Transformer Loading calculator
What the size you chose costs in losses, efficiency and top-oil temperature.
Power & Utilities · Professional Microtool
Transformer Sizing Calculator
Demand · Requirement · Standard rating
Turn connected load into a transformer requirement: demand after the demand factor, apparent power, an allowance for growth, a target loading — and the standard rating you can actually order against it.
- —Connected
- —Demand factor
- —Demand
- —Growth
- —Requirement
- —Standard rating
Start from an example: 1,200 kW connected, 70% demand factor, 25% growth, loaded to 80% →
Full output
- —Calculated requirement
- —Loading at the standard rating
- —Standard size over requirement
- —Demand including growth
- —Demand today
- —Demand today
- —Spare capacity
Standard transformer rating against connected load
This is the only staircase on the site, and it is the honest shape: ratings come in steps, so a small change in the load assumption can move you a whole size — and a size you did not need is paid for in core loss every hour for thirty years.
Keep it
Both carry the figures you entered, in the part of the address that is never sent to a server. Share only where that is appropriate. To keep a copy for a project file, print the page — it lays itself out as a document.
Want sizing driven by actual load history instead of assumptions?
We can work from real load profiles, forecast growth, motor starting and redundancy — and put the result into your planning workflow rather than a spreadsheet.
Calculation basis 5 steps · view the method →
Demand = connected load × demand factor—Apparent power = demand ÷ power factor—With growth = apparent power × (1 + growth allowance)—Requirement = demand with growth ÷ target loading—Standard rating = the next size available at or above the requirement—
Assumptions & limitations Screening estimate for preliminary selection and budgeting · 7 assumptions →
This is a preliminary selection aid, not a design. It does not consider motor starting or inrush, short-circuit contribution and downstream fault levels, harmonic derating for non-linear load, altitude and ambient derating, redundancy or N-1 arrangements, or the loss evaluation that often decides between two candidate ratings. Confirm against the applicable standard and the manufacturer's data.
- Standard ratings are common commercially available ANSI-style sizes; the IEEE C57.12.00 preferred sequence is slightly different — it has no 3,000 kVA — and IEC preferred numbers differ again.
- One demand factor is applied to the whole connected load rather than by load group, which is the assumption most worth challenging in any sizing argument.
- Motor starting, inrush and the resulting voltage dip are not considered.
- No derating is applied for ambient, altitude, harmonics or non-linear load.
- Redundancy is not considered: this sizes one unit for the whole demand.
- Growth is applied as a single multiplier rather than as a load forecast.
- Loss evaluation, which often justifies the next size up or down, is out of scope.
Worked examples
- 1,200 kW connected, 70% demand factor, 25% growth, loaded to 80%
Open with these figures → - The same site with no growth allowance — the same rating, more lightly loaded
Open with these figures → - Correcting the power factor to 0.95 instead
Open with these figures →
About this tool
Sizing arguments usually collapse into one number that nobody can reconstruct. This tool keeps the steps apart — diversity, power factor, growth, target loading — so each assumption can be challenged on its own, and then does the part the arithmetic cannot: rounds to a rating that exists. Written for electrical and facilities engineers preparing or reviewing a selection.
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