Wire gauge / amp chart (NEC 310.16)
A wire gauge to amp chart answers the most common electrical question there is: how many amps can this wire carry? The figures below come straight from NEC Table 310.16, the table electricians actually use, for copper and aluminum at the three insulation temperature ratings (60, 75, and 90 °C). This guide explains how to read the chart, the small-conductor breaker rule that trips people up, why aluminum needs to be bigger, and the one situation where the chart isn't enough on its own — voltage drop on long runs.
The chart: copper and aluminum ampacity
Values are allowable ampacities in amperes from NEC Table 310.16, based on an ambient temperature of 30 °C (86 °F) with not more than three current-carrying conductors in a raceway, cable, or earth. Sizes 14–4/0 are AWG; 250–500 are kcmil. Aluminum is rarely used below 12 AWG, so those cells are blank.
| Wire size | Copper | Aluminum | ||||
|---|---|---|---|---|---|---|
| 60 °C | 75 °C | 90 °C | 60 °C | 75 °C | 90 °C | |
| 1 AWG | 110 | 130 | 145 | 85 | 100 | 115 |
| 2 AWG | 95 | 115 | 130 | 75 | 90 | 100 |
| 3 AWG | 85 | 100 | 110 | 65 | 75 | 85 |
| 4 AWG | 70 | 85 | 95 | 55 | 65 | 75 |
| 6 AWG | 55 | 65 | 75 | 40 | 50 | 55 |
| 8 AWG | 40 | 50 | 55 | 35 | 40 | 45 |
| 10 AWG | 30 | 35 | 40 | 25 | 30 | 35 |
| 12 AWG | 20 | 25 | 30 | 15 | 20 | 25 |
| 14 AWG | 15 | 20 | 25 | — | — | — |
| 250 kcmil | 215 | 255 | 290 | 170 | 205 | 230 |
| 300 kcmil | 240 | 285 | 320 | 195 | 230 | 260 |
| 350 kcmil | 260 | 310 | 350 | 210 | 250 | 280 |
| 500 kcmil | 320 | 380 | 430 | 260 | 310 | 350 |
| 1/0 AWG | 125 | 150 | 170 | 100 | 120 | 135 |
| 2/0 AWG | 145 | 175 | 195 | 115 | 135 | 150 |
| 3/0 AWG | 165 | 200 | 225 | 130 | 155 | 175 |
| 4/0 AWG | 195 | 230 | 260 | 150 | 180 | 205 |
Source: NEC Table 310.16 (30 °C ambient, ≤3 current-carrying conductors).
How to read it
Reading the chart is three steps. First, pick the material — copper or aluminum — since they carry very different currents at the same size. Second, pick the temperature column that matches the lowest-rated termination in your circuit; for most modern breakers and lugs that's 75 °C, which is why 75 °C is the everyday default. Third, find the smallest wire size whose ampacity meets or exceeds your load current. For example, a 40 A copper circuit at 75 °C lands on 8 AWG (50 A), because 10 AWG only reaches 35 A. That's the ampacity answer; on a long run you'll still check voltage drop, which we get to below.
The 90 °C column deserves a caution. Those higher numbers are real, but you generally cannot use them as the final ampacity at the terminals, because most equipment is only rated to 60 or 75 °C and the connection is the weak link. The 90 °C column's real job is to be the starting point for derating — when high ambient temperature or many conductors in a conduit force you to reduce ampacity, you apply those factors to the 90 °C value (for 90 °C-rated wire like THHN/THWN-2) and then make sure the result still doesn't exceed the 75 °C terminal column. This calculator and chart default to 75 °C precisely to keep you out of trouble at the terminations.
The 12 AWG / 20 amp rule (NEC 240.4(D))
Look at copper 12 AWG: the chart shows 25 A at 75 °C and 30 A at 90 °C. Yet every electrician will tell you "12 AWG is 20 amps." Both are true. The ampacity of the wire is genuinely higher, but NEC 240.4(D) caps the overcurrent protective device for small conductors regardless of ampacity: 15 A for 14 AWG copper, 20 A for 12 AWG copper, and 30 A for 10 AWG copper (aluminum: 15 A for 12 AWG, 25 A for 10 AWG). So you can't put 12 AWG on a 25 A breaker even though the table says 25 A, except in the specific cases the code carves out. Our calculator applies these caps automatically, so it never recommends a small conductor for more current than its breaker is allowed to be.
