If you need the direct answer for standard residential branch circuits: a 15A breaker requires 14 AWG wire, a 20A breaker requires 12 AWG wire, and a 30A breaker requires 10 AWG wire. These pairings assume copper conductors, standard ambient temperatures, and no more than three current-carrying conductors in a raceway.
Sizing a circuit breaker to match your wire gauge is not about guessing; it is strictly governed by the National Electrical Code (NEC). The breaker's sole job is to protect the wire from melting and starting a fire inside your walls. If the breaker is too large for the wire, the wire becomes a heating element before the breaker ever trips. Below is the definitive reference chart, followed by the critical temperature and derating rules that dictate which numbers actually apply to your specific installation.
The Standard Circuit Breaker to Wire Size Chart
The following data is sourced directly from NEC 2023 Table 310.16 (Ampacities of Insulated Conductors) and NEC 240.4(B) (Overcurrent Devices Rated 800 Amps or Less). This table applies to copper conductors with common insulation types like THHN, THWN, and XHHW.
| Wire Size (AWG/kcmil) | 60°C Column (Amps) | 75°C Column (Amps) | 90°C Column (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 | 40A / 50A ** |
| 6 AWG | 55 | 65 | 75 | 60A |
| 4 AWG | 70 | 85 | 95 | 80A |
| 3 AWG | 85 | 100 | 110 | 100A |
| 2 AWG | 95 | 115 | 130 | 110A |
| 1 AWG | 110 | 130 | 145 | 125A |
| 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 | 225A |
* NEC 240.4(D) Small Conductor Rule: Regardless of the insulation rating or the 75°C/90°C ampacity, 14 AWG is strictly capped at 15A, 12 AWG at 20A, and 10 AWG at 30A for overcurrent protection in most general applications.
** Exception for 8 AWG: While the 60°C column lists 40A, NEC 240.4(B) allows you to round up to the next standard breaker size (50A) if the 75°C or 90°C ampacity supports it and the specific load calculations dictate it, but 40A is the standard conservative match for the 60°C column.
Which Temperature Column Applies to Your Installation
The most common mistake DIYers and junior apprentices make is looking at the 90°C column because they purchased 90°C-rated THHN wire from the hardware store. You almost never get to use the 90°C column for final breaker sizing.
NEC 110.14(C) dictates that the ampacity of a circuit is limited by the lowest temperature rating of any connected component, termination, or conductor in the entire run. This is known as the "weakest link" rule.
- The 60°C Column: You must use this column for 14, 12, and 10 AWG wires, regardless of their insulation rating. NEC 110.14(C)(1)(a) explicitly limits circuits rated 100 amps or less, using conductors sized 14 AWG through 1 AWG, to the 60°C ampacity column unless the equipment is specifically listed and identified for 75°C. Most standard residential receptacles and basic breakers fall into this 60°C termination limit.
- The 75°C Column: You can use this column for larger feeder wires (typically 1/0 AWG and larger, or specific 75°C rated equipment like modern main lugs and high-amperage breakers). Most modern commercial breakers and subpanel lugs are rated 75°C.
- The 90°C Column: You are only permitted to use the 90°C column for derating calculations (adjusting for heat and conduit fill), not for the final breaker sizing.
Derating Factors and What the Chart Cannot Tell You
The chart above assumes perfect conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors bundled together in a single raceway or cable. When real-world conditions deviate, the base ampacity drops.
How Derating Modifies the Base Value
If you pull four or more current-carrying conductors through a single conduit, the wires heat each other up. NEC Table 310.15(C)(1) requires you to multiply the wire's ampacity by a derating factor.
Worked Example: You are running four 12 AWG THHN current-carrying conductors in a conduit for two separate 20A circuits.
1. The 90°C ampacity of 12 AWG THHN is 30A.
2. Four conductors require an 80% derating factor.
3. 30A × 0.80 = 24A adjusted ampacity.
4. Because 24A is still above the 20A breaker limit, you can still use a 20A breaker. However, if you had nine conductors in that conduit (requiring a 50% derating factor), your adjusted ampacity would drop to 15A (30A × 0.50). You would then be legally forced to upsize your wire to 10 AWG to maintain the 20A circuit.
What the Table Cannot Tell You
An ampacity chart only solves for thermal limits. It completely ignores voltage drop. NEC 310.14(B) includes an informational note recommending that branch circuit voltage drop be limited to 3%, and total feeder-plus-branch drop to 5%.
If you are wiring a detached garage 120 feet away from your main panel on a 20A circuit, the chart says 12 AWG is perfectly safe from a fire perspective. However, under a full 20A load, 120 feet of 12 AWG copper will experience a voltage drop of roughly 4.1% (dropping your 120V down to 115V). While not a fire hazard, this will cause motors to run hot and lights to dim. In this scenario, you must upsize to 10 AWG wire to mitigate voltage drop, even though your breaker remains 20A. For comprehensive guidance on long-run calculations, consult resources like Electrical Contractor Magazine's code columns or the NFPA's NEC hub.
Furthermore, the chart does not account for specific appliance hardwiring rules. A water heater or HVAC condenser often requires a breaker sized at 125% of the continuous load or specific minimum circuit ampacities (MCA) listed on the manufacturer's nameplate, which may force you to use a larger wire than the standard chart implies. Always defer to the manufacturer's installation sheet and your local Authority Having Jurisdiction (AHJ) for final sign-off.
