Fuse I²t melting calculator
This fuse I²t melting calculator squares the prospective RMS fault current, multiplies it by the fault duration, and compares the resulting ampere-squared seconds with the fuse manufacturer's melting or pre-arcing I²t rating.
Run — free
It reports the calculated stress, the rating, their ratio, the percentage of the threshold reached, and the remaining or exceeded margin. Use it for an initial protection check, while remembering that a complete design must also consider the fuse's total-clearing I²t, time-current curve, voltage rating, and interrupting capacity.
What the I²t comparison means
A fuse responds to heating produced by current, and the simplified thermal measure used for short fault events is current squared multiplied by time. Enter the prospective RMS current that is expected to pass through the fuse and the duration of that current. The calculator returns fault I²t in ampere-squared seconds and divides it by the fuse's published melting or pre-arcing I²t. A ratio of one means the simplified fault pulse exactly reaches the stated melting threshold. A ratio above one means that the available current-time energy reaches or exceeds that threshold, so the result is marked melting_threshold_reached. A ratio below one means the modeled pulse does not supply the published melting energy and the fuse should not be assumed to melt from that pulse alone. This is an energy comparison, not a prediction of an exact opening time. Real fault current can vary during the event, while this calculator assumes the entered current remains constant for the entered duration.
Choose the correct manufacturer value
Use a melting or pre-arcing I²t value when the question is whether the fuse element begins to melt. Do not silently substitute a total-clearing I²t value: total-clearing energy includes the arcing interval after melting begins and answers a different coordination question. Datasheets may publish several values for one fuse family, often separated by rated current, operating voltage, or test condition, so select the row that matches the exact part and application. Enter current in amperes, duration in seconds, and the rating in ampere-squared seconds. Convert milliseconds to seconds before entering the duration; for example, divide the millisecond value by one thousand. The percentage output is the ratio multiplied by one hundred, and the signed margin is calculated fault I²t minus fuse rating. A positive margin indicates that the melting threshold is exceeded by that amount, while a negative margin shows the shortfall. Preserve realistic significant figures because fault studies and datasheet values both carry uncertainty.
Use the result in a complete protection check
Treat the threshold result as one piece of a protection study. Reaching melting I²t suggests that the element receives enough thermal energy to begin opening, but it does not by itself establish safe interruption. Confirm that the fuse interrupting rating exceeds the prospective fault current at the applicable system voltage. Then review the manufacturer's time-current curve and total-clearing I²t, especially when coordinating with upstream breakers, downstream semiconductors, cables, busbars, or contactors. A protected component may have its own withstand I²t, and that limit should normally remain above the fuse's total-clearing let-through energy with an engineering allowance for tolerances and operating conditions. Temperature, aging, current waveform, DC time constants, enclosure conditions, and manufacturer test assumptions can all change actual behavior. For a changing current waveform, calculate or obtain the integral of current squared over time instead of representing the entire event with one constant RMS value. The API calculation costs $0.002 per request and remains deterministic for the same inputs.
What you can do with it
Screen a prospective short circuit
Compare a fault-study current and clearing interval with a candidate fuse's melting I²t before detailed coordination work.
Check a pulse against the melting threshold
Determine whether a known constant-current pulse contains enough I²t to reach the manufacturer's pre-arcing value.
Review fuse and component coordination
Use the ratio and margin as an initial input when comparing fuse data with cable, busbar, or semiconductor withstand limits.
FAQ
What formula does the calculator use?
It uses I²t = I × I × t for a constant RMS fault current, then divides that result by the entered fuse melting I²t rating.
Does a ratio above one prove that the fault clears safely?
No. It shows that the simplified pulse reaches the melting threshold. Safe interruption also depends on total-clearing I²t, interrupting capacity, voltage rating, and the time-current curve.
Should I enter melting I²t or total-clearing I²t?
Enter melting or pre-arcing I²t for this threshold test. Use total-clearing I²t separately when checking let-through energy and protection coordination.
Can I enter fault duration in milliseconds?
The duration field uses seconds. Divide milliseconds by one thousand before entering the value.
What does a negative margin mean?
The calculated fault I²t is below the fuse rating by the magnitude shown, so the modeled pulse does not reach the stated melting threshold.
How much does an API calculation cost?
Each API request costs $0.002. The calculation uses no network service and returns the same result for the same inputs.
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/elec/fuse-i2t \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"fault_current_a":500,"duration_s":0.02,"fuse_i2t_a2s":4000}'const res = await fetch("https://api.kit.forhosting.com/elec/fuse-i2t", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"fault_current_a": 500,
"duration_s": 0.02,
"fuse_i2t_a2s": 4000
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/elec/fuse-i2t",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"fault_current_a": 500,
"duration_s": 0.02,
"fuse_i2t_a2s": 4000
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/elec/fuse-i2t", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"fault_current_a":500,"duration_s":0.02,"fuse_i2t_a2s":4000}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"fault_current_a":500,"duration_s":0.02,"fuse_i2t_a2s":4000}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/elec/fuse-i2t", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"fault_current_a": 500,
"duration_s": 0.02,
"fuse_i2t_a2s": 4000
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "elec.fuse_i2t",
"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. |