The ampacity of a wire—its maximum continuous current-carrying capacity under specific conditions—is not a single fixed number. It changes based on the conductor material (copper vs. aluminum), the insulation temperature rating, and the physical environment of the installation. In the United States, the governing standard for these values is the National Electrical Code (NEC), specifically Table 310.16 published by the NFPA.
Before you pull wire or size a breaker, you need to know how to read this table correctly. The chart is divided by material (Copper on the left, Aluminum on the right) and then by temperature rating (60°C, 75°C, and 90°C). The most common mistake DIYers and junior apprentices make is looking exclusively at the 90°C column because it offers the highest ampacity. In reality, your termination points (breakers, lugs, and receptacles) dictate which column you must use for your final breaker sizing. Use this reference to find your baseline, then apply the necessary corrections.
The Master Wire Amp Capacity Chart (NEC Table 310.16)
Below is the core data from NEC Table 310.16 for the most common residential and light-commercial wire sizes. This table assumes an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway or cable. Bookmark this section for quick bench-side lookups.
| AWG / kcmil | Copper 60°C (140°F) | Copper 75°C (167°F) | Copper 90°C (194°F) | Aluminum 60°C (140°F) | Aluminum 75°C (167°F) | Aluminum 90°C (194°F) |
|---|---|---|---|---|---|---|
| 14 | 15A* | 20A* | 25A* | - | - | - |
| 12 | 20A* | 25A* | 30A* | 15A | 20A | 25A |
| 10 | 30A | 35A | 40A | 25A | 30A | 35A |
| 8 | 40A | 50A | 55A | 30A | 40A | 45A |
| 6 | 55A | 65A | 75A | 40A | 50A | 60A |
| 4 | 70A | 85A | 95A | 55A | 65A | 75A |
| 3 | 85A | 100A | 115A | 65A | 75A | 85A |
| 2 | 95A | 115A | 130A | 75A | 90A | 100A |
| 1 | 110A | 130A | 145A | 85A | 100A | 115A |
| 1/0 | 125A | 150A | 170A | 100A | 120A | 135A |
| 2/0 | 145A | 175A | 195A | 115A | 135A | 150A |
| 3/0 | 165A | 200A | 225A | 130A | 155A | 170A |
| 4/0 | 195A | 230A | 260A | 150A | 180A | 205A |
Which Temperature Column Applies to Your Installation?
Knowing the numbers in the wire amp capacity chart is only half the battle; knowing which column to actually use is where installations pass or fail inspection. The rule of thumb is the weakest link principle, codified in NEC 110.14(C). Your final ampacity is limited by the lowest temperature rating of any component in the circuit, including the wire insulation, the breaker terminals, the lugs in your subpanel, and the receptacles.
The 60°C Column
You must use the 60°C column if you are using NM-B cable (commonly known as Romex) for residential branch circuits. Even though the individual THHN wires inside the yellow or white sheath might be rated for 90°C, the overall NM-B assembly is only rated for 60°C. Additionally, any circuit rated 100A or less using conductors sized 14 AWG through 1 AWG defaults to the 60°C column unless the equipment is explicitly marked otherwise.
The 75°C Column
This is the workhorse column for modern electrical work. If you are pulling individual THHN/THWN-2 conductors in EMT or PVC conduit, and terminating them on modern breakers and lugs rated for 75°C (which almost all standard Square D, Eaton, and Siemens breakers are), you use the 75°C column. This is also the column you use for sizing SER or SEU aluminum feeder cables for subpanels and ranges.
The 90°C Column
According to EC&M (Electrical Construction & Maintenance), the 90°C column is almost never used for final breaker sizing. Its primary purpose is to provide a higher baseline ampacity before you apply derating factors for ambient temperature or conductor bundling. You calculate your derated ampacity starting from the 90°C column, but the final result cannot exceed the 75°C or 60°C termination limits.
Derating Factors: When the Chart Lies
The wire amp capacity chart assumes ideal conditions: an ambient temperature of 30°C (86°F) and a maximum of three current-carrying conductors in a conduit. When you deviate from these conditions, the base ampacity drops. If you ignore this, your wire insulation will degrade prematurely, or worse, melt and cause a fault.
Conductor Bundling (NEC 310.15(C)(1))
When you pull more than three current-carrying conductors in a single raceway, they heat each other up. You must apply an adjustment factor to the 90°C column ampacity.
Imagine you are pulling a multi-wire circuit through a conduit that contains 6 current-carrying conductors (e.g., two 120V circuits and a shared neutral, plus another 240V circuit).
1. Identify the base: You are using 8 AWG THHN copper. The 90°C column gives you 55A.
2. Find the factor: NEC Table 310.15(C)(1) states that 4 to 6 conductors require an 80% derating factor.
3. Calculate: 55A × 0.80 = 44A.
4. Verify termination: Your 44A derated ampacity is lower than the 75°C termination limit (50A), so 44A is your final allowable ampacity. You would protect this with a 40A breaker.
Ambient Temperature Correction (NEC 310.15(B))
If your conduit runs through a hot attic, a boiler room, or across a sun-baked roof, the ambient temperature exceeds 30°C. You must multiply the base ampacity by a correction factor. For example, if the attic reaches 50°C (122°F), a 90°C rated wire must be multiplied by 0.82. A 10 AWG THHN wire (40A at 90°C) drops to 32.8A in that environment. Always check the highest expected ambient temperature along the entire run.
What This Wire Amp Capacity Chart Cannot Tell You
While NEC Table 310.16 is the bible for thermal limits, it is not a complete design tool. Relying on it blindly will lead to failures in three specific areas that the ampacity chart completely ignores.
1. Voltage Drop
Ampacity only tells you if the wire will melt. It does not tell you if the voltage at the end of the run will be sufficient to operate your equipment. The NEC recommends a maximum 3% voltage drop for branch circuits and 5% for the total feeder plus branch circuit. If you are running a 20A circuit to a detached workshop 150 feet away, 12 AWG wire is perfectly legal per the ampacity chart, but the voltage drop will be over 5%. You must upsize to 10 AWG or 8 AWG purely for voltage drop, even though your breaker remains 20A.
2. Conduit Fill Capacity (Chapter 9)
The chart tells you how many amps a wire can carry, but Chapter 9 of the NEC dictates how many wires physically fit inside a conduit. You cannot simply stuff twelve 10 AWG THHN wires into a 1/2-inch EMT conduit, even if you applied the correct derating factors. The physical cross-sectional area of the wires (including insulation) cannot exceed 40% of the conduit's internal area for three or more wires.
3. Short-Circuit Withstand Rating
Ampacity is about continuous, steady-state heat. It does not account for the massive magnetic and thermal forces generated during a short circuit. If your utility provides 22,000 amps of available fault current at your main panel, your wire and busbars must be able to withstand that瞬间 surge before the breaker clears it. This is governed by the equipment's Short-Circuit Current Rating (SCCR) and the breaker's Amps Interrupting Capacity (AIC), not the wire ampacity chart.
Always treat the wire amp capacity chart as your starting point, not your finish line. Calculate your base ampacity, apply your derating factors, check your voltage drop, and verify your conduit fill. When in doubt, or when dealing with service entrance conductors, consult your local Authority Having Jurisdiction (AHJ) or a licensed electrical contractor, as local amendments frequently override baseline NEC guidance.
