The Complete AWG Cable Table: Ampacity, Sizing, and NEC Derating

The AWG cable table—formally known as NEC Table 310.16 (formerly 310.15(B)(16))—dictates the maximum allowable ampacity for electrical conductors based on wire gauge, material, and insulation temperature rating. For standard 120V/240V residential branch circuits under 100A, you must use the 60°C column. For feeders and terminations rated 75°C, use the 75°C column. The 90°C column is strictly a starting point for derating calculations.

How to Read the AWG Cable Table (and Which Column Applies)

Before pulling wire, you need to understand how the National Electrical Code (NEC) structures ampacity charts. The table is divided by conductor material (Copper vs. Aluminum) and then by temperature rating (60°C, 75°C, 90°C).

Which column applies to your installation? This is governed by NEC 110.14(C). For branch circuits rated 100A or less, or utilizing 14 AWG through 1 AWG wire, you must size the overcurrent device based on the 60°C column. This is because standard residential receptacles, switches, and breakers are typically only tested and rated for 60°C terminations. Even if you pull 90°C THHN wire, the breaker and outlet limit your safe ampacity to the 60°C value.

For circuits over 100A, or wire sizes 1/0 AWG and larger, you can use the 75°C column, provided your terminations (like a main panel lug) are explicitly rated for 75°C. The 90°C column is almost never used for final breaker sizing; it exists solely to give you a higher baseline number before applying ambient temperature or conduit fill derating factors.

The Master AWG Cable Table (NEC Table 310.16)

The following data is sourced directly from the NFPA 70 National Electrical Code, Table 310.16. It assumes an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway.

Derating: When the Base Ampacity Drops

The table above assumes ideal conditions: exactly three current-carrying conductors in a conduit and an ambient temperature under 86°F (30°C). When you violate either condition, NEC Article 310.15 requires you to derate the wire's ampacity.

Conduit Fill Derating (NEC 310.15(C)(1)): Heat builds up when multiple wires are bundled. If you pull two separate 120V circuits through the same PVC conduit, you have four current-carrying conductors (two hots, two neutrals). You must apply an 80% adjustment factor.

Worked Example: You are running two 20A circuits in one conduit using 12 AWG THHN.

• Base ampacity from the 90°C column: 30A.
• Adjustment factor for 4-6 conductors: 80%.
• Derated ampacity: 30A × 0.80 = 24A.
• Result: 24A is greater than the 20A breaker, so 12 AWG THHN is legally compliant.

Ambient Temperature Derating: If your conduit runs across a hot attic or along a sun-baked exterior wall where temperatures regularly exceed 86°F, you must multiply the 90°C base ampacity by the temperature correction factors found in NEC Table 310.15(B)(1). For example, in a 110°F attic, the correction factor is 0.87. A 10 AWG THHN wire (40A base) drops to 34.8A.

What the AWG Table Cannot Tell You (Voltage Drop)

The most common mistake DIYers make is treating the AWG cable table as the final word on wire sizing. The table only guarantees the wire won't melt or start a fire. It completely ignores voltage drop over distance.

According to Southwire's engineering guidelines and NEC informational notes, voltage drop should not exceed 3% for branch circuits and 5% total for feeder plus branch combined. If you run a 50A hot tub circuit 150 feet from the panel using 6 AWG copper (rated 55A at 60°C), the wire won't overheat, but the voltage at the tub will drop by roughly 7.4V (over 3% of 240V). The tub's control board may fault or the heater will run inefficiently.

To fix this, you must upsize the wire purely for voltage drop, not ampacity. Moving from 6 AWG to 4 AWG copper drops the voltage loss to roughly 4.6V (1.9%), solving the problem while maintaining a massive safety margin for heat.

Decision Path: Sizing Your Next Circuit

Use this decision tree to terminate your wire-sizing process with a concrete, code-compliant pick. Do not guess; follow the logic path.