For standard residential branch circuits using copper NM-B (Romex), the direct rule is simple: match a 15A breaker to 14 AWG, a 20A breaker to 12 AWG, and a 30A breaker to 10 AWG. However, when pulling THHN in conduit, sizing subpanel feeders, or dealing with high-ambient environments, you must reference the exact ampacity tables. This breaker wire size chart is based on the National Electrical Code (NEC) Table 310.16, providing the exact data you need to pass inspection and prevent thermal failures.
The Master Breaker Wire Size Chart (NEC Table 310.16)
The table below outlines the allowable ampacities for insulated copper conductors rated up to 2000 volts. The values assume an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway.
| AWG Size | 60°C Copper (Amps) | 75°C Copper (Amps) | 90°C Copper (Amps) | Max Standard Breaker |
|---|---|---|---|---|
| 14 AWG | 15 | 20 | 25 | 15A * |
| 12 AWG | 20 | 25 | 30 | 20A * |
| 10 AWG | 30 | 35 | 40 | 30A * |
| 8 AWG | 40 | 50 | 55 | 50A |
| 6 AWG | 55 | 65 | 75 | 70A |
| 4 AWG | 70 | 85 | 95 | 90A |
| 3 AWG | 85 | 100 | 110 | 100A |
| 2 AWG | 95 | 115 | 130 | 125A |
| 1 AWG | 110 | 130 | 145 | 150A |
| 1/0 AWG | 125 | 150 | 170 | 150A |
| 2/0 AWG | 145 | 175 | 195 | 175A |
| 3/0 AWG | 165 | 200 | 225 | 200A |
| 4/0 AWG | 195 | 230 | 260 | 250A |
* Note: NEC 240.4(D) strictly limits overcurrent protection for small conductors. Regardless of the 75°C or 90°C column values, 14 AWG is capped at 15A, 12 AWG at 20A, and 10 AWG at 30A for standard branch circuits.
Which Ampacity Column Applies to Your Installation?
The most common mistake DIYers and junior apprentices make is looking at the 90°C column because it offers the highest ampacity. Under NEC 110.14(C), you cannot use the 90°C column for final breaker sizing unless every single termination point in the circuit is rated for 90°C. In the real world, almost no residential breakers, receptacles, or switches are rated above 75°C.
- 60°C Column: Use this for NM-B (Romex) cable assemblies, as the internal heat dissipation of the bundled jacket limits the wire. Also use this when terminating on older devices or specific industrial equipment explicitly marked 60°C.
- 75°C Column: Use this for THHN/THWN-2 wires pulled in conduit, as well as SER/SEU feeder cables, provided the breaker lugs and panel terminations are marked 75°C (which nearly all modern Square D, Eaton, and Siemens panels are).
- 90°C Column: Use this only as the starting baseline for derating calculations (explained below).
How Derating and Bundling Modify These Base Values
The ampacities in the chart above assume two things: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a single conduit. When you violate either assumption, the wire cannot dissipate heat as efficiently, and you must apply a derating factor.
This is where the 90°C column finally earns its keep. You apply the derating percentage to the 90°C ampacity, not the 75°C ampacity.
Real-World Derating Example
Imagine you are pulling two separate 20A circuits (four current-carrying hot wires, plus two grounds which do not count) through a single 3/4-inch EMT conduit. According to NEC Table 310.15(C)(1), 4 to 6 current-carrying conductors require an 80% derating factor.
- Start with the 90°C column for 12 AWG THHN: 30A.
- Multiply by the 80% derating factor: 30A × 0.80 = 24A.
- Because 24A is greater than the 20A breaker you intend to use, 12 AWG is still perfectly legal and safe for this run.
However, if you pulled four circuits (8 current-carrying conductors), the derating factor drops to 70%. 30A × 0.70 = 21A. You can still use a 20A breaker. But if you had 10 conductors (50% factor), 30A × 0.50 = 15A. You would be forced to either downsize to a 15A breaker or upsize the wire to 10 AWG.
What This Table Cannot Tell You
While this breaker wire size chart is the gold standard for thermal ampacity, it is entirely blind to three critical jobsite realities that will fail an inspection or cause operational issues if ignored:
- Voltage Drop: NEC Table 310.16 does not account for distance. A 6 AWG wire is perfectly legal for a 60A breaker at 50 feet. But if you are running a 60A EV charger 150 feet from the panel, that same 6 AWG wire will suffer a voltage drop exceeding the recommended 3% threshold. You must use Chapter 9, Table 8 (circular mils) and the voltage drop formula to upsize the wire, often jumping to 4 AWG or 3 AWG for long runs.
- Conduit Fill Capacity: Just because the ampacity chart allows you to cram nine 12 AWG wires into a 1/2-inch conduit doesn't mean it physically fits. You must cross-reference NEC Chapter 9, Table 1 to ensure the cross-sectional area of the wires does not exceed 40% of the conduit's internal area. Overstuffing conduit makes pulling impossible and damages wire insulation.
- Termination Torque: Since the 2017 NEC cycle (110.14(D)), you are legally required to use a calibrated torque screwdriver or wrench when terminating breakers and lugs. A 10 AWG wire on a 30A breaker torqued to 35 in-lbs instead of the manufacturer's specified 45 in-lbs will create a high-resistance connection, leading to localized melting and arc faults, regardless of how perfectly you sized the wire.
Disclaimer: This chart provides NEC-style guidance for standard copper installations. Always verify your local Authority Having Jurisdiction (AHJ) requirements, as local amendments can supersede national code. For aluminum wire sizing (common in 100A+ feeders), reference the separate aluminum columns in NEC 310.16; a general rule of thumb is to upsize aluminum by two AWG sizes compared to copper (e.g., use 1/0 AWG Aluminum where you would use 3 AWG Copper for a 100A feeder).
