The ampacity of wire gauges depends strictly on the conductor material (copper or aluminum), the insulation temperature rating (60°C, 75°C, or 90°C), and the installation method. For standard residential branch circuits using copper THHN/THWN-2 wire in conduit, you must use the 75°C column for termination sizing, yielding 15A for 14 AWG, 20A for 12 AWG, 30A for 10 AWG, and 40A for 8 AWG. However, NEC 240.4(D) caps the overcurrent protection for 14, 12, and 10 AWG copper at 15A, 20A, and 30A respectively, regardless of the insulation's higher thermal limits.
Below is the complete reference data derived from the National Electrical Code (NFPA 70) 2023 Edition, specifically Table 310.16 (formerly 310.15(B)(16)).
How to Read the NEC Ampacity Table
Before pulling wire, you must understand which column applies to your specific installation. The table is divided by material (Copper vs. Aluminum) and then by temperature rating. The temperature rating is determined by the weakest link in your circuit, which is almost always the termination point (breaker lug, receptacle screw, or wire nut).
Even if you pull 90°C rated THHN wire through your walls, modern residential breakers and receptacles are typically rated for 75°C terminations. Therefore, you must size your breaker and calculate your base ampacity using the 75°C column. The 90°C column is strictly reserved for calculating derating adjustments (like bundling or high ambient heat) before applying the final termination limit.
The Master Chart: Ampacity of Wire Gauges (Copper & Aluminum)
This table assumes an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway or cable. Rows marked with an asterisk (*) are the most frequently queried quick-jump values for residential and light commercial branch circuits.
| AWG / kcmil | 60°C (TW, UF) Copper | 75°C (THW, THWN) Copper | 90°C (THHN, XHHW) Copper | 75°C Aluminum | 90°C Aluminum |
|---|---|---|---|---|---|
| 14* | 15A | 20A | 25A | — | — |
| 12* | 20A | 25A | 30A | — | — |
| 10* | 30A | 35A | 40A | — | — |
| 8* | 40A | 50A | 55A | 40A | 45A |
| 6* | 55A | 65A | 75A | 50A | 60A |
| 4* | 70A | 85A | 95A | 65A | 75A |
| 3 | 85A | 100A | 110A | 75A | 85A |
| 2* | 95A | 115A | 130A | 90A | 100A |
| 1 | 110A | 130A | 145A | 100A | 115A |
| 1/0 | 125A | 150A | 170A | 120A | 135A |
| 2/0 | 145A | 175A | 195A | 135A | 150A |
| 3/0 | 165A | 200A | 225A | 155A | 170A |
| 4/0 | 195A | 230A | 260A | 180A | 205A |
Source: Adapted from NFPA 70 (National Electrical Code) 2023 Edition, Table 310.16. For comprehensive code interpretations, refer to Electrical Construction & Maintenance (EC&M) termination guidelines.
What the Table Cannot Tell You: Derating & Real-World Adjustments
The chart above provides the base ampacity of wire gauges under ideal conditions: 30°C (86°F) ambient temperature and no more than three current-carrying conductors bundled together. In the real world, you must apply adjustment factors (derating) that modify these base values.
1. Bundling (More than 3 Current-Carrying Conductors):
When you pull multiple circuits through a single conduit, the wires heat each other up. If you have 4 to 6 current-carrying conductors in a raceway, you must multiply the base ampacity by 80%. For 7 to 9 conductors, the factor drops to 70%.
2. Ambient Temperature Correction:
If your conduit runs through an environment hotter than 30°C (like an unventilated attic in summer or a boiler room), you must apply temperature correction factors from NEC Table 310.15(B)(1). For example, at 41-45°C (105-113°F), you multiply the 90°C column value by 0.82.
Worked Numeric Example:
You are pulling four 12 AWG THHN (90°C) current-carrying conductors through a single EMT conduit in a standard 30°C basement to feed two 20A receptacle circuits.
- Base 90°C ampacity for 12 AWG = 30A.
- Derating factor for 4-6 conductors = 80% (0.80).
- Adjusted ampacity = 30A × 0.80 = 24A.
Because 24A is greater than the 20A required by your breaker and the 75°C termination limit (25A), this installation is code-compliant. However, if you added two more circuits (totaling 8 current-carrying conductors), the derating factor becomes 70%. 30A × 0.70 = 21A. This is still above 20A, but leaves virtually no thermal headroom, prompting a wise electrician to upsize to 10 AWG.
Frequently Asked Questions
What is the exact ampacity of 8 AWG copper wire for a 40-amp circuit?
For a standard 40-amp circuit (like an electric range or large window AC unit), 8 AWG copper wire is the correct size. Looking at the 75°C column, 8 AWG copper has an ampacity of 50A. Because 50A exceeds the 40A breaker requirement and the termination limits, it is perfectly safe. Note that if you are using 8 AWG aluminum (such as SER cable for a feeder), the 75°C ampacity is only 40A, which is the absolute minimum for a 40A breaker and leaves no room for voltage drop over long distances.
Can I use the 90°C column to size my breaker if I'm using THHN wire?
No. This is one of the most common mistakes made by DIYers and apprentice electricians. While THHN wire insulation is rated for 90°C, the breakers, lugs, and receptacles you connect it to are almost universally rated for a maximum of 75°C. NEC 110.14(C) dictates that the ampacity of the circuit is limited by the lowest temperature rating of any connected component. You use the 90°C column only as the starting point for calculating derating adjustments; the final adjusted ampacity must still meet or exceed the breaker size based on the 75°C termination limits.
How does the ampacity of wire gauges change when running through attic insulation?
When NM-B (Romex) cable is buried under thick thermal insulation in an attic, the ambient temperature around the wire can easily exceed 30°C, and the insulation prevents heat dissipation. While the NEC does not have a single blanket derating table specifically for "attic insulation depth," it does require ambient temperature correction if the space exceeds 30°C. In extreme summer attics where ambient air reaches 50°C (122°F), the 90°C ampacity of the wire must be multiplied by 0.71. Furthermore, NEC 334.80 explicitly states that if NM cable is run through holes bored in framing members that are filled with thermal insulation, the ampacity must be adjusted according to the bundling tables, effectively treating the insulation as a heat trap.
Why does aluminum wire require a larger gauge than copper for the same ampacity?
Aluminum has a higher electrical resistance than copper—approximately 61% of the conductivity of copper by volume. Because it resists the flow of electrons more strongly, it generates more heat at the same current level. To achieve the same ampacity (current-carrying capacity without exceeding thermal limits), an aluminum conductor must have a larger cross-sectional area (a lower AWG number) than a copper conductor. For example, to safely carry 100A on a 75°C termination, you need 3 AWG copper, but you must step up to 1/0 AWG aluminum. Additionally, aluminum expands and contracts more than copper under thermal cycling, which is why it requires specific anti-oxidant paste and torque-rated terminations to prevent loose connections and arcing over time.
