AWG Wire Size Chart: NEC Ampacity, Breaker Sizing & Derating Guide

When selecting an AWG wire size for a circuit, the direct answer depends on your overcurrent protection device (breaker) and the insulation type. For standard residential copper branch circuits, the baseline pairings are: 14 AWG for 15A, 12 AWG for 20A, 10 AWG for 30A, 8 AWG for 40A, and 6 AWG for 55A. However, these numbers assume standard conditions (not more than three current-carrying conductors in a raceway, ambient temperature of 30°C/86°F). If you are pulling THHN in conduit or running long feeder lines, the base ampacity changes, and you must consult the temperature columns and derating factors outlined below.

The Master AWG Wire Size & Ampacity Chart (NEC Table 310.16)

This table is derived directly from NFPA 70 (National Electrical Code) Table 310.16. It lists the allowable ampacities for insulated conductors rated up to 2000 volts, in ambient temperatures of 30°C (86°F).

How to read this table: Locate your wire gauge in the first column. Move horizontally to the material (Copper or Aluminum) and the temperature rating printed on your wire's jacket (60°C, 75°C, or 90°C). The intersecting number is the maximum continuous current the wire can carry before the insulation degrades. Note: NEC 240.4(D) strictly limits overcurrent protection for 14, 12, and 10 AWG copper to 15A, 20A, and 30A respectively, regardless of the higher values shown in the 75°C and 90°C columns.

* Asterisks denote sizes subject to NEC 240.4(D) small conductor overcurrent protection limits.

Which Temperature Column Applies to Your Installation?

The most common mistake DIYers and junior electricians make is reading the 90°C column because modern THHN/THWN-2 wire is rated for 90°C. However, NEC 110.14(C) dictates that the ampacity of a circuit is limited by the lowest temperature rating of any connected component, termination, or conductor in that circuit.

Here is how to choose the correct column in practice:

• 60°C Column: Use this for Nonmetallic-Sheathed Cable (NM-B / Romex), UF-B cable, and any equipment explicitly marked for 60°C. Most residential branch circuits using NM-B fall here.
• 75°C Column: Use this for THWN/THHN wires in conduit terminating on modern breakers, lugs, and disconnects rated 100A or less (which are typically tested and listed for 75°C terminations), as well as SER/SEU cable for ranges and dryers.
• 90°C Column: Use this only as the starting baseline for derating calculations (bundling and ambient temperature adjustments). You almost never use the 90°C column for final breaker sizing because terminations rarely support it.

How Derating Modifies Your Base Ampacity

The ampacities in the master table assume ideal conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors (CCCs) bundled together. When you exceed these parameters, the wire cannot dissipate heat as effectively, and you must apply adjustment factors.

Worked Numeric Example: Bundled THHN in Conduit
Imagine you are pulling wire for a multi-wire branch circuit and a dedicated 240V line through the same EMT conduit. You have 6 current-carrying conductors (two hots and a neutral for the MWBC, plus two hots for the 240V line; grounds do not count as CCCs). You plan to use 10 AWG Copper THHN.

1. Find Base Ampacity: Because we are derating, NEC 310.15 allows us to start with the 90°C column for the wire itself. 10 AWG at 90°C = 40A.
2. Apply Bundling Factor: According to NEC Table 310.15(C)(1), 4 to 6 current-carrying conductors require an 80% adjustment factor.
3. Calculate Derated Ampacity: 40A × 0.80 = 32A.
4. Select Breaker: The derated capacity is 32A. You can safely protect this wire with a standard 30A breaker. (If the math resulted in 28A, you would be forced to step up to 8 AWG wire).

What the AWG Table Cannot Tell You

While Table 310.16 prevents wires from melting, it does not guarantee your equipment will function correctly. Ampacity charts ignore three critical real-world engineering constraints:

• Voltage Drop: The NEC recommends (but rarely mandates) a maximum 3% voltage drop for branch circuits and 5% total for feeder + branch. If you are running a 12 AWG wire 150 feet to a 15A window AC unit at the end of a driveway, the wire will not overheat, but the voltage at the receptacle will sag below 114V, potentially stalling the compressor motor. For long runs, use a voltage drop calculator and upsize the wire by one or two gauges, regardless of ampacity.
• Short-Circuit Withstand (Let-Through Current): Under a massive short circuit (e.g., 10,000A fault), a breaker takes milliseconds to trip. During that window, immense magnetic and thermal forces act on the wire. Extremely long runs of minimally sized wire can suffer mechanical damage at the terminations before the breaker clears the fault.
• Physical Termination Limits: You might calculate that 2 AWG aluminum is perfect for a 90A subpanel feeder. However, the physical lugs on a standard 100A main breaker might only be rated to accept up to 4 AWG. Always check the manufacturer's datasheet for the specific lug torque and wire-range specifications before buying cable.

Disclaimer: This guide provides NEC-style reference data for educational and planning purposes. The National Electrical Code is updated every three years, and local municipalities frequently adopt amendments. Your local Authority Having Jurisdiction (AHJ) or a licensed electrician has final authority on all wire sizing and overcurrent protection decisions.