Copper Wire Size Breaker Chart: NEC Ampacity Reference

For standard residential copper wiring, the baseline pairings are straightforward: match 14 AWG to a 15A breaker, 12 AWG to a 20A breaker, 10 AWG to a 30A breaker, 8 AWG to a 40A breaker, and 6 AWG to a 55A (or 60A) breaker. However, simply matching wire to a breaker without understanding temperature columns and derating factors is how circuits fail inspections or, worse, overheat in the walls. The chart below provides the exact ampacity limits based on the National Electrical Code (NEC), followed by the critical rules for applying these numbers to your specific installation.

The Master Copper Wire Size Breaker Chart (NEC Table 310.16)

This table is derived directly from NEC Table 310.16 for copper conductors. Before using this chart, you must understand how to read the temperature columns. The 60°C column applies primarily to older installations and specific cable types like NM-B (Romex). The 75°C column is the standard baseline for modern breaker terminations and THHN wire in conduit. The 90°C column is rarely used for final ampacity but is essential for calculating derating factors. Note that for 14, 12, and 10 AWG wires, NEC 240.4(D) imposes strict maximum breaker limits that override the higher ampacities found in the 75°C and 90°C columns.

Which Temperature Column Applies to Your Installation?

The most common mistake DIYers and junior apprentices make is using the 90°C column to size their breakers. You can almost never use the 90°C column for final ampacity sizing. The column you use is dictated by the weakest link in your circuit, which is usually the termination point (the breaker lug or the receptacle screw).

According to standard wiring practices and NEC 110.14(C), the temperature rating of the termination dictates the wire's allowable ampacity. Most modern residential breakers and receptacles are rated for 75°C terminations. Therefore, even if you pull 90°C THHN wire through your walls, the ampacity is capped at the 75°C column where it lands on the breaker.

The NM-B (Romex) Exception: If you are using standard non-metallic sheathed cable (NM-B, commonly known by the brand name Romex), NEC 334.80 explicitly states that the ampacity must not exceed the 60°C column, regardless of the fact that the internal conductors are technically rated for 90°C. This is why 10 AWG NM-B is strictly limited to 30A, and you cannot use it for a 35A load, even though 10 AWG in the 75°C column shows 35A.

Derating Factors and What the Chart Cannot Tell You

The wire size breaker chart above assumes ideal conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors bundled together in a single raceway or conduit. When real-world conditions deviate, the base ampacity drops.

How Derating Modifies the Base Value

When you pull more than three current-carrying conductors through a single conduit, the wires heat each other up. You must apply a derating factor from NEC Table 310.15(C)(1). This is the one scenario where you do use the 90°C column to do your math, before checking the result against your breaker size.

Worked Example: You are pulling four 12 AWG THHN current-carrying conductors in a conduit for a multi-wire branch circuit.
1. Base ampacity at 90°C for 12 AWG is 30A.
2. The derating factor for 4-6 conductors is 80%.
3. 30A × 0.80 = 24A.
4. Because 24A is greater than your 20A breaker, 12 AWG is still legally compliant. However, if you had 10 conductors in that pipe (50% derating), 30A × 0.50 = 15A. You would be forced to upsize to 10 AWG to maintain a 20A circuit.

What the Chart Cannot Tell You

A reference chart is a starting point, not a complete engineering document. This table will not tell you:

• Voltage Drop: The NEC recommends keeping voltage drop under 3% for branch circuits. If you are running a 240V circuit to a detached garage 150 feet away, 8 AWG might be legal for ampacity, but the voltage drop will exceed 3%. You will need to upsize to 6 AWG or 4 AWG purely for voltage retention.
• Conduit Fill Limits: Upsizing your wire to compensate for derating or voltage drop might push your conduit fill percentage past the NEC 40% limit, requiring you to pull a larger diameter PVC or EMT conduit.
• Short Circuit Ratings (AIC):strong> The chart assumes the breaker can safely interrupt the available fault current at your panel. If your utility transformer supplies 22,000 amps of fault current, your breakers must have an adequate Ampere Interrupting Capacity (AIC) rating, regardless of the wire size.