Transformer Size Calculator
Convert connected kW and power factor to base kVA, then show a reserve-adjusted planning requirement.
Use the Transformer Size Calculator
Convert connected kW and power factor to base kVA, then show a reserve-adjusted planning requirement.
Calculations happen locally in your browser. ToolLott does not send these inputs to a server for this tool.
Preliminary engineering calculation only. It does not replace applicable codes, manufacturer data, licensed electrical or mechanical design, protection studies, derating, fault checks, site conditions or professional verification.
How to use it
Replace the worked-example values with the actual project inputs. Review the intermediate quantities and assumptions before using the result elsewhere.
How the Transformer Size Calculator works
Convert connected kW and power factor to base kVA, then show a reserve-adjusted planning requirement. The methodology below exposes the production equation, a QA-verified worked example and the boundary between this screen and detailed design.
Calculation breakdown
ToolLott will explain the current inputs and displayed result here.
Transformer apparent-load screening
The production engine evaluates base kVA = load kW / PF; planning kVA = base x (1+reserve). It begins by convert connected real power to apparent load using entered power factor., then apply the selected planning reserve., and finally compare the planning result with available transformer ratings and project design requirements. This keeps the calculation auditable instead of hiding design assumptions behind a single number.
base kVA = load kW / PF; planning kVA = base x (1+reserve)ToolLott evaluates the relationship using the entered units and full numeric precision before display rounding.
| Symbol / input | Meaning | Unit |
|---|---|---|
loadKw | Connected load (kW) | user input |
powerFactor | Power factor | user input |
reserve | Planning reserve (%) | user input |
Step-by-step method
- Convert connected real power to apparent load using entered power factor.
- Apply the selected planning reserve.
- Compare the planning result with available transformer ratings and project design requirements.
Worked example
Leo, a project engineer, has 180 kW of three-phase connected load at an estimated 0.90 power factor and wants a preliminary transformer kVA check.
Example inputs
- Connected load (kW): 180
- Power factor: 0.9
- Planning reserve (%): 20
Calculation / processing
Base apparent load = 180 / 0.9 = 200 kVA.Reserve = 200 x 20% = 40 kVA.Planning requirement = 240 kVA.
Use the result as a screening or quantity-planning value and compare it with the project criteria, manufacturer data and applicable engineering requirements before making a design or procurement decision.
Assumptions
- The entered values are representative of the condition being screened and use the units shown on the page.
- The equation models only the stated relationship; omitted system effects are not silently estimated.
Limitations
- This page is not engineering certification and does not replace applicable standards, design loads, manufacturer data, site investigation or competent professional review.
- Results can change materially when real geometry, losses, transients, material properties, duty cycles, safety factors or regulatory criteria differ from the simplified inputs.
Common questions
Is this a final engineering design?
No. It is a transparent screening calculation using the entered assumptions and the stated equation.
Why does the page show assumptions and limitations?
Engineering formulas are only valid within their modelling assumptions; exposing them helps users decide what additional design checks are required.
Should I use the rounded displayed value in later design work?
Use suitable calculation precision and the governing project/standard requirements rather than relying on display rounding alone.
Methodology sources
Related ToolLott tools
A realistic way Leo could use this tool
Leo is a project engineer.
Leo has 180 kW of three-phase connected load at an estimated 0.90 power factor and wants a preliminary transformer kVA check.
In the example, Leo uses load kw = 180; power factor = 0.9; reserve = 20. The observable result is Base apparent load: 200 kVA; the worked case therefore connects the source data directly to the number, text or setting shown on the page.
What changes for Leo is now explicit: Base apparent load: 200 kVA. The result can be checked against the real source data before they convert connected kW and power factor to base kVA, then show a reserve-adjusted planning requirement. The page also keeps this limitation explicit: Preliminary screening or quantity calculation only.
Fictional scenario using realistic example data. For Ready tools, the worked result is tied to the tested example shown in the tool. Replace the figures with your own inputs and independently verify important professional, financial, legal, health or safety decisions.
What this calculator is for
Use it to convert connected kW and power factor to base kVA, then show a reserve-adjusted planning requirement.
It sits within ToolLott’s Electrical collection, where you can also estimate battery banks, cable sizes, generator sizes, motor current, Ohm’s law values, solar systems, transformers and voltage drop.