When sizing wire for a breaker, the direct answer for standard residential copper branch circuits is: 14 AWG is rated for 15 amps, 12 AWG for 20 amps, 10 AWG for 30 amps, 8 AWG for 40 amps, and 6 AWG for 55 or 65 amps depending on your termination temperature. However, simply memorizing these five numbers will eventually lead to a failed inspection or a melted terminal lug. To size wire correctly, you must understand how to read the official ampacity tables and apply adjustment factors.

How to read the table below: First, identify your conductor material (almost always copper for residential branch circuits). Second, locate your wire gauge in the left column. Third, and most importantly, choose the correct temperature column. The temperature rating is dictated by the weakest link in your circuit—usually the breaker lugs or device terminals, not the wire insulation itself. The values below represent the maximum continuous current the wire can carry before its insulation begins to degrade, assuming an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway.

The Master AWG Current Ratings Table (NEC 310.16)

The following data is sourced directly from NFPA 70 (National Electrical Code) Table 310.16. This table applies to copper conductors rated 0-2000 volts. Bookmark this section for quick reference on the jobsite.

AWG / kcmil Size 60°C (140°F) Column 75°C (167°F) Column 90°C (194°F) Column
14 AWG 15 A 20 A 25 A
12 AWG 20 A 25 A 30 A
10 AWG 30 A 35 A 40 A
8 AWG 40 A 50 A 55 A
6 AWG 55 A 65 A 75 A
4 AWG 70 A 85 A 95 A
3 AWG 85 A 100 A 110 A
2 AWG 95 A 115 A 130 A
1 AWG 110 A 130 A 145 A
1/0 AWG 125 A 150 A 170 A
2/0 AWG 145 A 175 A 195 A
3/0 AWG 165 A 200 A 225 A
4/0 AWG 195 A 230 A 260 A
Code Caveat (NEC 240.4(D)): For small conductors (14, 12, and 10 AWG copper), the NEC strictly caps overcurrent protection at 15A, 20A, and 30A respectively. Even if your wire has 90°C THHN insulation and the math allows a higher rating after derating, you cannot protect 12 AWG wire with a 25A breaker. The 15/20/30A limits are absolute for general branch circuits.

Which Temperature Column Applies to Your Installation?

The most common mistake DIYers and junior apprentices make is looking at the wire jacket, seeing "THHN 90°C," and immediately using the 90°C column to size their breaker. This is incorrect and violates NEC 110.14(C).

You must use the temperature column that matches the lowest-rated termination in your circuit. In modern residential and light-commercial panels, the lugs on circuit breakers and the terminals on standard receptacles are almost universally rated for 75°C. Therefore, the 75°C column is your baseline for determining wire ampacity in 95% of standard installations.

When do you use the 60°C column? You must default to the 60°C column if you are working on older equipment (pre-1990s panels or breakers) where the terminal temperature rating is unmarked, or if you are connecting to devices explicitly marked for 60°C only. Furthermore, NEC 240.4(D) mandates that the 60°C column values be used for overcurrent protection sizing for 14, 12, and 10 AWG conductors, regardless of the insulation type.

When do you use the 90°C column? You almost never use the 90°C column to determine your final breaker size. Instead, the 90°C column is used exclusively as the starting point for derating calculations (which we will cover next). Once you apply your derating factors to the 90°C value, you must compare the result to the 75°C column value. Your final allowable ampacity is the lower of the two numbers.

How Derating Modifies Your Base Ampacity

The ampacities in the master table assume two ideal conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors bundled together in a conduit or cable. When real-world conditions violate these assumptions, the wire cannot dissipate heat as efficiently, and you must reduce (derate) its allowable current.

Derating involves two distinct NEC tables (formerly in 310.15, now reorganized in the 2023/2026 NEC under 310.15(B) and 310.15(C)):

  1. Ambient Temperature Correction: If your conduit runs through a hot attic in the summer, the ambient temperature might reach 50°C (122°F). According to the ambient temperature correction factors, a 90°C wire in a 50°C environment must be multiplied by 0.82.
  2. Adjustment Factors for Bundling: If you pull four to six current-carrying conductors through a single conduit, you must apply an 80% adjustment factor (multiply by 0.80). For seven to nine conductors, the factor drops to 70%.

Worked Example: You are running a 240V baseboard heater circuit. You pull four current-carrying conductors (two separate 240V circuits, no neutral) through a conduit in an attic that reaches 45°C (113°F). You are using 8 AWG THHN copper.

  • Step 1: Start with the 90°C column for 8 AWG: 55A.
  • Step 2: Apply ambient correction for 45°C (0.87 factor): 55A × 0.87 = 47.85A.
  • Step 3: Apply bundling adjustment for 4 conductors (0.80 factor): 47.85A × 0.80 = 38.28A.
  • Step 4: Compare this to the 75°C column for 8 AWG (50A). The lower number is 38.28A.
  • Step 5: Select the breaker. The next standard breaker size down (or equal) is 35A. (Note: If the calculated value was 39A, you could round up to a 40A breaker per 240.4(B), but 38.28A requires stepping down to 35A to be strictly safe, or stepping up to 10 AWG wire if a 40A breaker is mandatory for the load).
Pro-Tip for NM-B Cable: Standard indoor Romex (NM-B) contains a 90°C insulation jacket, but NEC 334.80 strictly limits its ampacity to the 60°C column values. You cannot use the 90°C column for derating NM-B cable. If you need to derate for attic heat, you must start with the 60°C baseline.

What This Table Cannot Tell You (Voltage Drop & Local Code)

While the Copper Development Association Ampacity Tables and NEC 310.16 tell you how much current a wire can carry before the insulation melts or the breaker trips, they tell you absolutely nothing about voltage drop. A wire can be perfectly safe from a fire hazard perspective while simultaneously starving your equipment of voltage.

The Voltage Drop Blind Spot: If you run a 50-amp circuit to a detached garage 150 feet away using 6 AWG copper (rated 65A at 75°C), the wire will not overheat. However, at a full 50A load, you will experience roughly a 6.5% voltage drop. Your 120V tools will see only 112V, causing motors to run hot, draw more current, and fail prematurely. Industry best practice (and NEC informational note 210.19(A)) recommends limiting voltage drop to 3% on branch circuits and 5% total from the service entrance to the furthest outlet. For that 150-foot, 50-amp run, you actually need to upsize to 3 AWG copper to maintain a 3% drop, even though 6 AWG is legally allowed for ampacity. Always run long feeds through a reliable voltage drop calculator before pulling wire.

What else is missing?

  • Short-Circuit Withstand: The table assumes normal operating loads. It does not tell you if the wire can survive the magnetic and thermal forces of a 10,000-amp short circuit before the breaker clears the fault in 16 milliseconds.
  • Physical Fit: The table ignores the physical reality of panel gutters. You might calculate that 250 kcmil is required for a 255A feeder, but if your breaker lugs are only rated to accept a maximum of 3/0 AWG, you cannot physically terminate the wire. You must check the manufacturer's termination torque and wire-range specifications.
  • Local AHJ Amendments: The NEC is a model code. Your local Authority Having Jurisdiction (AHJ) or city inspector may have local amendments that override the baseline table. Some municipalities mandate a minimum of 12 AWG for all 15A and 20A general lighting circuits, effectively banning 14 AWG from residential construction entirely despite its 15A rating in the table.

Keep this chart bookmarked, but always pair it with a voltage drop calculation and a quick check of your local municipal amendments before buying wire by the spool.