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Transformer Sizing Calculator

Estimate the minimum transformer kVA rating needed for a given connected electrical load.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-25 · v1.0.0

Market
Imperial · sales tax

Recommended transformer size

15 kVA

Check your inputs

This is a planning estimate — final transformer sizing for a real installation should be confirmed by an electrical engineer or your utility, accounting for load diversity, future growth, and inrush current from large motors.

Minimum required capacity
12.5 kVA

At the values currently entered, the recommended transformer size works out to 15 kVA. The largest intermediate quantity is minimum required capacity, at 12.5 kVA — check that step first if the total looks off. Note that total connected load sits at the low end of the range this calculator was checked against, so treat the output as indicative rather than settled. Confidence is moderate: the method is sound, but real materials and site conditions vary. Figures are shown for United States, where IRC 2024 is the governing residential reference; switch the market above if you are building elsewhere.

Add the equipment this sizes

This result is a specification — 15 kVA — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • Required kVA = connected watts / 1,000 / power factor; standard distribution transformer sizes commonly include 5, 10, 15, 25, 37.5, 50, 75, 100, 150, and 225 kVA

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Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Frequently asked questions

Why does power factor matter for sizing?
A transformer is rated in kVA (apparent power), not kW (real power) — loads with a lower power factor (like motors) draw more apparent power for the same real wattage, so a lower power factor requires a larger transformer for the same connected watts.
Should I size for today's load or future growth?
Many electrical designers add headroom (often 20-25%) beyond the current connected load specifically to accommodate future additions, since replacing an undersized transformer later is far more disruptive than installing slightly larger capacity upfront.
Is this for a whole utility service or a specific piece of equipment?
This estimates general transformer capacity for a connected load — sizing for a specific application (like a dedicated machine, an outbuilding, or a full utility service) should also account for that context's specific code requirements and inrush current characteristics.