Wire resistance run calculator
30 meters of 12 AWG copper wire add 0.152322331834 ohms one way, and twice that for the go-and-return loop of a single-phase circuit. Enter gauge, one-way length and material to see the ohms a long run adds before you order cable. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Gauge
- 12
- Length
- 30
- Material
- Copper
- Awg
- 12
- L
- 30
- Resistivity
- 0.0000000168
- Rho
- 0.0000000168
Planning a branch circuit, subpanel feeder, or long appliance run often starts with a simple electrical question: how many ohms does this wire run add to the path? Homeowners, electricians, and DIY renovators usually know the AWG size on the spool, the approximate one-way distance from the panel to the load, and whether the conductor is copper or aluminum—not bulk resistivity and bare cross-section in square meters. This wire resistance run calculator accepts gauge, length, and material, maps AWG to solid-conductor diameter, applies standard room-temperature resistivity, and returns one-way resistance plus the go-and-return loop resistance used when estimating DC or single-phase voltage drop later. There is no network lookup, no randomness, and no dependence on the clock—only deterministic arithmetic you can pin in tests and permit-planning notes.
How wire-run resistance is computed from gauge and material
Electrical resistance of a uniform conductor follows R equals rho times L divided by A, where rho is the material resistivity, L is the conductor length along the current path, and A is the cross-sectional area perpendicular to flow. For American Wire Gauge sizes the solid diameter in millimeters is given by the standard relation d equals 0.127 times 92 raised to the power of (36 minus n) over 39, with n the AWG number (including negative n for 0000-style sizes). Area is then pi times radius squared.
This calculator uses fixed twenty-degree Celsius resistivities of about 1.68 times 10 to the minus 8 ohm-meters for copper and 2.65 times 10 to the minus 8 for aluminum—the same room-temperature constants used across related KIT wire tools—so answers stay comparable when you later multiply resistance by current for a rough voltage-drop estimate.
You enter the one-way run length in meters from source to load; the response includes that one-way resistance and a loop resistance equal to twice the one-way value, which models the go-and-return conductors of a typical single-phase circuit that share the same gauge and material. Stranded constructions, temperature rise, skin effect at high frequency, and multi-conductor cable fill are outside this first-order model: treat the result as a transparent planning number, not a substitute for code tables or engineered design.
Where a wire resistance run calculator earns its keep
Renovation planners use a wire resistance run calculator before ordering cable so a long kitchen island, garage workshop, or detached outbuilding feeder is not under-sized purely by ampacity tables without looking at path ohms. Electricians teaching apprentices can show how doubling the length doubles resistance while stepping two AWG sizes changes area and therefore ohms more sharply than intuition suggests. Home automation and energy-monitoring projects that model branch circuits in software need a deterministic oracle for R given gauge, length, and metal without standing up a full electrical CAD package.
Keep lengths honest (one-way meters, not cable-on-the-spool guesswork) and document ambient assumptions when you promote the number into a permit package or bill of materials.
What you can do with it
Long branch-circuit planning
Estimate one-way and loop ohms for a 12 AWG copper home run to a distant outlet or appliance before deciding whether to upsize the cable.
Copper vs aluminum feeder comparison
Compare resistance of aluminum versus copper at the same AWG and run length when budgeting subpanel or outbuilding feeders.
Teaching and automation oracles
Give apprentices and circuit-model scripts a deterministic R from gauge, length, and material without a full design spreadsheet.
FAQ
How do I calculate wire run resistance from gauge and length?
Send AWG gauge (−3…40), one-way length in meters (non-negative), and material copper or aluminum.
Which materials are supported?
Resistivities are fixed at room temperature (~1.68e-8 and ~2.65e-8 Ω·m). Other alloys are outside this model.
Does this include voltage drop?
No. This calculator returns resistance only. Multiply loop resistance by load current yourself, or use a dedicated voltage-drop tool when you need volts rather than ohms.
How much does going up a wire size lower the resistance?
Resistance falls with the conductor's cross-section, and AWG areas change quickly. Stepping two gauge numbers changes the ohms more sharply than intuition suggests, while doubling the length simply doubles them. Compare 12 and 10 AWG over the same run to see the difference.
Does the result account for stranded wire or a warm cable?
No. It uses solid-conductor diameters and fixed resistivities at 20 °C. Stranded construction, temperature rise and high-frequency effects are outside this first-order model, so treat the figure as a planning number, not as a code table.
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
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/home/wire-resistance-run \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"gauge":12,"length":30,"material":"copper"}'const res = await fetch("https://api.kit.forhosting.com/home/wire-resistance-run", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"gauge": 12,
"length": 30,
"material": "copper"
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/home/wire-resistance-run",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"gauge": 12,
"length": 30,
"material": "copper"
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/home/wire-resistance-run", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"gauge":12,"length":30,"material":"copper"}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"gauge":12,"length":30,"material":"copper"}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/home/wire-resistance-run", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"gauge": 12,
"length": 30,
"material": "copper"
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "home.wire_resistance_run",
"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. |
Technical notes
Input contract, response fields, and validation rules
Formula and equation strings restate R equals rho L over A with the substituted numbers, and a steps array walks through diameter, area, resistivity, and both resistance results for display next to the answer. All numeric results are rounded to twelve decimal places for stable goldens across runtimes. Example: twelve AWG copper over thirty meters yields a small one-way resistance on the order of a few tenths of an ohm and a loop resistance of twice that—useful when comparing whether upsizing to ten AWG is worth the copper cost on a long home run. POST a JSON body with three required fields. Gauge is the AWG size as a finite number from negative three through forty inclusive; aliases awg, wire_gauge, and size are accepted. Length is the one-way wire run in meters and must be non-negative; aliases include L, len, wire_length, run_length, and total_length. Material must be copper or aluminum (cu and al are accepted aliases; aluminium is normalized to aluminum). Missing fields, non-finite numbers, gauges outside the supported AWG band, negative length, or an unrecognized material fail with invalid_input and are not billed. On success the body echoes gauge, length, and material; reports resistivity, diameter in meters, and area in square meters; and returns R, resistance, and R_one_way for the one-way path, R_loop for the go-and-return path, and ohms_per_meter so you can scale to another distance without a second call.
Where a wire resistance run calculator earns its keep
Pair the loop resistance with load current in a separate voltage-drop step if you need volts rather than ohms—this endpoint returns resistance only, so it stays focused for SEO and for clients that already compute drop themselves.
Technical questions
Is length one-way or round-trip?
Enter one-way distance from source to load. The response includes R_one_way for that length and R_loop = 2×R for a typical single-phase go-and-return path of the same gauge and material.
What does the wire resistance run calculator cost?
The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input is not billed.