For standard residential copper wiring, the most common breaker size chart pairings are: a 15A breaker with 14 AWG wire, a 20A breaker with 12 AWG wire, and a 30A breaker with 10 AWG wire. These pairings are dictated by the National Electrical Code (NEC) Table 310.16 and the small conductor limits in NEC 240.4(D). However, picking the right wire and breaker isn't just about matching two numbers on a page. You must account for insulation temperature ratings, terminal limitations, and conduit derating to ensure your installation won't trip nuisance or, worse, melt inside the wall.
How to Read the NEC Breaker Size Chart
The definitive source for wire ampacity in the United States is NFPA 70 (the National Electrical Code), specifically Table 310.16. When you look at a standard spec sheet or reference card, you will see three distinct temperature columns: 60°C, 75°C, and 90°C. Here is how to read them:
- 60°C Column: Use this for NM-B (Romex), UF-B, and older equipment terminals. By default, NEC 110.14(C) requires circuits rated 100A or less to use the 60°C column unless the equipment is specifically marked otherwise.
- 75°C Column: Use this for THWN wire in conduit and modern breaker lugs/panel terminals, which are almost universally rated for 75°C.
- 90°C Column: Use this only as the starting point for derating calculations (adjusting for heat and conduit bundling). You almost never use the 90°C ampacity as your final breaker size because residential breaker terminals are rarely rated for 90°C.
Bookmark Quick-Jumps: The most queried residential circuits are 15A (14 AWG), 20A (12 AWG), 30A (10 AWG), and 50A (6 AWG). Note that NEC 240.4(D) strictly caps the overcurrent protection for small conductors, overriding the higher values found in the 75°C and 90°C columns.
| Wire Gauge (AWG) | 60°C Ampacity (NM-B) | 75°C Ampacity (THWN) | 90°C Ampacity (THHN) | Max Standard Breaker |
|---|---|---|---|---|
| 14 AWG | 15A | 20A | 25A | 15A (per 240.4(D)) |
| 12 AWG | 20A | 25A | 30A | 20A (per 240.4(D)) |
| 10 AWG | 30A | 35A | 40A | 30A (per 240.4(D)) |
| 8 AWG | 40A | 50A | 55A | 50A |
| 6 AWG | 55A | 65A | 75A | 60A |
| 4 AWG | 70A | 85A | 95A | 80A |
| 3 AWG | 85A | 100A | 110A | 100A |
| 2 AWG | 95A | 115A | 130A | 110A |
Notice that 12 AWG wire has a 90°C ampacity of 30A. Do not let this trick you into putting 12 AWG on a 30A breaker. NEC 240.4(D) explicitly limits 14 AWG to 15A, 12 AWG to 20A, and 10 AWG to 30A for overcurrent protection, regardless of the insulation's higher thermal rating. There are specific exceptions for motor circuits and HVAC equipment, but for general branch circuits, these hard caps apply.
Adjusting for Derating and Installation Conditions
The base ampacity values in the chart above assume an ambient temperature of 86°F (30°C) and no more than three current-carrying conductors bundled together in a raceway. When you deviate from these baseline conditions, you must apply correction and adjustment factors. This is where the 90°C column earns its keep.
Derating modifies the base value by multiplying the 90°C ampacity by two separate factors: an ambient temperature correction factor (NEC Table 310.15(B)(1)) and a conduit bundling adjustment factor (NEC Table 310.15(C)(1)). You then compare that final derated number against the ampacity in the 75°C column (assuming your terminals are rated 75°C). The lower of the two numbers dictates your maximum allowable ampacity.
Worked Numeric Example:
Imagine you are pulling four 8 AWG THHN current-carrying conductors through a conduit in an attic where the ambient temperature reaches 104°F (40°C). You want to know the maximum breaker size.
- Start with the 90°C base: 8 AWG THHN at 90°C is 55A.
- Apply Temperature Correction: At 104°F (40°C), the correction factor for 90°C wire is 0.94. 55A × 0.94 = 51.7A.
- Apply Bundling Adjustment: Four current-carrying conductors require an 80% adjustment factor. 51.7A × 0.80 = 41.36A.
- Check Terminal Limitations: Your breaker lugs are rated 75°C. The 75°C ampacity for 8 AWG is 50A.
- Final Determination: Compare the derated 90°C value (41.36A) with the 75°C termination limit (50A). The lower value is 41.36A. Therefore, you must drop down to the next standard breaker size, which is 40A. If you had blindly used the 75°C chart value, you would have installed a 50A breaker, creating a severe fire hazard in that hot attic.
For a deeper dive into the mathematical nuances of conduit fill and thermal dissipation, the OSHA electrical safety guidelines and NEC training manuals provide extensive worksheets for complex commercial runs.
What This Breaker Size Chart Cannot Tell You
While the NEC breaker size chart is the foundational reference for any wiring project, it is not a complete design tool. Relying on it blindly without considering the physical realities of your installation will lead to failed inspections or underperforming circuits. Here is what the chart leaves out:
1. Voltage Drop on Long Runs
Ampacity charts tell you what size wire will prevent the insulation from melting; they do not tell you if the voltage will actually reach the load. NEC 310.15(B)(1) Informational Note recommends keeping voltage drop under 3% for branch circuits. If you are running a 20A circuit to a detached shed 120 feet away, 12 AWG wire is perfectly legal per the ampacity chart, but it will suffer a voltage drop of roughly 4.5% at full load. Your tools will run hot and inefficiently. To fix this, you must upsize the wire to 10 AWG (or even 8 AWG) to reduce resistance, while keeping the breaker at 20A to match the load requirements.
2. Continuous vs. Non-Continuous Loads
The breaker sizes listed in the chart assume a non-continuous load (something that runs for less than three hours at a time). If you are wiring a circuit for a continuous load—like a baseboard heater, a server rack, or hardwired commercial lighting—NEC 210.20(A) requires you to derate the breaker's capacity by 125%. A standard 20A breaker can only safely carry 16A of continuous load. If your continuous load draws 17A, you cannot use a 20A breaker; you must step up to a 25A or 30A breaker and upsize the wire accordingly.
3. Aluminum vs. Copper Discrepancies
The chart provided above is strictly for copper conductors. If you are feeding a subpanel or running a heavy appliance circuit using aluminum SER or XHHW-2 cable (which is common and cost-effective for 2 AWG and larger), the ampacities are significantly lower. For example, 2 AWG copper handles 115A at 75°C, but 2 AWG aluminum only handles 90A at 75°C. Always verify the conductor material stamp on the insulation jacket before sizing your breaker, and consult the aluminum-specific columns in NEC Table 310.16.
