For standard residential branch circuits using copper wire, match your breaker size to the 60°C column of the NEC 310.16 table: use 14 AWG for 15A, 12 AWG for 20A, 10 AWG for 30A, 8 AWG for 40A, and 6 AWG for 50A/55A. This baseline assumes standard NM-B cable or THHN in conduit with no more than three current-carrying conductors and an ambient temperature of 86°F (30°C). If your run exceeds 100 feet, or if you are bundling more than three wires in a single conduit, you must apply derating factors and voltage drop calculations before finalizing your wire size.

How to Read the NEC 310.16 Ampacity Table

The authoritative source for wire ampacity in the United States is the National Electrical Code (NFPA 70), specifically Table 310.16 (formerly 310.15(B)(16) in older code cycles). This table dictates the maximum continuous current a conductor can carry without exceeding its insulation temperature rating.

How to read the columns: The table is split by conductor material (Copper vs. Aluminum) and then by insulation temperature rating (60°C, 75°C, and 90°C). The numbers in the cells represent the maximum allowable ampacity in Amps. Always read the copper section unless you are explicitly using aluminum feeders (like SER cable for a subpanel).

Complete AWG Table Amperage Chart (Copper, 60°C to 90°C)

Below is the complete reference chart for the most common residential and light-commercial copper wire sizes. Bookmark this section for quick jobsite lookups.

AWG / kcmil Size 60°C (140°F) Column 75°C (167°F) Column 90°C (194°F) Column Common Application
14 AWG15A20A25A15A Lighting/Receptacle circuits
12 AWG20A25A30A20A Kitchen/Bath/Basement circuits
10 AWG30A35A40A30A Dryer / Water Heater / EV L2
8 AWG40A50A55A40A Range / Subpanel feeder
6 AWG55A65A75A50A/60A Subpanel / EV Charger
4 AWG70A85A95A70A Subpanel feeder
3 AWG85A100A115A100A Subpanel feeder (Copper)
2 AWG95A115A130A100A+ Heavy feeders
1 AWG110A130A145A125A Service entrance
1/0 AWG125A150A170A150A Service entrance
2/0 AWG145A175A195A150A/200A Service entrance
3/0 AWG165A200A225A200A Service entrance (Copper)
4/0 AWG195A230A260A200A+ Heavy service entrance

Source: NFPA 70 (NEC) Table 310.16. Values assume not more than three current-carrying conductors in a raceway and an ambient temperature of 30°C (86°F).

Which Temperature Column Applies to Your Installation?

The most common mistake DIYers make is looking at a spool of 90°C THHN wire, seeing it is rated for 30A at 12 AWG, and putting it on a 30A breaker. This violates NEC 110.14(C), which enforces the "weakest link" rule for terminations.

You must size your overcurrent protection (breaker) based on the temperature rating of the terminations (the breaker lugs and the receptacle/device screws), not just the wire insulation. Here is the hard rule for selecting your column:

  • 14 AWG through 10 AWG: You must use the 60°C column for breaker sizing. Standard residential breakers and 15A/20A receptacles are rarely rated higher than 60°C for these smaller wire sizes.
  • 8 AWG and larger: You may use the 75°C column for breaker sizing, provided your breaker and equipment lugs are explicitly marked "AL/CU" and rated for 75°C (which almost all modern residential breakers are).
  • The 90°C column: This column is almost never used for final breaker sizing. It is used exclusively as the starting point for derating calculations (explained below).

Derating: How Bundle Size and Ambient Heat Modify Base Values

When you pull more than three current-carrying conductors through a single conduit, or when the ambient temperature exceeds 86°F (30°C), the wires cannot dissipate heat effectively. The NEC requires you to reduce (derate) the wire's ampacity. For a deeper dive into bundling rules, refer to EC Magazine's codes and standards resources.

Worked Numeric Example:
You are pulling four 12 AWG THHN current-carrying wires through a conduit in a standard 86°F basement to feed a multi-wire branch circuit.

  1. Start with the 90°C column: 12 AWG THHN at 90°C is rated for 30A.
  2. Apply the bundling derating factor: NEC Table 310.15(C)(1) states that 4 to 6 current-carrying conductors require an 80% adjustment factor.
  3. Calculate derated ampacity: 30A × 0.80 = 24A.
  4. Compare to termination limits: The derated value (24A) is higher than the 60°C termination limit for 12 AWG (20A). Therefore, the final allowable ampacity is capped at 20A.

If you had pulled nine wires in that conduit (70% derating), the math would be 30A × 0.70 = 21A. Still above 20A, so 12 AWG survives. But if you pulled ten wires (50% derating), 30A × 0.50 = 15A. You would be forced to upsize to 10 AWG wire to maintain a 20A circuit.

Decision Path: Pick Your Exact Wire Size in 4 Steps

Use this decision tree to lock in your exact wire and breaker size for any standard single-phase load.

Step Action Example Scenario: 24A Continuous Load (e.g., EV Charger or Server Rack)
1. Classify Load Is the load expected to run at maximum current for 3 hours or more? If yes, it is "Continuous." Yes, an EV charger or server rack runs for 3+ hours. It is a Continuous load.
2. Apply 125% Rule Multiply continuous load by 1.25 to find minimum circuit ampacity. (Non-continuous loads skip this step). 24A × 1.25 = 30A minimum circuit ampacity.
3. Select Breaker Choose the next standard breaker size equal to or greater than the minimum circuit ampacity (NEC 240.6). 30A is a standard breaker size. Pick a 30A breaker.
4. Select Wire (AWG) Look up the breaker size in the 60°C column (for 14-10 AWG) or 75°C column (for 8 AWG+). Pick the wire that meets or exceeds this value. Looking at the 60°C column, 10 AWG is rated for exactly 30A. Concrete Pick: 10 AWG Copper.

What the AWG Table Cannot Tell You

The NEC 310.16 table is strictly a thermal limit chart. It tells you when the wire insulation will melt or degrade. It does not account for power quality or physical fit. Before finalizing your install, you must verify three things the table ignores:

  1. Voltage Drop: The NEC table assumes a short run. If you are running a 20A circuit 150 feet to a detached garage, 12 AWG wire will suffer a voltage drop exceeding the recommended 3% limit. You must upsize to 10 AWG or 8 AWG purely to maintain voltage at the receptacle, even though the breaker is only 20A.
  2. Conduit Fill Capacity: NEC Chapter 9, Table 1 limits conduit fill to 40% for three or more wires. You might calculate that 8 AWG THHN is electrically sufficient, but if you are pulling four of them through a 1/2-inch EMT conduit, they physically will not fit without jamming and damaging the insulation. You must upsize the conduit or the wire.
  3. Short-Circuit Withstand Rating: The table does not tell you how much fault current the wire can survive before vaporizing. In high-availability industrial panels with 65kAIC fault currents, the let-through current of the breaker must be coordinated with the wire's short-circuit thermal rating.

Always treat the AWG table amperage chart as your starting baseline, not your final sign-off. Verify your termination ratings, apply your derating math, check your voltage drop, and confirm your physical conduit fill before pulling the wire.