What Size Wire Do I Need? An AWG Guide

Undersized wire is one of the most common and most dangerous DIY electrical mistakes. Wire that is too thin for the load overheats. Wire that is too thin for the distance lets the voltage sag. This guide explains how to pick the right gauge and flags where you need a licensed electrician.

A safety note up front: circuit wiring is governed by code and affects life safety. Use this to understand and plan, but have a licensed electrician confirm any circuit you’re not certain about.

The backwards scale that confuses everyone

Wire is sized in AWG (American Wire Gauge), and the scale runs backwards: the bigger the number, the thinner the wire.

14 AWG, 12, 10, 8, 6, 4, 2, 1, 1/0 (spoken “one-aught”)

A 14-gauge wire is thin lamp-cord territory; a 2-gauge is a thick service cable. Once you remember that a lower number means thicker wire, the rest follows.

Two things decide your wire size

Picking a gauge means answering two separate questions, then taking the bigger answer.

1. Ampacity: can the wire carry the current safely?

Every gauge has a maximum current it can carry without overheating. For common residential copper:

Wire (copper)	Max amps	Typical use
14 AWG	15 A	Lighting circuits
12 AWG	20 A	Outlets, kitchen
10 AWG	30 A	Water heater, dryer
8 AWG	40 A	Range, subpanel feed
6 AWG	55 A	Large appliances

This sets the floor. A 20-amp breaker must have at least 12 AWG behind it, never thinner, however short the run.

2. Voltage drop: is the run too long?

Distance adds a second limit. Electricity loses a little voltage to the wire’s resistance every foot it travels, and over a long run that adds up. Too much drop and lights dim, motors run hot, and electronics act strangely. The NEC recommends keeping drop under 3% on a branch circuit.

The Voltage Drop Calculator shows exactly how much you’re losing. For example, a 20-amp load on 12 AWG copper is fine at 25 feet, but at 100 feet it drops 6.6%, more than double the limit. At that distance you need 8 AWG. The wire could carry the current all day, so ampacity is not the issue. The distance is.

Worked example: a long garage circuit

You want a 20-amp, 120-volt circuit run 100 feet to a detached garage:

• Ampacity says: 12 AWG (fine for 20 amps)
• Voltage drop says: 12 AWG drops 6.58% at that distance, which fails. 8 AWG drops 2.60%, which passes.
• You use: 8 AWG, the larger of the two.

The Wire Size Calculator does this comparison for you: enter voltage, amps, distance, and your drop limit, and it returns the minimum gauge that satisfies both.

What about aluminum?

Aluminum wire has about 60% more resistance than copper for the same gauge, so it both carries less current and drops more voltage. As a rule, aluminum needs to be one to two sizes larger than copper for the same job. It’s common in service entrances and feeders but should be terminated with care, so leave it to a pro.

Don’t forget the conduit

If your wire runs in conduit, there’s a fill limit too: pack in too many conductors and they trap heat and become impossible to pull. The NEC caps fill at 40% of the conduit’s internal area for three or more wires. The Conduit Fill Calculator checks whether your bundle fits or needs a bigger pipe.

Common mistakes

1. Sizing for amps only. Fine for short runs, risky for long ones. Always check voltage drop on anything over ~50 feet.
2. Reading the gauge scale forward. Grabbing “bigger number = bigger wire” gets you dangerously thin conductors.
3. Forgetting aluminum needs upsizing. Copper tables don’t apply directly.
4. Overfilling conduit. Legal fill limits exist for heat and pullability.

Get your gauge

Enter your load and run length into the Wire Size Calculator for the minimum gauge, then confirm the voltage drop and conduit fill with the companion calculators. If you are not fully sure about a circuit, bring in a licensed electrician. The stakes are too high to guess.