# Wire Size Calculator — NEC Ampacity + Voltage Drop

> Free wire size calculator: find the smallest conductor that passes BOTH NEC 310.16 ampacity (with 110.14(C) terminals and the 240.4(D) small-conductor rule) and voltage drop — copper or aluminum, AWG, and it tells you which constraint governs. All in your browser.

Size a conductor the way the code does — the smallest wire that passes both NEC 310.16 ampacity and voltage drop, copper or aluminum, with the governing constraint spelled out. Runs entirely in your browser.

- **HTML page:** https://siteometry.com/en/tools/wire-size-calculator
- **Category:** [Electrical](https://siteometry.com/en/tools/category/electrical)
- **Figures live on the page:** tables, diagrams and worked figures the text refers to ("the table below") are drawn by the page and are not reproduced here. Each section below links to its anchor on the page.
- **Runs in the browser:** Siteometry calculators need no account and send no input anywhere — every number below is computed on the visitor's own device.

## How to size or check a conductor with this calculator

1. **Choose units and mode** — Pick imperial (AWG) or metric (mm²), then Size wire to find the smallest passing conductor or Check wire to test a size you already have.
2. **Enter the circuit** — Set the conductor material, the load current (or pick a load preset), the one-way length, the system voltage and the phase — 1-phase, 3-phase or DC.
3. **Set the code assumptions** — Keep the NEC 310.16 defaults or open Derating & code assumptions to change the terminal rating, insulation column, ambient band, conductor count and the continuous-load flag.
4. **Read the result** — Size returns the recommended conductor and the constraint that governs it; Check shows each gate's pass or fail and the smallest fixing size. The ampacity gauge, voltage-drop gauge and wire-size ladder visualize both.

_On the page: https://siteometry.com/en/tools/wire-size-calculator#howto_

## Wire size calculator

"What wire do I buy?" has two answers that must both be satisfied, and the bigger one wins. A conductor has to carry the load current without overheating — its ampacity — and it has to deliver that current to the load without losing too much voltage on the way. Pick a wire on ampacity alone and a long run arrives with dim lights and sluggish motors; pick it on voltage drop alone and a short, heavy circuit can cook the insulation. This calculator sizes for both at once and tells you which one drove the answer.

In Size mode it scans the conductor catalog from smallest to largest and returns the first size that passes both the NEC 310.16 ampacity gate — including the 110.14(C) terminal-temperature rule, the 240.4(D) small-conductor caps, and ambient/bundling/continuous derating — and the voltage-drop gate. In Check mode it takes a size you already have and reports each gate's pass or fail independently, plus the smallest size that would fix a failure. Copper or aluminum, AWG, imperial or metric — all in your browser. Metric (mm²) is sized on voltage drop only; ampacity there needs a national table this tool does not ship.

_On the page: https://siteometry.com/en/tools/wire-size-calculator#intro_

## What governs wire size: ampacity vs voltage drop

Ampacity is the current a conductor can carry continuously without its insulation exceeding its temperature rating. NEC Table 310.16 lists a base ampacity for every size at 60, 75 and 90 °C, and the usable figure is the smallest of three ceilings: the base derated for ambient temperature and conductor bundling; the ampacity of the column your terminals are rated for (110.14(C) — 60 °C at or below 100 A, 75 °C above, and never 90 °C at a lug); and, for 14, 12 and 10 AWG, the fixed 240.4(D) cap that stops a big breaker landing on small wire. A continuous load (on for three hours or more) sizes the conductor to 125% of the load under 210.19(A) / 215.2.

Voltage drop is separate physics: current times the round-trip resistance of the run, as a percentage of system voltage. It grows with length, current and a thinner wire, and it shrinks with a fatter one. Codes recommend keeping it to about 3% on a branch circuit and 5% overall. Voltage drop uses the actual load current, not the 125% ampacity figure, because the drop depends on the current that really flows.

The two limits rarely bind at the same size. Short, high-current circuits are usually ampacity-governed — the wire has to be thick enough to carry the amps whatever the length. Long or low-voltage runs are usually voltage-drop-governed — ampacity is satisfied early but the wire keeps growing to hold the voltage. The recommended size is the more restrictive of the two, and the tool names the winner so you understand the result instead of just trusting it.

