 20–25%) so the transformer is not run at 100% continuously. Third, round up to the next standard ANSI/NEMA kVA rating - you can never specify a non-standard size off the shelf. Full-load amps on each winding follow from I = kVA×1000 ÷ V (1φ) or ÷(√3·V) (3φ), used to size primary and secondary protection and conductors.

Worked Numeric Example

A panel draws 90 A at 480 V three-phase. Connected load = √3 × 480 × 90 ÷ 1000 = 74.8 kVA. Apply 25% headroom: 74.8 × 1.25 = 93.5 kVA required. The next standard rating above that is 100 kVA. For a 480 V primary the full-load current is 100,000 ÷ (√3 × 480) = 120.3 A; on a 208 V secondary it is 100,000 ÷ (√3 × 208) = 277.6 A. These currents set the primary and secondary breaker and conductor sizes per the applicable code. The lower-voltage winding always carries proportionally more current.

Real-World Use

This calculation specifies dry-type and pad-mounted distribution transformers for commercial buildings, industrial plants, and utility services: stepping 480 V down to 208/120 V for receptacles and lighting, feeding motor control centres, or isolating sensitive equipment. It is also used to verify an existing transformer is not overloaded after adding load, to estimate no-load and load losses for energy studies, and to coordinate primary and secondary overcurrent protection.

Common Mistakes

Sizing on kW instead of kVA underestimates the transformer for any low-power-factor load. Forgetting growth headroom leads to a transformer running hot at full load with no spare capacity for future circuits or motor inrush. Choosing a value below the calculated requirement because it "looks close" ignores that ratings are discrete - always round up. Mixing single- and three-phase formulas (omitting the √3) miscalculates current by 73%. Ignoring large motor starting kVA, which can momentarily be 5–7× running load and cause excessive voltage dip on an undersized unit.

Limitations & Related Tools

This is a steady-state apparent-power sizing tool. It does not apply demand factors or diversity (not all loads run simultaneously - the code permits sizing on the calculated demand load, often less than connected load), nor does it account for harmonic heating from non-linear loads, which may require a K-rated transformer derate. Ambient temperature, altitude, and impedance affect actual capacity and voltage regulation. Pair this with a load schedule / demand factor calculator, a voltage-drop tool for the feeders, a short-circuit / available fault current study, and the kVA-kW-PF calculator to convert real-power loads to apparent power before sizing.

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