ForHosting KIT · Engineering & construction

Wire ampacity derating calculator

A 30 A wire rated 90 °C, in 40 °C air with six current-carrying conductors, drops to 21.84 A. Enter the table rating, ambient temperature, insulation column and conductor count to see each factor and the result. Free in your browser.

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Worked example

If you enter

Base ampacity: 30
Ambient c: 40
Rating c: 90
Conductors: 6

You get

Temp factor: 0.91 A
Bundling factor: 0.8
Combined factor: 0.728
Derated ampacity: 21.84

Conductor ampacity tables assume a baseline ambient temperature and no more than three current-carrying conductors in a raceway or cable. Real installations are hotter, denser, or both, so the safe continuous current must be reduced with temperature and bundling correction factors before you size breakers, feeders, or branch circuits. This wire ampacity derating calculator multiplies a base table rating by an ambient factor for the insulation temperature column (60, 75, or 90 °C) and a bundling factor for the number of current-carrying conductors, returning the combined derating and the final derated ampacity in amperes. Use it as a fast design aid; always confirm against the current code edition and your authority having jurisdiction.

Why base ampacity must be derated for heat and bundling

Published ampacity tables list the maximum continuous current a conductor can carry under standardized conditions: typically not more than three current-carrying conductors, ambient near thirty degrees Celsius, and the insulation temperature rating of the wire. When ambient temperature rises, less heat can leave the conductor before the insulation approaches its thermal limit, so allowable current falls. When more than three current-carrying conductors share a raceway or cable, mutual heating reduces the capacity of each wire, so a separate bundling adjustment is applied.

Both corrections are multiplicative: you take the base table value for size, material, and insulation column, multiply by the ambient temperature correction factor for that column, then multiply again by the bundling factor for the conductor count. Skipping either step overstates safe load and can leave a feeder or branch circuit undersized for the true thermal environment.

This calculator encodes common NEC-style ambient bands for sixty, seventy-five, and ninety degree insulation columns and common bundling ranges (one to three conductors at full factor, then stepped reductions for denser bundles) so you get a transparent intermediate factor pair and a single derated ampacity. Continuous loads may still require an additional eighty percent service factor on the overcurrent device; that equipment rule is separate from conductor ampacity derating and is not applied here.

Where derating matters on real jobs

Electricians, designers, and estimators use a wire ampacity derating calculator when a conduit fills with more than three hots or when rooftop and attic ambients exceed the table baseline. Panel schedules and load calculations need a single defensible derated number before selecting conductor size and breaker trip ratings. Training and exam prep benefit from a deterministic oracle that returns both intermediate factors and the final amperes, making it obvious which band of the ambient or bundling table applied.

Keep requests small, pin the rounded factors in tests, and treat results as design estimates that still need code-book confirmation for the jurisdiction and edition you serve. The free browser path and the paid automation path share the same module, so classroom demos and production scripts stay aligned.

What you can do with it

Hot attic or rooftop feeders

Apply ambient temperature correction to a table ampacity when conduit runs through high-heat spaces, then size the conductor to the derated amperes.

Bundled multi-circuit raceways

Derate when more than three current-carrying conductors share a conduit or cable so each circuit’s continuous load stays within the adjusted ampacity.

FAQ

What ambient temperatures work?

Each insulation column has a supported ambient band (roughly 10–55 °C for 60 °C insulation, wider for 75 °C and 90 °C). Ambients outside that band for the chosen rating are rejected.

Does this include continuous-load 80% rules?

No. This tool applies conductor ambient and bundling derating only. Continuous-load and equipment-terminal rules are separate code requirements you apply when selecting breakers and terminations.

How do I derate a 30 A wire at 40 °C with six conductors?

Enter 30 as the base rating, 40 °C as the ambient, the 90 °C column and six conductors. The temperature factor is 0.91 and the bundling factor 0.8, so the combined 0.728 gives 21.84 A.

When does a bundling factor apply?

When more than three current-carrying conductors share a raceway or cable. Up to three keep the full table rating; beyond that, each wire heats its neighbours and the allowed current steps down as the count grows.

