The Complete Wiring Gauge Chart: NEC Ampacity & Breaker Sizing

When sizing conductors for residential and commercial branch circuits or feeders, the direct answer for standard copper wire ampacity is governed by NEC Table 310.16. For the most common general-purpose installations: 14 AWG is rated for 15 amps, 12 AWG for 20 amps, 10 AWG for 30 amps, 8 AWG for 40 amps, and 6 AWG for 55 amps (commonly protected at 60 amps when terminations are rated 75°C). However, simply matching a breaker to a wire size without checking the temperature column and installation conditions is the most frequent cause of failed electrical inspections and overheated terminals.

This reference guide provides the exact ampacity values for copper conductors, explains how to read the temperature columns, and details the derating factors that force you to upsize your wire.

The Master Wiring Gauge Chart (NEC Table 310.16)

The table below outlines the allowable ampacities for insulated copper conductors rated up to 2000 volts. This data is sourced directly from the National Fire Protection Association (NFPA) NEC Table 310.16 (formerly 310.15(B)(16)), assuming an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway or cable.

* Note on Small Conductors: Per NEC 240.4(D), the overcurrent protective device (OCPD) for 14 AWG shall not exceed 15A, 12 AWG shall not exceed 20A, and 10 AWG shall not exceed 30A, regardless of the higher ampacities listed in the 75°C or 90°C columns.

Which Temperature Column Actually Applies to Your Installation?

The most common mistake DIYers and junior apprentices make is looking at a spool of THHN wire, seeing it rated for 90°C, and using the 90°C column to size the breaker. This violates the 'weakest link' rule of the NEC.

Your allowable ampacity is dictated by the lowest temperature rating of any component in the entire circuit. This includes the wire insulation, the breaker terminals, the receptacle terminals, and any junction block lugs.

• The 60°C Column: Applies when using NM-B (Romex), UF-B, or TW cable. It also applies to any circuit rated 100 amps or less where the termination equipment (like a standard 15A or 20A duplex receptacle) is not explicitly marked with a higher temperature rating.
• The 75°C Column: Applies to most modern commercial and residential breakers rated 15A through 100A, as well as THW and XHHW wire. If you pull 6 AWG THHN (90°C insulation) through conduit to a 60A breaker with 75°C rated terminals, your final ampacity is limited to the 75°C column value: 65A.
• The 90°C Column: Almost never used for final breaker sizing in standard branch circuits. The 90°C column is primarily used as the starting baseline for calculating derating factors (explained below) before you apply the final termination temperature limit.

Derating Factors: When Base Ampacity Needs Adjustment

The wiring gauge chart above assumes ideal conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors bundled together. When you deviate from these conditions, the physical insulation cannot dissipate heat as efficiently, and you must 'derate' (reduce) the wire's ampacity.

1. Bundling (More Than 3 Current-Carrying Conductors)

When you pull multiple circuits through a single conduit, the heat from adjacent wires compounds. Per NEC Table 310.15(C)(1), if you have 4 to 6 current-carrying conductors in a raceway, you must multiply the base ampacity by 80%. For 7 to 9 conductors, the multiplier drops to 70%.

2. Ambient Temperature Corrections

If your conduit runs through an attic in a southern climate where ambient temperatures regularly exceed 30°C (86°F), you must apply temperature correction factors from the bottom of NEC Table 310.16. For example, in a 50°C (122°F) attic, the correction factor for 90°C wire is 0.82. A 10 AWG THHN wire (40A at 90°C) derates to 32.8A. You must then check this derated value against your terminal temperature column limits.

What This Chart Cannot Tell You: Voltage Drop and Material Limits

Ampacity charts only tell you the maximum current a wire can carry before the insulation melts or the conductor overheats. They do not account for voltage drop over distance, nor do they apply to aluminum conductors without a separate chart.

The Voltage Drop Blindspot

The NEC recommends (in Informational Note to 210.19(A)) that branch circuit voltage drop be limited to 3%, and the total feeder-plus-branch drop limited to 5%. If you run a 30-amp, 120-volt RV outlet 150 feet away from your panel using 10 AWG copper wire, the chart says the wire can handle 30 amps safely. But the physics of resistance disagree.

Using the standard voltage drop formula VD = (2 × L × I × R) / 1000 (where R for 10 AWG copper is ~1.24 Ω/kft):
VD = (2 × 150 × 30 × 1.24) / 1000 = 11.16 Volts.
11.16V is a 9.3% drop on a 120V circuit. Your RV's air conditioner compressor will overheat and fail prematurely due to low voltage. To fix this, you must ignore the ampacity chart's minimum and upsize to 6 AWG copper (or 4 AWG aluminum) to bring the drop under 3%, even though a 30A breaker only legally requires 10 AWG for thermal safety.

Copper vs. Aluminum

The table above is strictly for copper. If you are sizing aluminum or copper-clad aluminum (common for 2/0 and 4/0 service entrance feeders), the ampacity is significantly lower for the same physical gauge. For example, 2/0 AWG copper handles 175A at 75°C, while 2/0 AWG aluminum (like XHHW-2) only handles 135A at 75°C. Always verify you are looking at the correct material chart before purchasing feeder wire, as mistaking aluminum ampacity for copper is a direct path to a melted service lug.

For deeper guidance on specific installation conditions, refer to the Copper Development Association's building wire guidelines and always consult your local Authority Having Jurisdiction (AHJ), as local amendments can supersede baseline NEC tables.