If you need a direct answer for standard residential branch circuits: use 14 AWG copper for 15A, 12 AWG copper for 20A, and 10 AWG copper for 30A. However, selecting the correct wire gauge for feeders, subpanels, or high-load appliances requires consulting the official AWG cable size chart derived from the National Electrical Code (NEC). Sizing wire isn't just about matching a breaker; it requires balancing the load, the termination temperature ratings, and the physical installation environment.

The Master AWG Cable Size Chart (NEC Table 310.16)

The following data is sourced directly from NEC Table 310.16 (formerly 310.15(B)(16)), the definitive standard for allowable ampacities of insulated conductors rated up to 2000 volts.

How to read this table: The chart is divided by conductor material (Copper vs. Aluminum/Copper-Clad) and temperature rating (60°C, 75°C, 90°C). The values represent the maximum continuous current the wire can carry in an ambient temperature of 30°C (86°F) before the insulation begins to degrade. Always start your lookup in the 75°C column for standard modern installations, but verify termination ratings before finalizing your pick.
Source: NFPA 70 (NEC) Table 310.16 - Ambient Temperature 30°C
AWG Size Copper 60°C Copper 75°C Copper 90°C Aluminum 75°C Aluminum 90°C
14 AWG15A20A25A--
12 AWG20A25A30A--
10 AWG30A35A40A--
8 AWG40A50A55A40A45A
6 AWG55A65A75A50A60A
4 AWG70A85A95A65A75A
3 AWG85A100A110A75A85A
2 AWG95A115A130A90A100A
1 AWG110A130A145A100A115A
1/0 AWG125A150A170A120A135A
2/0 AWG145A175A195A135A150A
3/0 AWG165A200A225A155A170A
4/0 AWG195A230A260A180A195A

Which Temperature Column Applies to Your Installation?

The most common mistake DIYers and junior electricians make is looking at the 90°C column because modern THHN/THWN-2 wire is rated for 90°C. You almost never use the 90°C column for final breaker sizing.

Under NEC 110.14(C), the allowable ampacity of a circuit is limited by the lowest temperature rating of any connected termination, device, or conductor. Most modern residential breakers, lugs, and receptacles are rated for 75°C. Therefore, the 75°C column is your baseline for circuits larger than 10 AWG.

The 60°C Rule for Small Wires: For 14, 12, and 10 AWG conductors, the NEC explicitly mandates using the 60°C column, regardless of the wire's insulation rating or the termination's 75°C rating. This is why 12 AWG THHN (rated 90°C) is still strictly limited to a 20A breaker.

When to use the 90°C column: The 90°C column is only used as the starting point for calculating derating adjustments (discussed below) or when terminating to equipment explicitly listed and marked for 90°C, which is rare in residential work but common in industrial control panels.

Derating Factors: How Bundling and Heat Modify the Base Value

The AWG cable size chart above assumes two things: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway or cable. When you violate either assumption, you must derate the wire's ampacity.

If you pull multiple circuits through a single conduit, the wires heat each other up. According to NEC 310.15(C)(1), you must apply an adjustment factor to the 90°C column (for THHN/THWN-2) before comparing the result to your breaker size.

  • 4 to 6 current-carrying conductors: Multiply base 90°C ampacity by 80%.
  • 7 to 9 current-carrying conductors: Multiply base 90°C ampacity by 70%.
  • 10 to 20 current-carrying conductors: Multiply base 90°C ampacity by 50%.
Worked Example: You are pulling four current-carrying 12 AWG THHN wires in a single conduit for two 20A circuits. The 90°C ampacity for 12 AWG is 30A. Applying the 80% derating factor (30A x 0.80) gives you 24A. Because 24A is greater than your 20A load and breaker size, 12 AWG is still legally compliant. If you added a third circuit (6 current-carrying wires), the math remains the same (80%), but if you added a fourth (8 wires), you'd drop to 70% (30A x 0.70 = 21A), which is still barely acceptable for a 20A breaker, but leaves zero margin for error.

Note: Equipment grounding conductors and neutral wires that only carry unbalanced current do not count as current-carrying conductors for derating purposes.

Quick Decision Path: Pick Your Wire Size in 4 Steps

Stop guessing and use this decision tree to lock in your exact AWG pick for standard copper installations.

Step Action Rule / Math
1. Calculate True Load Identify if the load is continuous (on for 3+ hours). If continuous, multiply the load amperage by 1.25. (e.g., A 16A continuous load becomes 20A).
2. Size the Breaker Select the standard overcurrent protective device (OCPD). Breaker must be ≥ the True Load calculated in Step 1. Standard sizes: 15, 20, 30, 40, 50, 60A.
3. Lookup Base AWG Find the wire size in the 75°C Copper column (or 60°C for 14-10 AWG). The wire's ampacity must be ≥ the Breaker size. (e.g., 40A breaker requires 8 AWG Cu at 50A).
4. Apply Derating Check conduit fill and ambient temperature. If >3 wires in conduit, derate the 90°C column. If the derated value drops below the Breaker size, go up one AWG size and repeat.

Default Concrete Picks (Copper, ≤3 wires, 30°C ambient):

  • 15A Breaker: 14 AWG (NM-B / THHN)
  • 20A Breaker: 12 AWG (NM-B / THHN)
  • 30A Breaker: 10 AWG (NM-B / THHN)
  • 40A Breaker: 8 AWG (THHN / SER)
  • 50A Breaker: 6 AWG (THHN / SER)
  • 60A Breaker: 4 AWG (THHN / SER)
  • 100A Subpanel Feeder: 3 AWG Copper or 1 AWG Aluminum (XHHW-2)

What This AWG Cable Size Chart Cannot Tell You

While the NEC 310.16 chart is the law of the land for thermal limits, it is blind to two critical real-world physics problems: voltage drop and conduit fill.

1. Voltage Drop (The Long Run Problem)
The table assumes your wire is short enough that resistance doesn't matter. In reality, copper has inherent resistance. If you are running a 50A circuit to a detached garage 150 feet away, 6 AWG copper will result in a voltage drop exceeding the NEC's recommended 3% threshold (Informational Note in NEC 310.15(B)). The wire won't melt, but your tools and appliances will suffer from low voltage, causing motors to overheat and draw excess current. For runs over 100 feet, always run a voltage drop calculation and upsized the wire by 1 or 2 AWG sizes regardless of what the ampacity chart dictates.

2. Physical Conduit Fill (Chapter 9)
The ampacity chart tells you the electrical limit, but NEC Chapter 9, Table 1 dictates the physical limit. You cannot stuff more than 40% of a conduit's cross-sectional area with wires (for 3 or more wires). If you calculate that you need four 4/0 AWG conductors for a 200A service, the ampacity chart says it's fine, but Chapter 9 will force you to use a minimum of 2-inch PVC or EMT conduit to prevent jamming and physical damage during the pull.

Final Verdict: Use the 75°C column of the NEC 310.16 AWG cable size chart as your absolute baseline for sizing. Apply the 125% continuous load multiplier, derate for conduit bundling using the 90°C column, and upsized for voltage drop on any run exceeding 100 feet. When in doubt, the next larger AWG size is always legally permissible and physically safer, provided it physically fits into the breaker lugs.