Transformer Loading calculator
What the size you chose costs in losses, efficiency and top-oil temperature.
Power & Utilities · Interactive Engineering Model
Transformer Sizing Calculator
Demand · Requirement · Standard rating
Change the connected load and the judgements around it — demand factor, power factor, growth allowance, target loading — and watch the requirement move continuously while the transformer you can order jumps to the next standard size.
Sum of the nameplate loads to be supplied, before the demand factor is applied.
Maximum coincident demand as a share of connected load — at or below 100%. This is the demand factor; diversity factor is its near-inverse (the sum of individual maxima over the coincident maximum) and is greater than one, so the two must not be swapped.
Expected power factor of the demand, after any correction you intend to install.
How heavily you intend to load the unit at the design condition. 80% is common; higher accepts less margin for growth and heat.
Start from an example: 1,200 kW connected, 70% demand factor, 25% growth, loaded to 80% →
Advanced parameters1
- —Connected
- —Demand factor
- —Demand
- —Growth
- —Requirement
- —Standard rating
- —Standard rating
- —Calculated requirement
- —Loading at the standard rating
- —Standard size over requirement
- —Demand including growth
- —Demand today
- —Demand today
- —Spare capacity
Move a parameter and the substation answers: the demand the site actually makes, the capacity that demand asks for, and the size anyone would actually install.
The link carries the figures you entered, in the part of the address that is never sent to a server.
Change one assumption at a time. The model updates from the same calculation used for the reported result.
Try this
Increase connected load slowly. Watch the calculated requirement move continuously — and the selected transformer jump discretely to the next standard size.
How the model works
- The demandNot everything connected to a site runs at once. The demand factor is the judgement about how much of it does, and it turns a schedule of connected load into the load the transformer actually sees.
- The kVAA transformer is rated in kVA, not kW, because it is the current that heats it. Dividing by the power factor converts the real power the site uses into the apparent power the transformer has to carry.
- The room left overGrowth is capacity bought today for load that has not arrived yet, and the target loading is the room left for losses, heat and the next surprise. Both raise the requirement above the demand.
- The size that existsTransformers are made in standard sizes. The requirement is rounded up to the next one, and the gap between the two is capacity you own and pay for in core loss every hour it is energised.
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 →
What this model reveals
Sizing is a chain of judgements and then a rounding. Demand factor, power factor, growth allowance and target loading each scale the requirement; the answer is then taken up to a size that is actually manufactured. The step between the two is capacity you own and pay for in core loss every hour it is energised. The model shows the chain and the rounding separately, because they are argued about separately.
Go deeper
Power Grid & Utility Operations challenge
Transformers, substations, feeders and protection coordination.
GridModel case study
A pilot-ready enterprise platform for the electric-utility industry.