Wire Gauge (AWG) Explained

The backwards numbering

American Wire Gauge, or AWG, is the standard we use for conductor size. The confusing part for beginners is that the number goes down as the wire gets thicker. A 14 gauge wire is thinner than a 10 gauge, and a 2 gauge is huge compared to both.

Once conductors get bigger than 1 AWG, the scale switches to naming like 1/0, 2/0, and so on, said aught. More aughts means more copper.

Gauge sets the ampacity

Thicker wire carries more current safely, and that limit is called ampacity. The code publishes ampacity tables you must follow. As rough copper landmarks at common temperature ratings, 14 gauge handles around 15 amps, 12 gauge around 20, and 10 gauge around 30.

• 15 amp circuit: 14 gauge copper.
• 20 amp circuit: 12 gauge copper.
• 30 amp circuit: 10 gauge copper.

Always confirm against the actual table for your insulation type and conditions, but memorize those three because you will use them constantly.

Copper versus aluminum

Aluminum carries less current than copper of the same gauge, so an aluminum feeder is usually one or two sizes larger for the same job. Aluminum is common and legal for feeders and services when you use listed connectors and the right antioxidant paste, but it demands respect at terminations.

Never mix copper and aluminum under a screw unless the device is rated for it. Loose or mismatched aluminum terminations are a classic fire cause.

Derating and temperature

Ampacity is not a fixed number. Bundle many conductors together or run them through a hot attic and their safe capacity drops. The code makes you apply correction factors for high ambient temperature and adjustment factors when more than three current carrying conductors share a raceway.

Plan for these before you pull wire. Upsizing a conductor on paper is cheap. Ripping out an undersized pull after the inspector fails you is not.