To size a breaker, match the wire's ampacity to the breaker's standard rating. For standard residential branch circuits using copper THHN/THWN-2 wire in a 75°C termination environment, a 15A breaker requires 14 AWG, a 20A breaker requires 12 AWG, and a 30A breaker requires 10 AWG. The circuit breaker amp chart below maps standard overcurrent protective device (OCPD) ratings to the minimum wire gauge required, based on NEC Table 310.16 and NEC 240.4(B). Always verify your local codes, as the Authority Having Jurisdiction (AHJ) has final say over national standards.
The Standard NEC Circuit Breaker Amp Chart
This table provides the minimum allowable wire gauge for standard breaker sizes. It is derived from NFPA 70 (National Electrical Code) Table 310.16 for ampacities and NEC 240.4(B) for the "next standard size up" rule.
How to read this table: Locate your breaker size in the first column. The subsequent columns list the minimum American Wire Gauge (AWG) required based on the temperature rating of the weakest link in your circuit—usually the breaker terminal or the receptacle. Smaller AWG numbers indicate thicker wires with higher ampacity. The values assume an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway.
| Breaker Size (Amps) | Min Copper AWG (60°C Column) | Min Copper AWG (75°C Column) | Min Aluminum AWG (75°C Column) |
|---|---|---|---|
| 15A | 14 | 14 | 12 |
| 20A | 12 | 12 | 10 |
| 30A | 10 | 10 | 8 |
| 40A | 8 | 8 | 6 |
| 50A | 6 | 8 | 6 |
| 60A | 6 | 6 | 4 |
| 70A | 4 | 4 | 2 |
| 90A | 2 | 2 | 1/0 |
| 100A | 1 | 3 | 1 |
| 125A | 1/0 | 1/0 | 2/0 |
| 150A | 2/0 | 1/0 | 3/0 |
| 200A | 4/0 | 3/0 | 250 kcmil |
Quick-Jump Reference for Common Branch Circuits
For the most frequently installed residential circuits, bookmark these baseline rules:
- 15A Breaker (Lighting/General Use): Minimum 14 AWG copper. (Note: Many modern jurisdictions and electricians mandate 12 AWG for all general-purpose 15A/20A circuits to prevent future overloading and reduce voltage drop).
- 20A Breaker (Kitchen/Bathroom Receptacles): Minimum 12 AWG copper. Required for small-appliance and bathroom branch circuits per NEC 210.11(C).
- 30A Breaker (Dryers/Water Heaters): Minimum 10 AWG copper. Standard for older electric dryers and many 120/240V water heaters.
- 50A Breaker (Ranges/EV Chargers): Minimum 6 AWG copper (using 60°C column) or 8 AWG copper (if 75°C terminations are verified). Typical for standard electric ranges and Level 2 EV charging stations.
Which Column Applies to Your Installation
The most common mistake DIYers make is using the 90°C column to size their wire because THHN/THWN-2 wire is rated for 90°C. This is a code violation that can lead to melted terminals. You must use the temperature column that matches the lowest-rated termination point in the circuit.
The 60°C Column: According to NEC 110.14(C)(1)(a), circuits rated 100A or less, or marked for 14 AWG through 1 AWG conductors, must be sized using the 60°C column unless the equipment is specifically listed and identified for 75°C. In residential settings, standard 15A and 20A duplex receptacles and older breakers are typically rated for 60°C terminations. Furthermore, NEC 240.4(D) explicitly restricts 15A breakers to 14 AWG copper and 20A breakers to 12 AWG copper, regardless of the wire's insulation rating.
The 75°C Column: Modern breakers (like Square D QO or Homeline manufactured after the late 1990s), subpanel lugs, and larger appliances (ranges, dryers) are almost universally rated for 75°C terminations. For feeder wires and circuits over 100A, the 75°C column is the standard baseline. As noted by OSHA electrical safety guidelines, ensuring termination compatibility is critical to preventing thermal expansion and contraction that loosens lugs over time.
The 90°C Column: You can only use the 90°C ampacity of THHN/THWN-2 wire as the starting point for derating calculations (adjusting for ambient heat or wire bundling). The final adjusted ampacity must still be protected by a breaker sized to the 60°C or 75°C termination limits.
How Derating Modifies Base Values and Chart Limitations
The circuit breaker amp chart above assumes ideal conditions: an ambient air temperature of 30°C (86°F) and no more than three current-carrying conductors (CCCs) in a single conduit. When real-world conditions deviate, you must apply derating factors that effectively reduce the wire's ampacity, potentially forcing you to upsize the wire while keeping the same breaker.
Ambient Temperature Correction: If you run conduit across a hot attic in Texas where ambient temperatures reach 50°C (122°F), you must apply a correction factor. For 90°C THHN wire at 50°C ambient, the NEC Table 310.15(B)(1) correction factor is 0.82. A 12 AWG wire with a 90°C baseline ampacity of 30A derates to 24.6A. Since 24.6A is still above the 20A breaker requirement, 12 AWG is acceptable. However, if the attic hits 60°C, the factor drops to 0.71 (30A × 0.71 = 21.3A), leaving virtually no safety margin.
Conductor Bundling Adjustment: When you pull more than three CCCs through a single conduit (e.g., two multi-wire branch circuits sharing a neutral, totaling 4 CCCs), the wires heat each other up. NEC Table 310.15(C)(1) requires an 80% adjustment factor for 4-6 conductors. If you are pulling four 10 AWG THHN wires for two 30A circuits, the 90°C baseline is 40A. Multiplied by 0.80, the derated ampacity is 32A. Because 32A is still greater than the 75°C termination limit of 30A, you can still use a 30A breaker. But if you pulled six wires, the factor drops to 70% (40A × 0.70 = 28A), forcing you to upsize to 8 AWG wire to maintain a 30A breaker.
What the Chart Cannot Tell You
While this chart is the definitive starting point for OCPD sizing, it has three critical blind spots that require separate calculations:
- Voltage Drop: The NEC recommends (and some local codes mandate) a maximum 3% voltage drop on branch circuits and 5% overall. If you are running a 20A circuit to a detached garage 150 feet away, 12 AWG wire will suffer severe voltage drop under load. You must upsize to 8 AWG or 6 AWG copper to maintain voltage, even though the breaker remains 20A.
- Continuous Loads: If a load will run for 3 hours or more (e.g., commercial lighting, EV chargers, space heaters), NEC 210.20(A) requires the breaker to be sized at 125% of the continuous load. A 16A continuous EV charger requires a 20A breaker (16 × 1.25 = 20), and the wire must be sized for 20A continuous, meaning you need wire with an ampacity of at least 20A after derating.
- Motor and Appliance Specifics: Motors have high inrush currents. NEC Article 430 allows motor branch circuit short-circuit and ground-fault protective devices to be sized much higher than the wire's ampacity (often 250% of full-load current) to prevent nuisance tripping during startup. Always check the manufacturer's nameplate and NEC Article 422 for specific appliances.
