Voltage drop calculator
20 A through 50 m of 12 AWG copper loses a little over 10 V there and back. Enter the gauge, one-way length, current and copper or aluminium to get the round-trip voltage drop with each step shown. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- American Wire Gauge
- 12
- Length
- 50
- Load current in amperes
- 20
- Conductor material: copper
- Copper
- Awg
- 12
- L
- 50
- I
- 20
- Resistivity
- 0.0000000168
A voltage drop calculator answers a practical wiring question: how many volts are lost along a copper or aluminum cable of a given American Wire Gauge size, one-way run length, and load current. Enter AWG gauge, length in meters, current in amperes, and material; the tool maps gauge to conductor diameter with the standard AWG formula, builds cross-sectional area, applies fixed 20 °C resistivity for copper or aluminum, and returns the round-trip DC or single-phase drop Vd equals two times absolute current times rho times L over A, plus intermediate resistance, area, diameter, and step-by-step substitution.
What voltage drop means on a wire run
Every real conductor has resistance. When load current flows through the go conductor and returns through the neutral or return path, the product of current and loop resistance appears as a voltage drop between the supply terminals and the load terminals. For a uniform round wire, one-way resistance is R₁ equals rho times L divided by A, where rho is the material resistivity in ohm-meters, L is the one-way length in meters, and A is the metal cross-sectional area in square meters.
Round-trip DC and single-phase AC models use twice that resistance because the supply and return legs both sit in series with the load, so Vd equals two times absolute I times R₁, or equivalently Vd equals two times absolute I times rho times L over A. American Wire Gauge (AWG) sizes the diameter: the standard closed formula d in millimeters equals zero point one two seven times ninety-two raised to the power of thirty-six minus n over thirty-nine, with n from negative three for four-aught (0000 AWG) through forty.
Larger n means thinner wire, higher resistance, and more drop for the same length and current. Copper at twenty degrees Celsius is modeled with rho equals one point six eight times ten to the minus eight ohm-meters; aluminum uses two point six five times ten to the minus eight. This tool does not replace code ampacity tables, temperature derating, or three-phase specific factors; it answers the core ohmic drop for a simple two-wire path so you can compare candidate gauges before opening a full design spreadsheet.
Practical tips, limits, and related searches
Treat length as the one-way run from panel to load; the formula already doubles that path for go-and-return. Prefer thicker gauges (smaller AWG numbers) when drop exceeds a few percent of system voltage on long outdoor or barn circuits; this calculator does not take system voltage as an input, so compute percent yourself as one hundred times Vd over supply volts if you need a code-style percentage.
Aluminum needs a larger cross-section than copper for the same drop because its resistivity is higher—compare the same gauge side by side before buying. Gauge outside negative three through forty is rejected so the AWG diameter formula stays in its conventional tabulated range. Zero length yields zero drop, which is a valid edge case for a point load at the source.
Absolute current is used so a negative sign on I does not invent a negative voltage magnitude. Prefer this capability when you already know gauge, run length, current, and metal and only need ohmic Vd; if you only know resistance and need R equals rho L over A without AWG, use a dedicated wire-resistance calculator instead.
What you can do with it
Long branch-circuit pre-check
Electricians and DIY installers enter AWG, run length, expected load current, and copper or aluminum to see round-trip volts lost before choosing a thicker gauge.
Feeder gauge comparison scripts
Automation loops over candidate AWG sizes and materials with the same length and current, storing deterministic Vd for design notes or bill-of-materials tables.
FAQ
Why is length doubled?
The model assumes a go-and-return path (supply and return conductors). One-way length L is the run to the load; loop resistance is 2 · ρ · L / A.
What happens if length is negative?
Negative length is not physical for a cable run.
Copper or aluminum — which values are used?
Fixed 20 °C resistivities: copper ρ = 1.68×10⁻⁸ Ω·m, aluminum ρ = 2.65×10⁻⁸ Ω·m. Temperature derating is out of scope.
Is the voltage drop calculator free?
The browser tool is free and runs locally.
How do I turn the voltage drop into a percentage?
Divide the drop by your supply voltage and multiply by 100. The calculator does not ask for the system voltage, so that last step is yours; if the figure passes a few percent on a long run, try a thicker gauge.
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/elec/voltage-drop-wire \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"gauge":12,"length":50,"current":20,"material":"copper"}'const res = await fetch("https://api.kit.forhosting.com/elec/voltage-drop-wire", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"gauge": 12,
"length": 50,
"current": 20,
"material": "copper"
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/elec/voltage-drop-wire",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"gauge": 12,
"length": 50,
"current": 20,
"material": "copper"
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/elec/voltage-drop-wire", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"gauge":12,"length":50,"current":20,"material":"copper"}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"gauge":12,"length":50,"current":20,"material":"copper"}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/elec/voltage-drop-wire", 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": 50,
"current": 20,
"material": "copper"
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "elec.voltage_drop_wire",
"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 |
gauge_min | -3 |
gauge_max | 40 |
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
Negative length is rejected with invalid_input because a physical cable run cannot have negative meters.
How to call the API and what comes back
Send a JSON body with four required fields: gauge (aliases awg, wire_gauge, size) as a finite AWG number between negative three and forty inclusive; length (aliases L, len, wire_length, run_length) as non-negative meters; current (aliases I, i, amperage, amps) in amperes; and material (aliases conductor, metal, wire_material) as copper, cu, aluminum, aluminium, or al. Absolute values of numeric fields must stay within the published one-trillion limit so intermediate products remain finite in IEEE double precision on the Worker and in the browser. Negative length is always invalid_input. On success the response echoes rounded gauge, length, current, and normalized material; sets diameter_m and area_m2 from the AWG geometry; sets resistivity and rho to the material constant; R_one_way and R_loop to the one-way and round-trip resistances in ohms; voltage_drop and Vd to the same rounded drop in volts; formula and equation to the closed form and substituted line; unit tags for volts, ohms, meters, amperes, and square meters; message to a short English summary; and steps to an ordered array of geometry and substitution lines you can assert in tests. Failed validation—missing fields, non-finite numbers, out-of-range gauge, unsupported material, or negative length—returns invalid_input and is never charged. Pricing is a flat $0.002 per successful request with unit item and zero marginal unit price.
Practical tips, limits, and related searches
When teaching or testing, pair examples: twelve AWG copper at fifty meters and twenty amperes for a typical branch-circuit sketch; ten AWG aluminum at one hundred meters for a longer feeder comparison; and a deliberately negative length to demonstrate the hard invalid_input path. This landing targets people searching for a voltage drop calculator, AWG wire voltage drop, or copper aluminum cable drop online; it is not an ampacity table, not a three-phase power-factor tool, and not a full NEC design worksheet. Failed tasks that return invalid_input do not consume prepaid balance, which keeps exploratory scripts cheap when you probe edge cases.
Integration use cases
Classroom and unit-test fixtures
Instructors and coding kata authors generate known-good voltage-drop cases plus deliberate negative-length rejections for worksheets and golden tests.
Technical questions
How is voltage drop calculated?
AWG gauge maps to diameter with d_mm = 0.127 × 92^((36−n)/39), area A = π(d/2)², then round-trip Vd = 2 · |I| · ρ · L / A using copper or aluminum resistivity at 20 °C.