Transformer sizing calculator
15,000 W of connected load at 240 V single-phase, with a 0.9 power factor, 0.8 demand and the 1.25 continuous factor, calls for a 25 kVA transformer. You also see secondary current to check wire and breaker. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Load watts
- 15,000
- Voltage
- 240
- Phases
- 1
- Power factor
- 0.9
- Demand factor
- 0.8
- Continuous factor
- 1.25
- Effective load w
- 15,000
- Kva required
- 16.6666666667
- Recommended kva
- 25
- Secondary current a
- 104.1666666667
Choosing a transformer that is too small overheats windings and trips protection under continuous load; choosing one that is far too large wastes money, panel space, and magnetizing losses. Homeowners, landscape contractors, and electrical estimators need a transparent kVA number they can match to catalog ratings for twelve-volt path lighting, twenty-four-volt systems, or low-voltage residential single-phase and three-phase secondaries. This transformer sizing calculator takes the connected load in watts, the secondary voltage, optional demand and continuous factors, and power factor, then converts effective load to required kVA and rounds up to standard commercial sizes. It also reports secondary current so you can check wire ampacity and breaker coordination.
How connected load, voltage, and factors set transformer kVA
Transformer nameplate capacity is stated in kilovolt-amperes of apparent power, not in real watts alone. Real power in watts equals apparent power times power factor, so a lagging motor or magnetic load needs more kVA than a purely resistive heater of the same wattage. Effective watts divided by one thousand times power factor yields the minimum required kVA.
That raw value is rounded up to the next standard commercial rating such as half, one, three, five, ten, fifteen, twenty-five, or fifty kVA, including small landscape-class sizes starting at fifty volt-amperes. Secondary voltage is required because current, not only kVA, drives conductor size: for single-phase, amperes equal recommended kVA times one thousand divided by volts; for three-phase, divide by volts times the square root of three using line-to-line voltage.
Landscape jobs often use twelve, fifteen, or twenty-four volts with hundreds of watts of LEDs; residential low-voltage secondaries more often use one hundred twenty or two hundred forty volts with multi-kilowatt connected loads. Either way the math is the same deterministic chain: demand-adjusted continuous watts to kVA to catalog size to secondary amperes.
Who sizes transformers this way, and when
Home-improvement and landscape sites that rank for transformer sizing calculator need a live tool whose free browser path matches the paid automation path on the same pure module. Landscape contractors batch-size path-light transformers by posting summed LED wattage and twelve-volt secondary voltage with a continuous factor for dusk-to-dawn runs. Property managers and low-voltage remodel crews compare single-phase two-forty-volt subfeeds against three-phase options by toggling phases on the same connected load.
Failed validation does not consume balance.
What you can do with it
Residential low-voltage panel and subfeed quotes
Enter connected branch load, one-twenty or two-forty volts, demand diversity, and power factor to recommend single-phase transformer kVA before ordering gear.
FAQ
How does the transformer sizing calculator choose kVA?
It multiplies connected watts by demand and continuous factors, divides by one thousand times power factor to get required kVA, then rounds up to the next standard commercial transformer rating.
Does voltage change the kVA result?
Required kVA comes from watts, demand, continuous factor, and power factor. Voltage is used to compute secondary current so you can check conductors and overcurrent devices on the load side.
What is the difference from a turns-ratio calculator?
Turns-ratio tools solve Np/Ns and ideal primary/secondary voltages. This tool sizes capacity in kVA from connected load for purchase and wire planning on LV or landscape circuits.
Why does a lower power factor call for a bigger transformer?
Transformers are rated in kilovolt-amperes of apparent power, and real watts equal apparent power times power factor. So a motor or magnetic load with a lagging power factor needs more kVA than a resistive heater of the same wattage.
What is the continuous factor and when can I set it to 1?
It multiplies the demand-adjusted watts before they are turned into kVA. The default 1.25 follows the common 125% rule for loads that run for hours, such as dusk-to-dawn lighting. Use 1.0 only when the load is known to be non-continuous and your code allows it.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/home/transformer-sizing \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"load_watts":15000,"voltage":240}'const res = await fetch("https://api.kit.forhosting.com/home/transformer-sizing", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"load_watts": 15000,
"voltage": 240
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/home/transformer-sizing",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"load_watts": 15000,
"voltage": 240
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/home/transformer-sizing", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"load_watts":15000,"voltage":240}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"load_watts":15000,"voltage":240}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/home/transformer-sizing", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"load_watts": 15000,
"voltage": 240
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "home.transformer_sizing",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
Input fields, defaults, aliases, and response shape
Optionally set phases to one for single-phase (default) or three for three-phase so secondary current uses the correct formula. POST a JSON body with load_watts as a required positive number for total connected secondary load in watts, and voltage as a required positive secondary RMS voltage in volts. Optionally set power_factor between zero point one and one (default one for resistive and modern LED loads), demand_factor between zero point zero one and one (default one for full connected load), and continuous_factor between one and two (default one point two five). Aliases such as connected_load, watts, secondary_voltage, pf, diversity, and margin are accepted so forms and spreadsheets can map without renaming every column. Missing load or voltage, non-positive values, out-of-range factors, or phases other than one or three fail with invalid_input and are not billed. On success the body echoes load_watts, voltage, phases, power_factor, demand_factor, and continuous_factor; reports effective_load_w and kva_required before round-up; gives recommended_kva and matched_standard after catalog sizing; returns secondary_current_a at the recommended rating plus kva_required_current_a at the raw requirement; includes utilization, headroom_kva, and headroom_percent; and adds method, formula, message, and step strings for display. Pair the recommendation with local electrical code, manufacturer temperature ratings, inrush, and primary-side coordination before you buy: this endpoint is a transparent sizing aid, not a sealed design or permit package.
Where a transformer sizing API earns its keep
Electrical estimators embed the call in quote forms so customers see a recommended kVA and secondary amperes next to wire and breaker language before a site visit. Spreadsheet plugins and home-audit apps call the API to fill a recommended_kva column without shipping opaque vendor tables.
Integration use cases
Landscape lighting transformer selection
Sum LED fixture watts on a twelve- or fifteen-volt run, apply continuous factor, and pick the next standard kVA (or VA) rating with secondary current for cable sizing.
SEO widgets and contractor lead forms
Embed the free browser path on a transformer sizing calculator page so visitors get the same kVA and amperes your API later uses in automation.
Technical questions
Why is continuous_factor default 1.25?
Many continuous loads are planned at one hundred twenty-five percent of continuous amperes. Set continuous_factor to 1.0 when the load is non-continuous and your code path allows sizing without that margin.
What does the transformer sizing calculator cost?
The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input is not billed.