Documentation sections

Cable Voltage Drop Calculator

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

Slug voltage-drop. Calculate voltage drop across a cable run. Enter supply voltage, load current, wire gauge (AWG), and one-way distance to get voltage drop in volts and percent, power loss, and NEC 3%/5% compliance status. Supports copper and aluminum conductors.

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 voltage-drop
Name	Unit	Range	Default	Description
voltagenumber	V	1 … 10000	120	Supply Voltage
currentnumber	A	0.1 … 1000	15	Load Current
wireGaugenumber	none	0 … 24	12	12 AWG typical for 20A circuits
cableLengthnumber	m	0.5 … 5000	30	Distance from panel to load (not round-trip)
conductorTypenumber	none	0 … 1	0	ConductorOne of: 0 Copper · 1 Aluminum

Outputs

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

Outputs of voltage-drop
Field	Unit	Description
voltageDropheadline	V	Voltage Drop
voltageDropPercent	%	Voltage Drop
voltageAtLoad	V	Voltage at Load
powerLoss	W	Cable Power Loss
totalResistance	Ω	Round-Trip Resistance
necStatus	none	NEC Status (1=OK, 2=Marginal, 3=Fail)
maxLengthFor3Pct	m	Max Length for 3% Drop

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": "voltage-drop",
    "inputs": {
      "voltage": 120,
      "current": 15,
      "wireGauge": 12,
      "cableLength": 30,
      "conductorType": 0
    }
  }'

import rftools

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

result = client.calculate("voltage-drop", {
    "voltage": 120,
    "current": 15,
    "wireGauge": 12,
    "cableLength": 30,
    "conductorType": 0,
})
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": "voltage-drop",
    "inputs": {
      "voltage": 120,
      "current": 15,
      "wireGauge": 12,
      "cableLength": 30,
      "conductorType": 0
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "voltage-drop",
	  "inputs": {
	    "voltage": 120,
	    "current": 15,
	    "wireGauge": 12,
	    "cableLength": 30,
	    "conductorType": 0
	  }
	}`)
	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": "voltage-drop",
  "values": {
    "voltageDrop": 4.689900000000001,
    "voltageDropPercent": 3.9082500000000007,
    "voltageAtLoad": 115.3101,
    "powerLoss": 70.34850000000002,
    "totalResistance": 0.31266000000000005,
    "necStatus": 2,
    "maxLengthFor3Pct": 23.028209556706962
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:voltage-drop",
    "version": "api@3f2c1a9b04de",
    "formulaRef": null,
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "voltage": {
          "min": 1,
          "max": 10000,
          "unit": "V"
        },
        "current": {
          "min": 0.1,
          "max": 1000,
          "unit": "A"
        },
        "wireGauge": {
          "min": 0,
          "max": 24,
          "unit": ""
        },
        "cableLength": {
          "min": 0.5,
          "max": 5000,
          "unit": "m"
        },
        "conductorType": {
          "min": 0,
          "max": 1,
          "unit": ""
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "voltage": 120,
      "current": 15,
      "wireGauge": 12,
      "cableLength": 30,
      "conductorType": 0
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "voltage"
      ],
      "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 pageCable Voltage Drop Calculator, at these values