Documentation sections

PCB Trace Fusing Current Calculator

POSThttps://rftools.io/api/py/v1/calculate

Slug fusing-current. Calculate the current that will fuse a PCB trace using Onderdonk's equation. Includes etch-factor trapezoid correction, time-to-fuse at your operating current, and safety margin.

Inputs

Send every one of these in inputs, in the units shown. Most are required: a call without one is refused (400) and costs nothing; one the calculator can do without takes the default shown. An input outside its range is still computed and flagged in the provenance.

Inputs of fusing-current
Name	Unit	Range	Default	Description
traceWidthnumber	mm	at least 0.02	0.5	Width at the base of the trace, where it meets the laminate
copperThicknessnumber	μm	at least 5	35	Copper Weight
durationnumber	s	at least 0.001	1	Fault Duration
ambientTempnumber	°C	−55 … 150	25	Ambient Temperature
etchFactornumber	none	0 … 5	0	Etch depth divided by lateral undercut per side (IPC-T-50). A larger number means straighter sidewalls; enter 0 for perfectly vertical walls
operatingCurrentnumber	A	at least 0	5	Optional — used to report time-to-fuse and safety margin

Outputs

Returned in values, by key. A value that is unbounded or undefined is null.

Outputs of fusing-current
Field	Unit	Description
fusingCurrentheadline	A	Fusing Current
timeToFuse	s	Time to Fuse at Operating Current
currentMargin	×	Margin to Fusing
crossSectionArea	sq mil	Cross-Sectional Area
effectiveWidth	mm	Effective (Mean) Width

Calling it

Calculate: one call, the request and response in full, and its errors.
Batch: this calculator with others, or a sweep, in one request.
Solve for a target: the value of one input that brings an output to a target.

curl -X POST https://rftools.io/api/py/v1/calculate \
  -H "X-API-Key: $RFTOOLS_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "slug": "fusing-current",
    "inputs": {
      "traceWidth": 0.5,
      "copperThickness": 35,
      "duration": 1,
      "ambientTemp": 25,
      "etchFactor": 0,
      "operatingCurrent": 5
    }
  }'

import rftools

client = rftools.Client()  # reads RFTOOLS_API_KEY, or the key `rftools login` saved

result = client.calculate("fusing-current", {
    "traceWidth": 0.5,
    "copperThickness": 35,
    "duration": 1,
    "ambientTemp": 25,
    "etchFactor": 0,
    "operatingCurrent": 5,
})
print(result.values)
print(result.warnings, result.provenance.formula_ref)

const response = await fetch("https://rftools.io/api/py/v1/calculate", {
  method: "POST",
  headers: {
    "X-API-Key": process.env.RFTOOLS_API_KEY,
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    "slug": "fusing-current",
    "inputs": {
      "traceWidth": 0.5,
      "copperThickness": 35,
      "duration": 1,
      "ambientTemp": 25,
      "etchFactor": 0,
      "operatingCurrent": 5
    }
  }),
});
console.log(response.status, await response.json());

package main

import (
	"bytes"
	"fmt"
	"io"
	"net/http"
	"os"
)

func main() {
	body := []byte(`{
	  "slug": "fusing-current",
	  "inputs": {
	    "traceWidth": 0.5,
	    "copperThickness": 35,
	    "duration": 1,
	    "ambientTemp": 25,
	    "etchFactor": 0,
	    "operatingCurrent": 5
	  }
	}`)
	req, err := http.NewRequest("POST", "https://rftools.io/api/py/v1/calculate", bytes.NewReader(body))
	if err != nil {
		panic(err)
	}
	req.Header.Set("X-API-Key", os.Getenv("RFTOOLS_API_KEY"))
	req.Header.Set("Content-Type", "application/json")
	res, err := http.DefaultClient.Do(req)
	if err != nil {
		panic(err)
	}
	defer res.Body.Close()
	out, _ := io.ReadAll(res.Body)
	fmt.Println(res.Status, string(out))
}

application/json

{
  "slug": "fusing-current",
  "values": {
    "fusingCurrent": 5.052590095074653,
    "crossSectionArea": 27.125054250108505,
    "timeToFuse": 1.0211466667538598,
    "currentMargin": 1.0105180190149308,
    "effectiveWidth": 0.5
  },
  "warnings": [
    "Only 1.01× margin to fusing — this is a survival limit, not a continuous rating. Size continuous current with IPC-2152 instead"
  ],
  "errors": [],
  "provenance": {
    "method": "calculator:fusing-current",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "I. M. Onderdonk, as published in E. R. Stauffacher, \"Short-time Current Carrying Capacity of Copper Wire\", General Electric Review, 1928; IPC-2152 Appendix A",
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "traceWidth": {
          "min": 0.02,
          "max": null,
          "unit": "mm"
        },
        "copperThickness": {
          "min": 5,
          "max": null,
          "unit": "μm"
        },
        "duration": {
          "min": 0.001,
          "max": null,
          "unit": "s"
        },
        "ambientTemp": {
          "min": -55,
          "max": 150,
          "unit": "°C"
        },
        "etchFactor": {
          "min": 0,
          "max": 5,
          "unit": ""
        },
        "operatingCurrent": {
          "min": 0,
          "max": null,
          "unit": "A"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "traceWidth": 0.5,
      "copperThickness": 35,
      "duration": 1,
      "ambientTemp": 25,
      "etchFactor": 0,
      "operatingCurrent": 5
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "traceWidth"
      ],
      "msg": "Input should be a valid number, unable to parse string as a number",
      "input": "abc"
    }
  ]
}

An input sent as text. Nothing was counted.

{
  "detail": "More than 120 requests in 60 seconds from this address.",
  "errorKind": "rate_limited",
  "limit": 120,
  "windowSeconds": 60,
  "upgradeUrl": "https://rftools.io/pricing/"
}

Too many requests in a minute, with Retry-After.

Same inputs on the calculator pagePCB Trace Fusing Current Calculator, at these values