Wire Ampacity Size Chart: NEC 310.16 Copper Sizing Guide

The definitive wire ampacity size chart for US residential and commercial copper wiring is governed by NEC Table 310.16 (formerly 310.15(B)(16)). If you are sizing a branch circuit or a subpanel feeder, the direct answer for standard copper wire ampacities is found in the 60°C, 75°C, and 90°C columns of this table, assuming an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway.

However, picking the right American Wire Gauge (AWG) is not just about reading the highest number on the chart. It requires understanding the "weakest link" rule of termination temperatures, applying derating factors for bundled wires, and distinguishing between conduit runs and sheathed cable assemblies. This guide provides the complete data table, the decision logic for column selection, and the exact breaker pairings to keep your installation safe and code-compliant.

How to Read the NEC 310.16 Ampacity Table

Before looking at the numbers, you must know which column applies to your specific installation. The NEC defines ampacity based on the thermal limits of the wire's insulation and the equipment it connects to.

• 60°C Column (TW, UF, NM-B): Use this column for standard non-metallic sheathed cable (Romex/NM-B) and underground feeder (UF) cable. Even if the cable jacket says 90°C, NEC 334.80 restricts NM-B ampacity to the 60°C column.
• 75°C Column (THHW, THWN, XHHW): Use this column for most individual conductors pulled through conduit. More importantly, NEC 110.14(C) dictates that standard residential breakers and lugs rated 100A or less are generally tested and rated for 75°C terminations. This is your "weakest link" baseline.
• 90°C Column (THHN, THWN-2): This column is almost never used for final breaker sizing. It is strictly used as the starting baseline for calculating derating adjustments (temperature and bundling) before you compare the final number back to the 75°C or 60°C limits.

Copper Wire Ampacity Size Chart (NEC 310.16)

The following table covers the most queried AWG sizes for copper conductors. Bookmark this section for quick reference on the bench. Source: NFPA 70 (National Electrical Code), Table 310.16.

*Note: NEC 240.4(D) strictly limits 14 AWG copper to a 15A maximum overcurrent device, regardless of the 75°C or 90°C column values. Green highlighted rows indicate the most common DIY branch circuit and feeder sizes.

Derating: When Your Base Ampacity Drops

The wire ampacity size chart above assumes optimal conditions: an ambient temperature of 86°F (30°C) and a maximum of three current-carrying conductors in a single conduit. When you exceed these parameters, you must apply derating multipliers to the 90°C column, then compare the result to the 75°C or 60°C limits.

Adjustment Factors for Bundled Conductors (NEC 310.15(C)(1))

• 4 to 6 conductors: Multiply 90°C ampacity by 80%
• 7 to 9 conductors: Multiply 90°C ampacity by 70%
• 10 to 20 conductors: Multiply 90°C ampacity by 50%

Worked Example: You are pulling four 12 AWG THHN current-carrying conductors through a single EMT conduit for a multi-wire branch circuit.
1. Base 90°C ampacity for 12 AWG = 30A.
2. Apply 80% derating for 4 conductors: 30A × 0.80 = 24A.
3. Compare to 75°C termination limit (25A). 24A is less than 25A, so your final allowable ampacity is 24A.
4. Result: You can still safely use a standard 20A breaker, as 24A > 20A.

Decision Tree: Sizing Your Wire and Breaker

Use this if-then decision path to terminate your sizing process with a concrete pick. Always de-energize the panel, verify dead with a tested meter, and torque lugs to manufacturer specifications before energizing.

What the Ampacity Chart Cannot Tell You

Relying solely on the wire ampacity size chart will result in a safe thermal installation, but it ignores two critical physical realities of electrical design: voltage drop and physical lug constraints.

1. Voltage Drop Limits

Ampacity measures how much heat a wire can safely dissipate before the insulation melts. It does not measure how much voltage is lost over distance. Per industry voltage drop guidelines, you should limit voltage drop to 3% on branch circuits and 5% overall. If you are running a 12 AWG wire 150 feet to a 15A workshop receptacle, the wire won't overheat (ampacity is fine), but the voltage at the outlet will drop below 110V, causing motors to stall and overheat. Rule of thumb: If your one-way wire run exceeds 100 feet, bump up one AWG size to compensate for resistance, regardless of what the ampacity chart dictates.

2. Physical Lug Limitations

A 100A breaker might have an ampacity rating that accepts 3 AWG wire, but the physical lug might only be rated to accept a maximum of #2 AWG or a minimum of #8 AWG. If you strip a 4 AWG wire and try to jam it into a lug designed for a maximum of #6, you will damage the conductor strands or fail to achieve the required torque. Always check the breaker manufacturer's datasheet (e.g., Square D, Eaton, Siemens) for the specific "Wire Range" printed on the breaker label.

3. Local AHJ Overrides

The National Electrical Code (NFPA 70) is a baseline standard. Your local Authority Having Jurisdiction (AHJ) or municipal inspector may have local amendments that require larger minimum wire sizes (e.g., banning 14 AWG entirely in favor of 12 AWG minimums for all branch circuits). Always defer to your local inspector's requirements over general chart guidance.