_On the page: https://siteometry.com/en/tools/wire-size-calculator#two-constraints_

## NEC 310.16 base ampacity

The base ampacities the ampacity gate starts from, for a representative spread of sizes. These are the table's own baseline values — no more than three current-carrying conductors at 30 °C ambient — before the 110.14(C) terminal column, 240.4(D) cap and any ambient or bundling derating are applied. A larger conductor and a hotter temperature column both carry more; copper carries more than aluminum of the same size.

In practice the 60 °C or 75 °C column usually sets the usable number, because terminations are the weak link: even a 90 °C-rated conductor is limited to its 75 °C (or 60 °C) ampacity wherever it lands on a lug. That is why the recommended wire is often one column — or one gauge — larger than a bare 90 °C reading would suggest.

_On the page: https://siteometry.com/en/tools/wire-size-calculator#ampacity-table_

## Derating & code factors

Two adjustments reduce the base ampacity before it is compared with the required amps. Ambient temperature: NEC 310.15(B)(1) corrects for heat around the conductor — the 26–30 °C band is the 1.00 baseline, hotter bands cut the ampacity, cooler bands raise it. Conductor bundling: NEC 310.15(C)(1) cuts ampacity when more than three current-carrying conductors share a raceway or cable, because they can't shed heat as freely.

Both tables below are editable in the calculator, along with the terminal rating, insulation column, continuous-load flag and a per-size base-ampacity override. The ambient multipliers in particular are the weakest-sourced numbers in the tool and are flagged for a one-time diff against a primary NEC reprint — treat them as planning values and confirm against your adopted code.

_On the page: https://siteometry.com/en/tools/wire-size-calculator#derating_

## Worked examples

Two circuits that fall on opposite sides of the split, each recommended by the same engine the tool uses live. The gauges and the wire-size ladder update as you change the inputs — these are just the starting points.

_On the page: https://siteometry.com/en/tools/wire-size-calculator#worked-examples_

## Frequently asked questions

### Does this check ampacity, or just voltage drop?

Both, in imperial. Every candidate size must pass the NEC 310.16 ampacity gate — base ampacity derated for ambient and bundling, capped by the 110.14(C) terminal column and the 240.4(D) small-conductor rule — and the voltage-drop gate, and the recommended size is the smallest passing both. That is the difference from a plain voltage-drop calculator, which only checks the drop. Metric (mm²) is voltage-drop only, because this release ships no verifiable IEC ampacity table.

### Which one decides the wire size?

Whichever needs the bigger conductor. Short, high-current circuits are usually governed by ampacity; long or low-voltage runs are usually governed by voltage drop. The tool reports the winner in plain words — for example "12 AWG — governed by ampacity (voltage drop alone would allow 14 AWG)" — so you can see why the size is what it is.

### Why is the recommended wire bigger than the breaker suggests?

A few code rules push the size up. A continuous load is sized at 125% of the current. The 110.14(C) terminal rule limits you to the 60 °C or 75 °C column even on 90 °C wire. Ambient heat and bundled conductors derate the ampacity. And on a long run, voltage drop alone can force a far larger conductor than the current rating implies. Any of these can make the correct wire larger than a quick breaker-to-gauge lookup.

### Does it handle copper and aluminum?

Yes. Choose the material and the calculator uses the matching ampacity and resistance. Aluminum carries less current and has higher resistance than copper of the same size, so an aluminum conductor is typically one or more sizes larger than the copper equivalent for the same circuit. Note there is no standard 14 AWG aluminum, so that size is skipped for aluminum.

### What size wire do I need for a given number of amps?

Enter the load current (or a preset), the run length and the voltage, and Size mode returns the smallest conductor that carries the amps and holds the voltage drop. There is no single "amps to gauge" answer, because the length, voltage, material, temperature rating and derating all move the result — which is exactly what the calculator resolves for you.

### Why is metric (mm²) voltage-drop only?

Ampacity in the metric world comes from IEC 60364-5-52 and national tables whose values depend on installation method and are not freely reproducible. Rather than ship an unverified safety number, the tool sizes metric conductors on voltage drop alone and says so, and asks you to confirm the current rating against your local table. The voltage-drop math, catalog and ladder work identically in mm².

_On the page: https://siteometry.com/en/tools/wire-size-calculator#faq_

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