For standard residential branch circuits, the direct wire-to-breaker sizing rule is straightforward: use 14 AWG copper for 15A breakers, 12 AWG for 20A, 10 AWG for 30A, 8 AWG for 40A, and 6 AWG for 50A. However, these quick-reference numbers assume standard conditions—specifically, no more than three current-carrying conductors in a raceway and an ambient temperature of 86°F (30°C). When you move beyond basic plug-and-play receptacle circuits into subpanel feeders, HVAC disconnects, or high-ambient attic runs, you must consult the master ampacity tables and apply the correct temperature and derating adjustments.

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

The following chart is derived directly from NEC Table 310.16 (formerly 310.15(B)(16)), the authoritative standard published by the National Fire Protection Association (NFPA) for allowable ampacities of insulated conductors. This table is your primary reference for matching wire gauge to overcurrent protection.

How to Read This Table: The columns represent the temperature rating of the wire's insulation (60°C, 75°C, 90°C). The values listed are the maximum continuous current the wire can safely carry before the insulation begins to degrade. Always cross-reference these values with the termination temperature ratings of your breakers and lugs, which dictate the maximum allowable ampacity regardless of the wire's higher insulation rating.
NEC Table 310.16 Allowable Ampacities (Copper & Aluminum, up to 3 conductors in raceway, 86°F ambient)
AWG / kcmil 60°C Copper (NM-B) 75°C Copper (THWN) 90°C Copper (THHN) 75°C Aluminum (XHHW)
14 AWG 15A* 20A* 25A* —
12 AWG 20A* 25A* 30A* —
10 AWG 30A 35A 40A —
8 AWG 40A 50A 55A 40A
6 AWG 55A 65A 75A 50A
4 AWG 70A 85A 95A 65A
3 AWG 85A 100A 115A 75A
2 AWG 95A 115A 130A 90A
1 AWG 110A 130A 145A 100A
1/0 AWG 125A 150A 170A 120A
2/0 AWG 145A 175A 195A 135A
3/0 AWG 165A 200A 225A 155A
4/0 AWG 195A 230A 260A 180A

*Note on Small Conductors: NEC 240.4(D) strictly limits overcurrent protection for 14 AWG to 15A, 12 AWG to 20A, and 10 AWG to 30A, regardless of the higher ampacities shown in the 75°C and 90°C columns. You cannot put a 12 AWG wire on a 25A breaker, even if the insulation is rated for 30A.

Which Temperature Column Applies to Your Installation?

The most common mistake DIYers and junior apprentices make is sizing wire using the 90°C column simply because the wire jacket (like THHN) is printed with a 90°C rating. The Cerrowire ampacity guidelines and NEC Article 110.14(C) dictate that the lowest temperature rating of any connected component dictates the column you must use for final ampacity sizing.

  • The 60°C Column: Use this column for Non-Metallic Sheathed Cable (NM-B, commonly known as Romex) and for any circuit terminating on equipment rated 100A or less where the terminal temperature rating is not explicitly marked. Most standard residential receptacles, switches, and breakers fall into this category.
  • The 75°C Column: Use this column for THWN/THHN wires terminating in equipment explicitly rated for 75°C, which is standard for most modern panelboards, disconnects, and breakers rated over 100A. It is also the standard column for sizing aluminum feeder conductors (like XHHW-2 or SER cable).
  • The 90°C Column: You may only use the 90°C column for derating calculations (adjusting for heat and bundling). The final derated ampacity must still be compared against the 60°C or 75°C termination limits, and the lowest resulting number wins.

Derating Factors: When the Chart Isn't Enough

The chart above assumes ideal conditions: an ambient temperature of 86°F (30°C) and no more than three current-carrying conductors in a single conduit. When you bundle multiple circuits together or run wire through hot attics, the wire cannot dissipate heat effectively, and you must apply derating factors found in NEC Table 310.15(C)(1).

Real-World Derating Example: You are pulling four separate 20A circuits (8 current-carrying conductors total) through a single 1-inch EMT conduit in an attic that reaches 110°F (43°C).

1. Base Ampacity: 12 AWG THHN is rated 30A at 90°C.
2. Bundling Derating: 7-9 conductors require a 70% adjustment factor. (30A × 0.70 = 21A).
3. Temperature Derating: 110°F ambient requires an 87% adjustment factor. (21A × 0.87 = 18.27A).

Because your final derated ampacity (18.27A) is less than your breaker size (20A), 12 AWG THHN is now a code violation. You must upsize to 10 AWG THHN (40A base × 0.70 × 0.87 = 24.36A) to safely protect the circuit with a 20A breaker.

Note that equipment grounding conductors and neutral conductors that only carry unbalanced return current do not count as current-carrying conductors for bundling derating. However, on multi-wire branch circuits (MWBC) or 3-phase circuits, the neutral does count.

What This Chart Cannot Tell You (Edge Cases & Code Limits)

While NEC Table 310.16 is the bible for thermal limits, it does not account for voltage drop or physical conduit constraints. Relying solely on ampacity charts can lead to underperforming or physically impossible installations.

  • Voltage Drop: The NEC recommends (via Informational Notes in Article 310.15) a maximum 3% voltage drop for branch circuits and 5% for the total feeder-plus-branch system. If you are running a 12 AWG wire to a 20A receptacle 150 feet away, the wire will not melt (it meets the ampacity chart), but the voltage drop will exceed 5%, causing motors to overheat and electronics to brown out. For long runs, always use a voltage drop calculator and upsize the wire accordingly.
  • Conduit Fill Capacity: Chapter 9, Annex C of the NEC dictates how many wires can physically fit inside a specific trade-size conduit. You might calculate that 10 AWG THHN is required for your derated 30A circuit, but if you are pulling four of them through a 1/2-inch conduit, you will exceed the 40% fill capacity limit, making the pull impossible and violating code.
  • Short-Circuit Withstand: Ampacity charts measure continuous thermal loading. They do not tell you if a wire can survive the extreme magnetic and thermal forces of a 10,000-amp short circuit before the breaker trips. Proper breaker AIC (Ampere Interrupting Capacity) ratings and matched wire sizes handle this, but it is a separate engineering calculation from standard ampacity.

Always verify your final wire and breaker selection against the specific terminal torque requirements and the manufacturer's installation instructions, which NEC 110.3(B) mandates you follow. When in doubt, especially for service entrance conductors or complex multi-family feeders, consult your local Authority Having Jurisdiction (AHJ) or a licensed electrical contractor.