How to Read the NEC Circuit Breaker Ampacity Chart
The standard circuit breaker ampacity chart is based on NFPA 70 (National Electrical Code) Table 310.16. Before you pick a wire gauge or snap in a breaker, you need to know which temperature column applies to your specific installation. The chart is divided into three temperature ratings: 60°C (140°F), 75°C (167°F), and 90°C (194°F).
Here is the golden rule for residential and light commercial wiring: Your circuit ampacity is limited by the weakest link in the chain. Most modern breakers, lugs, and receptacles are rated for 75°C terminations. However, NEC 240.4(D) legally restricts the overcurrent protection for small conductors (14, 12, and 10 AWG copper) to the 60°C column values, regardless of the wire's actual insulation rating.
The Complete Circuit Breaker Ampacity Chart (NEC Table 310.16)
Below is the complete reference table for copper and aluminum conductors. We have included standard breaker mappings based on NEC 240.6 standard ampere ratings. Use the quick-jump links to navigate directly to the most queried wire sizes.
Quick Jump: 14 AWG | 12 AWG | 10 AWG | 8 AWG | 6 AWG | 4 AWG | 2 AWG
| AWG / kcmil | Copper 60°C (140°F) | Copper 75°C (167°F) | Copper 90°C (194°F) | Aluminum 75°C (167°F) | Max Standard Breaker (Copper) |
|---|---|---|---|---|---|
| 14 AWG | 15A | 20A | 25A | N/A | 15A * |
| 12 AWG | 20A | 25A | 30A | N/A | 20A * |
| 10 AWG | 30A | 35A | 40A | N/A | 30A * |
| 8 AWG | 40A | 50A | 55A | 40A | 40A / 50A |
| 6 AWG | 55A | 65A | 75A | 50A | 60A |
| 4 AWG | 70A | 85A | 95A | 65A | 70A / 80A |
| 3 AWG | 85A | 100A | 110A | 75A | 100A |
| 2 AWG | 95A | 115A | 130A | 90A | 110A / 125A |
| 1 AWG | 110A | 130A | 145A | 100A | 125A |
| 1/0 AWG | 125A | 150A | 170A | 120A | 150A |
| 2/0 AWG | 145A | 175A | 195A | 135A | 175A |
| 3/0 AWG | 165A | 200A | 225A | 155A | 200A |
| 4/0 AWG | 195A | 230A | 260A | 180A | 225A / 250A |
* Per NEC 240.4(D), 14, 12, and 10 AWG copper conductors are strictly limited to 15A, 20A, and 30A breakers respectively for general branch circuits, overriding the 75°C and 90°C column values.
Derating Factors: When the Chart Doesn't Tell the Whole Story
A common mistake on the jobsite is treating the base ampacity chart as the final word. The chart assumes two things: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors bundled in a raceway or cable. When real-world conditions violate these assumptions, you must apply derating factors from NEC 310.15.
How Derating Modifies the Base Value
Derating forces you to start with the 90°C column (for THHN/THWN-2) and multiply by an adjustment factor. Let's walk through a real-world scenario:
- The Setup: You are pulling four current-carrying conductors (two 240V circuits sharing a neutral) through a conduit in an attic where the ambient temperature hits 40°C (104°F) in the summer.
- The Wire: You want to use 8 AWG THHN copper to feed a 40A breaker.
- Step 1 (Base 90°C Ampacity): 8 AWG in the 90°C column is 55A.
- Step 2 (Conductor Adjustment): 4 to 6 conductors in a raceway requires an 80% multiplier (NEC Table 310.15(C)(1)). 55A × 0.80 = 44A.
- Step 3 (Temperature Correction): At 40°C ambient, the 90°C column correction factor is 0.91. 44A × 0.91 = 40.04A.
Your final derated ampacity is 40.04A. Because this is above the 40A breaker rating, 8 AWG THHN is legally acceptable here. If you had added a fifth conductor, the 70% adjustment factor would have dropped your ampacity to 35A, forcing you to upsize to 6 AWG wire.
What the Table Cannot Tell You
The ampacity chart is blind to voltage drop. A 6 AWG copper wire might be perfectly legal on a 60A breaker for a 50-foot run, but if you are feeding a subpanel 250 feet away, the voltage drop under full load will exceed the recommended 3% threshold. For long feeder runs, always calculate voltage drop independently and upsize your wire accordingly. Furthermore, the chart does not account for continuous loads (running at max capacity for 3 hours or more), which require the wire and breaker to be sized at 125% of the actual load.
Circuit Breaker Ampacity Chart FAQ
Can I use the 90°C column to size my circuit breaker directly?
No. The 90°C column is almost exclusively used as a baseline for derating calculations. NEC 110.14(C) requires that the final ampacity of the circuit be limited by the temperature rating of the terminations (lugs, breakers, and receptacles). Since nearly all standard residential breakers and receptacles are rated for 75°C (and 14-10 AWG are legally bound to 60°C per 240.4(D)), your final wire size must be validated against those lower columns after derating is complete.
Why is 10 AWG wire limited to 30 amps when the 75°C chart shows 35 amps?
This is due to NEC 240.4(D), often called the "small conductor rule." The physical mass of 14, 12, and 10 AWG wires is small enough that they can heat up rapidly under short-circuit or overload conditions before a standard thermal-magnetic breaker trips. To prevent the wire insulation from melting or starting a fire inside a wall cavity, the NEC hard-caps 14 AWG at 15A, 12 AWG at 20A, and 10 AWG at 30A for general branch circuits, regardless of the insulation's higher thermal rating.
Does the neutral wire count for derating in a multi-wire branch circuit?
It depends on the circuit type. For a standard single-phase, 3-wire multi-wire branch circuit (two hots sharing one neutral), the neutral only carries the unbalanced load. Per NEC 310.15(E), it is not counted as a current-carrying conductor for derating purposes. However, if you are running a 3-phase, 4-wire wye circuit where the major load is non-linear (like LED drivers or computer power supplies generating triplen harmonics), the neutral carries additive harmonic current and must be counted as a current-carrying conductor.
What size breaker and wire do I need for a 50-amp EV charger?
An EV charger is classified as a continuous load because it operates at maximum current for three hours or more. NEC 210.20(A) requires the branch circuit to be rated at 125% of the continuous load. Therefore, a 50-amp EV charger requires a circuit rated for at least 62.5 amps (50 × 1.25). You must step up to the next standard breaker size, which is 70 amps. To feed a 70A breaker using copper wire in a standard 75°C termination setup, you need 4 AWG copper (rated 85A). If you are using aluminum, you must use 2 AWG aluminum (rated 90A).
