Transformer Sizing Calculator

Free Transformer Sizing Calculator — estimate the kVA rating required for single or three-phase loads.

883.7K uses Updated · 2026-05-11 Runs locally · zero upload
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The Transformer Sizing Calculator estimates the kVA rating required for a load based on voltage, current or real power, automatically applying a configurable safety factor.

How to Use Transformer Sizing Calculator

  1. Pick the phase — single-phase or three-phase.
  2. Pick the calculation mode — V & I, or P & PF.
  3. Enter the load parameters — voltage, current, real power and power factor as required.
  4. Pick a safety factor — typically 1.2–1.5×.
  5. Read the recommended size — both the raw load kVA and the rounded-up standard size are shown.

Formula & Theory — Transformer Sizing Calculator

Single-phase:  kVA = V · I / 1000
Three-phase:  kVA = √3 · V · I / 1000
From real power:  kVA = kW / PF
Rated = kVA × Safety Factor

Use Cases for Transformer Sizing Calculator

  • Electrical installations — first-pass sizing for distribution transformers.
  • Industrial machinery — sizing dedicated step-down transformers for motors.
  • HVAC systems — estimating transformer needs for chillers and AHUs.
  • Energy audits — verifying that existing transformers are not overloaded.

Frequently asked questions about Transformer Sizing Calculator

Why is the result in kVA and not kW?

Transformers are rated in apparent power because they must supply both real and reactive components of the load.

Why round up to a standard size?

Commercial transformers come in fixed kVA ratings; choosing the next standard size simplifies procurement and spares.

Does inrush current change the sizing?

Yes. Motor and transformer inrush can be 6–10× nominal; for those loads the safety factor or selection table must be increased.

Is my data stored?

No. All calculations run locally in your browser; nothing is sent to any server.