Wire Gauge Chart: What Size Wire for Every Amperage

This is the NEC's ampacity table (Table 310.16) reduced to the gauges you'll actually touch: 14 AWG through 4/0. Ampacity is the maximum current a conductor can carry continuously without exceeding its temperature rating. The numbers assume no more than three current-carrying conductors in a raceway or cable, at 30°C (86°F) ambient — a normal room. Hot attics and bundled cables get derated from here.

Picked your gauge? Here's how to read the printing on the jacket to make sure the roll in your hand matches what you need.

Based on NEC Table 310.16 (allowable ampacities of insulated conductors rated 0–2000V, not more than three current-carrying conductors, 30°C ambient). Aluminum building wire starts at 12 AWG — that's why the top row is blank on the aluminum side.

The chart has three columns because wire insulation comes in three temperature ratings. But you don't get to just pick the biggest number. The column you read is decided by the weakest link: the terminals the wire lands on. That's NEC 110.14(C), and it's the rule that trips up more DIYers than anything else in this article.

The short version:

Most residential work uses the 60°C column. Breakers, receptacles, and switches in homes are generally rated 60°C (or unmarked, which defaults to 60°C). So even if your THHN is good for 90°C, a 12 AWG conductor landing on a standard breaker is a 20-amp wire, not a 30-amp wire.

The 75°C column applies when the equipment is marked for it — common on commercial gear, subpanels, and larger breakers (circuits over 100A default to 75°C-rated terminations).

The 90°C column almost never sets your circuit size. Its real job is derating math: when you have to reduce ampacity for bundled conductors or hot ambient temperatures, you start from the 90°C number and derate down — but the final answer still can't exceed the 60°C or 75°C column your terminations allow.

Romex (NM-B) is always 60°C for sizing. NEC 334.80 says NM cable is used at the 60°C ampacity even though the conductors inside are 90°C rated. So 12-2 Romex is a 20-amp wire, period — the 90°C column is only for derating calculations.

Chicago note: the City of Chicago requires conduit for most residential wiring — Romex generally isn't allowed inside city limits. So in the city, your sizing usually runs through THHN in conduit, where the 75°C column often applies if the terminations are marked for it. The suburbs are a different story, and local amendments vary — another reason the electrician gets the final word.

Look at the chart: 12 AWG copper shows 25 amps in the 60°C column. So why is every 12 AWG circuit on a 20-amp breaker? Because NEC 240.4(D) puts a hard ceiling on overcurrent protection for small conductors, regardless of what the ampacity table says:

14 AWG copper → 15 amps max. 12 AWG copper → 20 amps max. 10 AWG copper → 30 amps max.

Same logic for small aluminum and copper-clad aluminum: NEC 240.4(D) caps 12 AWG aluminum at 15 amps and 10 AWG aluminum at 25 amps — after any derating math. (The rule only names those sizes; anything larger goes by the ampacity table and your terminal ratings.)

This is the difference between ampacity (what the wire itself can carry) and breaker size (what protects it). They're related but they're not the same number, and the breaker is what the inspector checks. There are narrow exceptions for specific equipment like motors and HVAC units — your electrician knows them — but for general branch circuits, those ceilings are the law.

A continuous load — anything expected to run at full current for three hours or more — gets special treatment under NEC 210.19: the conductors must be sized at 125% of the continuous load. Flip it around and you get the version most people know: a circuit's continuous load can't exceed 80% of the circuit rating.

The classic example is the EV charger. A 48-amp charger runs for hours, so it's continuous: 48 × 1.25 = 60 amps. That means a 60-amp breaker and wire good for 60 amps — which is 6 AWG copper. And here's where the columns bite: 6 AWG in the 60°C column is only 55 amps, so NM-B (Romex) can't do this job. 6 AWG THHN in conduit at the 75°C column is 65 amps — that works, if the terminations are 75°C rated.

Other continuous loads that catch people: electric heat, grow lights, pool pumps, commercial lighting. When in doubt, multiply by 1.25 first, then pick the wire.

Wire has resistance, and resistance over distance eats voltage. The NEC recommends keeping voltage drop under 3% on a branch circuit (5% total from service to outlet) — it's guidance, not hard code, but it's the number electricians design to.

Here's the practical math for a copper run (single phase). Voltage drop = (2 × 12.9 × amps × one-way distance in feet) ÷ circular mils of the wire. (Use 21.2 instead of 12.9 for aluminum.) 12 AWG is 6,530 circular mils; 10 AWG is 10,380; 8 AWG is 16,510.

Worked example: a 120-volt, 20-amp circuit running 100 feet to a garage, loaded to 16 amps (the 80% continuous max). Drop = (2 × 12.9 × 16 × 100) ÷ 6,530 = 6.3 volts — that's 5.3% of 120V, over the 3% guideline. Bumping to 10 AWG drops it to 4.0 volts, or 3.3%. Close enough that most electricians would upsize and move on.

The rule of thumb: once a 120V run passes about 75–100 feet at near-full load, go up a gauge. At 240V you get twice the distance for the same percentage drop, so dryers and subpanels are more forgiving. Long runs are also the most common reason a job ends with a half-used spool of the bigger wire — which is exactly the kind of leftover worth selling instead of scrapping.

Sized the circuit and now you've got half a roll left? We buy usable leftover wire — anywhere in the country. Fast, free quote.

Copper is expensive, so upsizing "just to be safe" has a real cost — and most of the time it buys you nothing. If your terminations are 60°C rated, a 90°C wire doesn't carry a single extra amp for sizing purposes; the higher rating only helps in derating math. A 12 AWG THHN conductor on a standard 20-amp breaker is a 20-amp wire whether it cost you the THHN price or twice that.

The legitimate reasons to upsize: distance (voltage drop, above), future capacity (running 6 AWG to the garage today because the EV charger comes next year), and aluminum economics (aluminum feeder is legitimately cheaper than copper at large sizes — 1/0 aluminum does a 100-amp subpanel for less than 2 AWG copper, with proper terminations). Everything else is just paying for copper you can't use.

This chart is general guidance based on the NEC ampacity table. Sizing wire for a real installation also depends on ambient temperature, how many conductors share a conduit, your terminals' temperature ratings, and — critically — your local code amendments, which in the Chicago area can be stricter than the NEC. This article teaches you how the sizing works; it doesn't replace a licensed electrician or your local inspector. When in doubt, the electrician wins.