Documentation sections

Winding Resistance vs Temperature

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

Slug motor-winding-resistance. Calculate motor winding resistance at any temperature using copper or aluminum TCR. Get resistance change percentage for motor derating. Free, instant results.

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 motor-winding-resistance
Name	Unit	Range	Default	Description
resistance25number	Ω	at least 0.001	1	Resistance at 25°C
temperaturenumber	°C	−40 … 200	80	Operating Temperature
tempCoeffnumber	/°C	at least 0	0.00393	Copper: 0.00393/°C, Aluminium: 0.00429/°C

Outputs

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

Outputs of motor-winding-resistance
Field	Unit	Description
resistanceAtTempheadline	Ω	Resistance at Temp
resistanceChange	Ω	Resistance Change
resistanceChangePercent	%	Change

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": "motor-winding-resistance",
    "inputs": {
      "resistance25": 1,
      "temperature": 80,
      "tempCoeff": 0.00393
    }
  }'

import rftools

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

result = client.calculate("motor-winding-resistance", {"resistance25": 1, "temperature": 80, "tempCoeff": 0.00393})
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": "motor-winding-resistance",
    "inputs": {
      "resistance25": 1,
      "temperature": 80,
      "tempCoeff": 0.00393
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "motor-winding-resistance",
	  "inputs": {
	    "resistance25": 1,
	    "temperature": 80,
	    "tempCoeff": 0.00393
	  }
	}`)
	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": "motor-winding-resistance",
  "values": {
    "resistanceAtTemp": 1.21615,
    "resistanceChange": 0.21615000000000006,
    "resistanceChangePercent": 21.615000000000006
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:motor-winding-resistance",
    "version": "api@3f2c1a9b04de",
    "formulaRef": null,
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "resistance25": {
          "min": 0.001,
          "max": null,
          "unit": "Ω"
        },
        "temperature": {
          "min": -40,
          "max": 200,
          "unit": "°C"
        },
        "tempCoeff": {
          "min": 0,
          "max": null,
          "unit": "/°C"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "resistance25": 1,
      "temperature": 80,
      "tempCoeff": 0.00393
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "resistance25"
      ],
      "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 pageWinding Resistance vs Temperature, at these values