For standard residential branch circuits, use the 60°C column of the copper ampacity chart for 14, 12, and 10 AWG wire (rated 15A, 20A, and 30A respectively), and the 75°C column for 8 AWG and larger. This is mandated by NEC 110.14(C) based on standard termination ratings. Always size your breaker to the lowest temperature rating in the circuit chain. If you are running standard THHN in conduit, your wire insulation is rated for 90°C, but your breaker lugs are likely only rated for 75°C, capping your usable ampacity at the 75°C value.
The Master NEC Copper Ampacity Chart
How to read this table: This data is sourced directly from NEC Table 310.16 (2020/2023 editions) for copper conductors. The table is divided into three temperature columns based on the insulation type of the wire. The 60°C column applies to older insulation types like TW and UF-B cable. The 75°C column covers THHW and THWN. The 90°C column applies to modern THHN and XHHW-2 wires. The values below assume an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors bundled in a single raceway or cable.
| Wire Size (AWG/kcmil) | 60°C (140°F) TW, UF |
75°C (167°F) THHW, THWN |
90°C (194°F) THHN, XHHW-2 |
|---|---|---|---|
| 14 AWG | 15A | 20A | 25A |
| 12 AWG | 20A | 25A | 30A |
| 10 AWG | 30A | 35A | 40A |
| 8 AWG | 40A | 50A | 55A |
| 6 AWG | 55A | 65A | 75A |
| 4 AWG | 70A | 85A | 95A |
| 3 AWG | 85A | 100A | 115A |
| 2 AWG | 95A | 115A | 130A |
| 1 AWG | 110A | 130A | 145A |
| 1/0 AWG | 125A | 150A | 170A |
| 2/0 AWG | 145A | 175A | 195A |
| 3/0 AWG | 165A | 200A | 225A |
| 4/0 AWG | 195A | 230A | 260A |
Which Column Applies to Your Installation?
The 90°C column is a trap for beginners. You might look at 12 AWG THHN wire, see it rated for 30A in the 90°C column, and assume you can put it on a 30A breaker. You cannot. According to NEC 110.14(C), the ampacity of a circuit is limited by the lowest temperature rating of any connected termination, conductor, or device.
Standard residential circuit breakers, receptacles, and switches are typically rated for 60°C or 75°C terminations. Therefore, even if the wire inside the wall can handle 90°C heat, the breaker lug will overheat if you push 90°C-level current through it.
The Rule of Thumb:
- 14, 12, and 10 AWG: Always use the 60°C column for final breaker sizing, regardless of your wire's insulation rating. NEC 240.4(D) strictly caps these small wires at 15A, 20A, and 30A respectively.
- 8 AWG and larger: Use the 75°C column, assuming your breaker and lugs are rated for 75°C (which almost all modern Square D, Eaton, and Siemens breakers are).
A 40A continuous EV charger requires wire sized at 125% of the continuous load (40A × 1.25 = 50A). You need wire with an ampacity of at least 50A. Looking at the chart, 8 AWG copper in the 75°C column is rated for exactly 50A. Because modern 50A breakers have 75°C rated lugs, 8 AWG THHN/THWN-2 is the correct, code-compliant choice. If you were using UF-B cable (60°C rated), you would have to step up to 6 AWG (55A at 60°C).
How Derating Modifies Your Base Ampacity (And What the Chart Misses)
The ampacities in the table above represent ideal conditions: 30°C ambient air and no more than three current-carrying conductors in a raceway. Real-world jobsites rarely stay ideal. When conditions change, you must apply correction factors that reduce (derate) the base ampacity.
1. Conductor Bundling (More than 3 wires in a conduit)
When current flows through a wire, it generates heat. When you bundle multiple current-carrying wires tightly inside a PVC or EMT conduit, they heat each other up. If you pull four to six current-carrying conductors through a single conduit, NEC Table 310.15(C)(1) requires you to derate the 90°C ampacity to 80%.
Calculation: If you have five 12 AWG THHN wires in a conduit, you start with the 90°C base of 30A. Multiply by 0.80, yielding 24A. Since 24A is still above the 20A breaker limit, 12 AWG remains safe. If you had eight wires (derated to 70%), 30A × 0.70 = 21A, which is still acceptable for a 20A breaker, but leaves very little margin for error.
2. Ambient Temperature Corrections
If you are running conduit across a hot attic in the summer or along a rooftop where ambient temperatures hit 110°F (43°C), the wire cannot dissipate heat as efficiently. According to NEC Table 310.15(B)(1), a 90°C THHN wire in a 110°F ambient environment must be derated to 87% of its base value. A 10 AWG wire (40A base) drops to 34.8A. You can no longer use it on a 35A or 40A circuit.
What the Table Cannot Tell You: Voltage Drop
The most critical limitation of the NEC copper ampacity chart is that it only dictates thermal safety—it tells you the maximum current the wire can carry before the insulation melts or causes a fire. It tells you absolutely nothing about performance.
According to the Copper Development Association, a 2 AWG copper wire is thermally rated for 115A at 75°C. However, if you use 2 AWG wire for a 100A subpanel feeder that is 150 feet long, your voltage drop will exceed 4%. While this won't start a fire, it will cause lights to flicker and motors to run hot and inefficiently.
For long feeder runs, you must calculate voltage drop using the formula:
VD = (2 × L × I × R) / 1000
Where L is one-way length in feet, I is current in amps, and R is the resistance per 1000 ft (found in NEC Chapter 9, Table 8).
As a standard engineering practice, keep voltage drop under 3% for branch circuits and under 5% for the total feeder plus branch circuit combined. If your thermal ampacity chart says 6 AWG is fine for 65A, but your run is 120 feet, your voltage drop calculation might force you to pull 4 AWG or even 3 AWG to maintain usable voltage at the load. Always calculate thermal ampacity first to satisfy the inspector, then calculate voltage drop to satisfy the physics.
