Transformer Sizing Calculator
Size a distribution transformer's kVA rating from connected load, demand factor, power factor and a future-growth margin — rounded up to the nearest standard IEC/ANSI kVA rating, with the resulting secondary full-load current.
Input
The sum of the nameplate ratings of everything that could be switched on at once.
The share of the connected load actually drawn simultaneously in practice.
Lagging power factor of the demand load, between 0.05 and 1.
Spare capacity added on top for future load growth.
Line-to-line voltage on a three-phase secondary, line-to-neutral on single-phase.
Output
| Quantity | Value |
|---|---|
| No data yet | |
Guides
A transformer sized for the sum of every nameplate rating in a building is almost always oversized — real loads rarely all run at once, and power factor pulls the true kVA above the kW figure on top of that. This calculator walks the load through demand factor, power factor, and a growth margin, then rounds up to the nearest standard distribution-transformer rating instead of leaving you with an odd fractional kVA number nobody stocks.
How to use it
- Enter the connected load in kW — the sum of every device's nameplate rating.
- Set the demand factor — the share of that load actually running at the same time. A mixed commercial load rarely exceeds 70–85%.
- Enter the power factor of that demand load. Motors and other inductive equipment commonly sit around 0.8–0.9 lagging.
- Add a growth margin for future expansion — 20% is a common starting point.
- Pick single-phase or three-phase and the secondary voltage.
The tool reports the demand load, the required apparent power, the margin-adjusted kVA, the nearest standard transformer rating, and the secondary full-load current at that rating.
The formulas
Demand load = Connected load × demand factorRequired kVA = Demand load ÷ power factorWith margin = Required kVA × (1 + growth margin)- Rounded up to the nearest rating on the standard IEC/ANSI list (10, 15, 25, 37.5, 50, 75, 100, 150, 167, 200, 225, 250, 300, 400, 500, 750, 1000 kVA and so on)
I = kVA × 1000 ÷ (√3 × V)three-phase, orkVA × 1000 ÷ Vsingle-phase
Why round up to a standard size instead of the exact kVA?
Transformers are manufactured in a fixed set of ratings — ordering an exact 169.4 kVA unit isn't an option. Rounding up guarantees the unit you actually buy can carry the calculated load with the margin already built in.
What demand factor should I use?
It depends heavily on the load type. Continuous industrial process loads can run 90–100%; a mixed office or residential feeder is often 50–70%; consult the applicable electrical code or a load study for a real installation rather than guessing.
Is this enough to order a transformer?
It's a sizing starting point. Inrush current, ambient and altitude derating, harmonic loading, impedance and utility interconnection requirements all need a proper engineering review before specifying a unit.
Privacy
Everything is calculated in your browser. No values are sent to a server.
Related tools
To size the capacitor bank a poor power factor calls for, use the Power Factor Correction Calculator. For the feeder that connects to this transformer, the Wire Size Calculator covers current-carrying capacity.
Use it from code
From 3 credits per callREST API
curl -X POST https://api.iotools.cloud/v1/tool/transformer-sizing-calculator \
-H "Authorization: Bearer YOUR_API_KEY" \
-H "Content-Type: application/json" \
-d '{
"connectedLoadKw": "150",
"demandFactor": "80",
"loadPowerFactor": "0.85",
"growthMargin": "20",
"system": "three",
"voltage": "400"
}'Swap in your own key from your account. The tool's fields are the body — no wrapper.
Ask an AI agent
Use the IOTools `transformer-sizing-calculator` tool (Transformer Sizing Calculator) on this input:
YOUR_INPUT_HEREPaste this at any agent connected to the IOTools MCP server, then add your input.