Amp to wire size calculator
A 20 A load on a 30 ft run at 12 V, allowing a 3 % drop, needs 4 AWG copper. Enter current, one-way length, voltage and drop budget to get the smallest gauge that fits. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Current a
- 20
- Length
- 30
- Wire gauge
- 4
- Material
- copper
- System v
- 12
- Length unit
- Ft
Choosing a wire gauge from current alone is how low-voltage projects end up with dim lights, hot cables and unhappy inspectors. The correct size depends on three things working together: how many amperes the load draws, how far the power has to travel, and how much voltage drop you can tolerate along the way. The computation is fully deterministic — fixed arithmetic, no network, no randomness, no dates — and the same code runs free in your browser on this page. Use it while planning a run, then confirm ampacity and insulation rating separately before you buy cable.
Why amperes alone cannot pick a wire size
Every wire sizing question starts with current, but current alone never finishes it. A 20 A load does not have a single correct wire gauge: on a two-foot run inside a control panel, a thin conductor carries it without measurable loss, while the same 20 A pushed fifty feet to a garden shed can lose several volts on that same wire — and on a 12 V system, several volts is a quarter of your supply.
The reason is Ohm's law applied to the round trip: current flows out through one conductor and back through the other, so the total resistance grows with twice the one-way length and shrinks with the conductor's cross-sectional area. Voltage drop therefore rises linearly with both amperes and distance, which means doubling the run or doubling the load each demands twice the copper to hold the same drop. Because AWG sizes step in fixed ratios rather than continuously, a modest change in either input can jump the requirement several gauges.
That is why amp charts printed on cable spools quietly assume a length you may not have.
How the minimum gauge is computed
The algorithm is the standard round-trip circular-mil method used across the marine, automotive and solar trades. The system voltage defaults to 12 V and accepts any positive value, so 24 V vehicles, 48 V battery banks and 120 or 230 V mains runs all work. The required conductor area in circular mils is then 2 × 10.79 × current × length-in-feet ÷ allowed drop in volts, where 10.79 is the resistivity constant for copper at room temperature.
The code walks the AWG scale from 40 up to 4/0, computes each size's area from the defining diameter formula, and selects the smallest gauge that meets the requirement. The response includes the gauge, both areas in circular mils and square millimeters, and the actual drop in volts and percent.
Using the result responsibly
Treat the returned gauge as the voltage-drop answer to your question, and remember a complete wire selection has two more gates. The first is ampacity: a size chosen for drop on a long run is usually generous, but on short runs the drop budget can be satisfied by wire too thin to carry the current without overheating, so always cross-check the gauge against an ampacity table for your insulation temperature rating and bundling conditions — NEC, ABYC or your local code as applicable.
Practical habits matter too: measure the actual routing path rather than the straight-line distance, include both conductors of the circuit in your mental model (the round-trip factor is already in the formula, so enter the one-way length only), round up a size when the run passes through heat, and fuse for the wire, not the load. For anything that will be inspected or carries safety-critical loads, confirm the final selection against the current code edition and a licensed electrician.
What you can do with it
Size a boat, RV or van circuit
A 20 A fridge on a 30 ft one-way run at 12 V and 3% drop resolves to a concrete AWG size — check it before you buy a spool of the wrong gauge.
FAQ
Is the length one-way or round trip?
One-way, from source to load. The round trip out and back is handled internally by the factor of two in the formula.
What does it return?
The minimum copper AWG gauge that keeps the round-trip drop within your budget, plus the required and actual conductor areas (cmil and mm²) and the actual voltage drop that size produces.
Does it check ampacity too?
No. It sizes for voltage drop only. On short runs, cross-check the gauge against an ampacity table for your insulation rating before installation.
What does it cost?
The same computation is free to run in your browser on this page.
Why does a longer run need a thicker wire?
Because voltage drop grows in step with both the current and the round-trip distance. Double the run or the load and you need twice the copper cross-section to hold the same drop.
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/phys/amp-to-wire-size \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"current_a":20,"length":30}'const res = await fetch("https://api.kit.forhosting.com/phys/amp-to-wire-size", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"current_a": 20,
"length": 30
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/phys/amp-to-wire-size",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"current_a": 20,
"length": 30
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/phys/amp-to-wire-size", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"current_a":20,"length":30}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"current_a":20,"length":30}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/phys/amp-to-wire-size", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"current_a": 20,
"length": 30
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "phys.amp_to_wire_size",
"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.
Limits
max_abs | 1000000000000 |
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
This API turns amps into a wire size the way an electrician does: enter the current draw in amperes, the one-way run length in feet or meters, your system voltage — 12, 24, 48, 120, 230 V or any other — and the voltage drop limit you accept, either as a percentage of the system voltage (3% by default) or as an absolute value in volts. It returns the minimum copper AWG gauge that keeps the round-trip drop within your budget, along with the required and actual conductor areas and the exact drop the recommended size will produce, so the answer is auditable rather than a bare chart lookup.
Why amperes alone cannot pick a wire size
A trustworthy amp to wire size calculation must take current, length and a drop budget together — exactly what this endpoint does — and show you the margin the chosen gauge actually leaves, instead of a rule of thumb calibrated for somebody else's installation.
How the minimum gauge is computed
First the inputs are coerced and validated: current must be a positive number — plain strings like '20' or '20 A' are accepted — and so must the one-way length, in feet by default or meters when length_unit is 'm', with meters converted internally. The drop budget comes from drop_v in volts if you provide it, otherwise from drop_pct as a percentage of the system voltage, defaulting to the customary 3%. If even 4/0 is not enough, it raises a typed invalid_input error stating the required area instead of silently returning the largest size. Everything is fixed arithmetic — no network, no randomness, no clock — so the free browser tool and the paid API return byte-identical answers.
Using the result responsibly
The second gate is the environment: engine compartments and conduit runs call for temperature correction factors, tinned marine-grade conductors resist corrosion far better than plain building wire, and many codes impose stricter drop limits — 3% for critical circuits, 5% or 10% for feeder-plus-branch totals — which you can reproduce here by setting drop_pct or drop_v accordingly. Integrators embed this endpoint in quoting and design tools because the deterministic output is auditable — the same input always yields the same gauge, making it safe to cache, diff and regression-test.
Integration use cases
Plan 24/48 V solar and battery runs
Set voltage to 24 or 48 and drop_pct to 2 for sensitive charge-controller runs, and see how much copper the tighter budget demands.
Embed sizing in a quoting tool
Call the API from a configurator so sales staff get a deterministic, auditable minimum wire gauge for any current, run length and system voltage.
Technical questions
What inputs does it need?
Current draw in amperes and one-way wire length (feet by default, or meters with length_unit 'm'). Optionally the system voltage (default 12 V) and a drop budget: drop_pct as a percentage (default 3) or drop_v in volts.
Does it work for 24, 48 or 120 V systems?
Yes. Set voltage to any positive value; drop_pct is then read as a percentage of that voltage, or use drop_v for an absolute budget in volts.
What happens with impossible runs?
If even 4/0 copper cannot meet the budget, you get a typed invalid_input error stating the required conductor area, so you can shorten the run or relax the drop.