Where do I get the base ampacity to start from?

From the published ampacity table for the wire's size, material and insulation column. This calculator does not look that rating up: it only applies the temperature and bundling corrections to the figure you already have.

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

POSThttps://api.kit.forhosting.com/elec/wire-ampacity-derating

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/wire-ampacity-derating \
  -H "Authorization: Bearer $KIT_KEY" \
  -H "Content-Type: application/json" \
  -d '{"base_ampacity":30,"ambient_c":40,"rating_c":90,"conductors":6}'

const res = await fetch("https://api.kit.forhosting.com/elec/wire-ampacity-derating", {
  method: "POST",
  headers: {
    "Authorization": `Bearer ${process.env.KIT_KEY}`,
    "Content-Type": "application/json"
  },
  body: JSON.stringify({
    "base_ampacity": 30,
    "ambient_c": 40,
    "rating_c": 90,
    "conductors": 6
  })
});
const { task_id } = await res.json();

import os, requests

res = requests.post(
    "https://api.kit.forhosting.com/elec/wire-ampacity-derating",
    headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
    json={
    "base_ampacity": 30,
    "ambient_c": 40,
    "rating_c": 90,
    "conductors": 6
},
)
task_id = res.json()["task_id"]

<?php
$res = file_get_contents("https://api.kit.forhosting.com/elec/wire-ampacity-derating", false, stream_context_create([
  "http" => [
    "method"  => "POST",
    "header"  => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
    "content" => '{"base_ampacity":30,"ambient_c":40,"rating_c":90,"conductors":6}',
  ],
]));
$task = json_decode($res, true);

body := bytes.NewBufferString(`{"base_ampacity":30,"ambient_c":40,"rating_c":90,"conductors":6}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/elec/wire-ampacity-derating", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)

Example request

{
  "base_ampacity": 30,
  "ambient_c": 40,
  "rating_c": 90,
  "conductors": 6
}

Example response

{
  "task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
  "type": "elec.wire_ampacity_derating",
  "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

Per request$0.002

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

The same pure logic powers the free browser tool and the API path, so design notes and automation never disagree on a factor band.

Input contract, factor tables, and response fields

Example: base thirty amperes, ambient forty degrees Celsius, ninety degree rating, six conductors yields temperature factor zero point nine one, bundling factor zero point eight, combined zero point seven two eight, and derated ampacity twenty-one point eight four amperes. POST a JSON body with four required finite numbers: base_ampacity, the positive table ampacity in amperes before correction; ambient_c, the ambient temperature in degrees Celsius used to select the temperature factor band; rating_c, the insulation temperature rating column which must be sixty, seventy-five, or ninety; and conductors, an integer count of current-carrying conductors from one through one thousand. Missing fields, non-finite values, non-positive base ampacity, unsupported rating columns, non-integer conductor counts, or ambient temperatures outside the published band for that rating fail with invalid_input and are not billed. On success the body echoes the inputs, reports temp_factor and bundling_factor, their product as combined_factor, and derated_ampacity equal to base times combined factor, with unit set to A and formula restating I_derated equals I_base times F_temp times F_bundle. Pair this tool with a separate ampacity table lookup when you need the base rating from gauge and material; this endpoint only applies the derating math to a rating you already know.

Where a derating API earns its keep in real work

Prefabrication shops and BIM plug-ins can call the API with base ampacity and environment parameters so every drawing revision uses the same factors.

Integration use cases

Design notes and automation

Call the API from estimators, BIM tools, or panel-schedule scripts to attach temp and bundling factors to every feeder row consistently.

Technical questions

How is derated ampacity calculated?

I_derated = I_base × F_temp × F_bundle. Supply base_ampacity, ambient_c, rating_c (60, 75, or 90), and conductors; the API returns both factors, their product, and derated_ampacity in amperes.

Which insulation ratings are supported?

rating_c must be 60, 75, or 90 °C — the standard ampacity-table columns. Other values fail with invalid_input.

What does the wire ampacity derating calculator cost?

The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input is not billed.

Read the full KIT documentation →

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