National Electrical Code Wire Gauge Chart: Sizing Guide & Ampacity Table

The definitive source for wire sizing in the United States is the National Electrical Code wire gauge chart, officially published as NEC Table 310.16 (formerly 310.15(B)(16)). If you are sizing conductors for a branch circuit, feeder, or service entrance, this table dictates the maximum allowable ampacity based on wire material, gauge, and insulation temperature rating. However, reading the chart incorrectly is the most common cause of failed electrical inspections and overheated terminals.

How to Read the NEC Wire Gauge Chart

Before looking at the numbers, you must understand the temperature columns. Table 310.16 is divided into 60°C, 75°C, and 90°C columns for both copper and aluminum. The most critical rule in residential wiring is found in NEC 110.14(C): the ampacity of your wire is limited by the temperature rating of the weakest link in the circuit, which is almost always the breaker or receptacle terminal.

Even if you pull 90°C THHN wire through your conduit, standard residential breakers and receptacles rated 100A or less are typically only tested and listed for 60°C or 75°C terminations. Therefore, you must use the 60°C column to determine your final breaker size for most standard branch circuits. The 90°C column is strictly used as your starting baseline for calculating derating factors (explained below) or for equipment explicitly marked for 75°C/90°C terminations, such as large industrial lug panels.

Assumptions for the table below: Ambient temperature of 30°C (86°F), not more than three current-carrying conductors in a single raceway, and standard AC wiring.

The Master Ampacity Table (NEC 310.16)

The following spec-sheet-table provides the allowable ampacities for insulated conductors rated up to 2000 volts. This data is sourced directly from the National Fire Protection Association (NFPA) 70 National Electrical Code.

Decision Path: Picking Your Exact Wire Size

Use this decision-tree-table to terminate your sizing process with a concrete material and gauge pick. These recommendations assume standard residential environments, NM-B (Romex) for indoor branch circuits, and THHN/THWN-2 for conduit runs.

Derating and Edge Cases: What the Chart Doesn't Tell You

NEC Table 310.16 is a baseline, not a universal guarantee. The chart assumes ideal conditions. When real-world physics and installation geometry intervene, you must apply adjustment factors. Here is what the base chart cannot tell you, and how to handle it.

1. Conductor Bundling and Derating (NEC 310.15(C)(1))

If you pull more than three current-carrying conductors through a single conduit, the wires heat each other up, and you must derate their ampacity. You apply the multiplier to the 90°C column (for THHN), then compare that result to your termination limit.

• 4 to 6 conductors: Multiply base ampacity by 80%.
• 7 to 9 conductors: Multiply base ampacity by 70%.
• 10 to 20 conductors: Multiply base ampacity by 50%.

Example: You pull four 12 AWG THHN wires (two circuits sharing a conduit). The 90°C base ampacity for 12 AWG is 30A. Multiply 30A by 0.80 (80%) = 24A. Since 24A is still greater than your 20A breaker termination limit, 12 AWG is legally sufficient. However, if you pulled 10 wires (50% factor), 30A x 0.50 = 15A. You would be forced to upsize to 10 AWG wire to legally protect a 20A circuit.

Pro-Tip: Equipment grounding conductors (bare copper or green) do not count toward the current-carrying conductor total for derating purposes. Furthermore, a neutral conductor that only carries the unbalanced load from the same phases (like a standard 120/240V split-phase circuit) does not count as a current-carrying conductor per NEC 310.15(E).

2. Voltage Drop on Long Runs

Table 310.16 dictates thermal limits (when the wire melts or insulation fails), but it ignores voltage drop. If you run a 50A EV charger circuit 150 feet from your panel using 6 AWG copper, the wire will not overheat, but the voltage at the charger will drop below 230V, causing the charger to fault or charge slower.

While the NEC only recommends a maximum 3% voltage drop for branch circuits and 5% total for feeder+branch in Informational Note 310.15(B), equipment manufacturers mandate it. Per NEC 110.3(B), you must follow manufacturer instructions. If your EV charger manual requires less than 3% voltage drop, it becomes a code violation to ignore it. For any 240V run exceeding 100 feet, use a voltage drop calculator and expect to upsize your wire by at least one or two AWG steps.

3. Conduit Fill Capacity (Chapter 9, Table 1)

Just because the ampacity chart allows you to stuff nine 10 AWG THHN wires into a 1/2-inch EMT conduit doesn't mean it's legal or physically possible. NEC Chapter 9 limits conduit fill to 40% for three or more wires to prevent jamming and insulation damage during the pull. Always cross-reference your wire count and insulation type with a conduit fill calculator before purchasing your raceway.

When sizing your next circuit, default strictly to the 60°C column for termination limits on standard residential gear, apply derating math from the 90°C column when bundling wires in conduit, and always upsize for voltage drop on runs exceeding 100 feet. For final authority on localized amendments, always verify your specific installation plan with your local Authority Having Jurisdiction (AHJ) or a licensed master electrician, as referenced by OSHA electrical safety guidelines.