Copper vs aluminum
For the same size, aluminum carries less current and has higher resistance than copper. Compare the columns: copper 2 AWG is 115 A at 75 °C, while aluminum 2 AWG is only 90 A. To match a copper circuit, aluminum usually has to go up one or two sizes — which is why a 100 A copper feeder might be 3 AWG copper but 1 AWG aluminum. Aluminum is lighter and cheaper per amp, so it's common on large feeders and service entrances, but it needs terminations and antioxidant compound rated for aluminum (look for AL or CU-AL markings). When you compare cost, compare the installed result — conductor, terminations, and drop — not just the price per foot.
When ampacity isn't enough: voltage drop
The chart sizes for heat — keeping the conductor from overheating at a given current. It says nothing about whether the load still gets enough voltage after a long trip down the wire. That second question is voltage drop, and on long runs it routinely demands a larger conductor than the ampacity chart alone. A classic example: 50 A of copper needs 8 AWG by ampacity (50 A at 75 °C), but over 100 feet at 240 V single-phase, 8 AWG drops about 3.2% — past the common 3% target — so you'd step up to 6 AWG. The breaker says one size; the distance says another. The correct approach is always to size for both ampacity and voltage drop and use the larger result, which is exactly what the wire size calculator does for you.
What the chart leaves out
Table 310.16 is a baseline, and several real-world factors can move the required size up. High ambient temperature and having more than three current-carrying conductors in a raceway both reduce ampacity through derating factors. Continuous loads (running three hours or more) require sizing the conductor and overcurrent device to 125% of the load. Conduit fill, terminal temperature ratings, and the difference between DC resistance and AC impedance on large conductors (NEC Table 9) all matter too. This chart and calculator don't model those — so treat the result as a well-grounded starting point, then confirm the final design against the current NEC and your local code.
FAQ
How do I read a wire gauge / amp chart?
Find your conductor material (copper or aluminum), then the column for your terminal temperature rating — usually 75 °C. The number where your wire size meets that column is the allowable ampacity from NEC Table 310.16 at 30 °C ambient with three or fewer current-carrying conductors. Pick the smallest size whose ampacity meets or exceeds your load, then check voltage drop separately for long runs.
Why does 12 AWG copper say 20 amps when the table shows more?
The 75 °C and 90 °C columns list higher figures (25 A and 30 A) for 12 AWG copper, but NEC 240.4(D) caps the overcurrent device for small conductors: 15 A for 14 AWG, 20 A for 12 AWG, and 30 A for 10 AWG copper. So you protect 12 AWG copper at 20 A even though its raw ampacity is higher. This chart notes those caps.
Which temperature column should I use?
Match the column to the lowest-rated termination in the circuit. Most modern breakers and lugs are rated 75 °C, so the 75 °C column is the usual choice. The 90 °C column is generally used only as a starting point for derating calculations, not as the final ampacity at the terminals. The 60 °C column applies to some terminations and certain small conductors.
Does a bigger amp rating always mean a bigger wire?
For ampacity, yes — more current needs more copper or aluminum. But two same-current circuits can need different sizes because of voltage drop: a long run needs a larger conductor than ampacity alone to keep the drop within about 3%. Always size for both and use the larger result.
Put the chart to work in the wire size & ampacity calculator, or jump to a common load like what size wire for 50 amps.
Source: NEC Table 310.16; overcurrent caps per NEC 240.4(D).
Educational reference only, based on NEC Table 310.16 and 240.4(D). Not a substitute for a licensed electrician or your local code (AHJ). It does not apply ambient or conductor-count derating, the 125% continuous-load factor, conduit fill, or terminal-rating logic. Verify against the current NEC and local code before wiring. Based on NEC 310.16.