Wire Size to Amp Load Chart: NEC 310.16 Copper & Aluminum Sizing Guide

The correct wire size for any electrical circuit depends strictly on the amp load, conductor material (copper vs. aluminum), and the temperature rating of your terminations. For a standard 15A lighting circuit, use 14 AWG copper. For a 20A receptacle circuit, use 12 AWG copper. For a 100A subpanel feeder, use 3 AWG copper or 1/0 AWG aluminum. For a 200A residential service, use 2/0 AWG copper or 4/0 AWG aluminum. These baseline values assume a standard 30°C (86°F) ambient temperature and no more than three current-carrying conductors in a raceway.

The Master Wire Size to Amp Load Chart (NEC Table 310.16)

This chart is derived directly from NEC Table 310.16 (formerly 310.15(B)(16)). It lists the allowable ampacities for insulated conductors rated up to 2000 volts.

How to read this table: The columns are divided by conductor material (Copper vs. Aluminum) and then by insulation temperature rating (60°C, 75°C, and 90°C). The 60°C column generally applies to older homes and smaller breakers (100A and below). The 75°C column applies to most modern residential breakers and panels. The 90°C column is almost exclusively used as the starting baseline for calculating derating factors, not for final termination sizing. Bookmark-friendly quick-jump rows for the most common residential loads (15A, 20A, 50A, 100A, 200A) are highlighted below.

* 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 or 90°C columns.

Which Temperature Column Applies to Your Installation?

The most common mistake DIYers make is sizing wire using the 90°C column because it allows for smaller, cheaper wire. In almost all residential scenarios, you cannot use the 90°C column for your final wire size.

The rule of thumb dictated by NEC 110.14(C) is based on the equipment terminations (the lugs on your breaker or panel):

• 100 Amps and Below: You must use the 60°C column, unless the equipment is explicitly marked for 75°C. Most modern breakers are marked 75°C, but standard NM-B (Romex) cable is rated 90°C for derating but its ampacity is legally capped at the 60°C column for termination purposes.
• Over 100 Amps: You must use the 75°C column, unless the equipment is marked otherwise. This applies to your main service panel lugs and large subpanel feeders.

Derating Factors: When the Base Chart Isn't Enough

The chart above assumes two perfect conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a single conduit. If your installation deviates from this, you must apply derating factors to the 90°C column base value to find your true allowable ampacity.

1. Bundling (More than 3 conductors): When you pull multiple circuits through a single conduit, the wires heat each other up. According to NEC 310.15(C)(1), if you have 4 to 6 current-carrying conductors, you must multiply the 90°C base ampacity by 80%. If you have 7 to 9 conductors, multiply by 70%.

2. Ambient Temperature: If your conduit runs through a hot attic (e.g., 45°C / 113°F), you must apply a temperature correction factor from NEC Table 310.15(B)(1). At 45°C, the correction factor for 90°C wire is 0.87.

Worked Numeric Example:
You are running a 30A circuit using 10 AWG THHN (90°C base = 40A) in a conduit with 5 total current-carrying conductors in a 30°C attic.
1. Base ampacity (90°C column): 40A
2. Bundling derating (5 conductors = 80%): 40A × 0.80 = 32A.
3. Final allowable ampacity: 32A. Because 32A is greater than your 30A load and breaker size, 10 AWG THHN is legally sufficient for this run.

Decision Path: Sizing Your Wire in 4 Steps

Use this decision tree to lock in your exact wire size and material for common residential and light commercial projects. Follow the path down until you hit a concrete pick.

What This Chart Cannot Tell You

While NEC Table 310.16 is the law of the land for thermal limits and fire prevention, it is not a complete design tool. Relying on it blindly will lead to two major failures on the jobsite:

1. Voltage Drop: The NEC ampacity chart does not account for distance. If you run 10 AWG wire to a 30A load that is 250 feet away, the wire won't catch fire, but the voltage at the receptacle will drop below 110V, causing motors to overheat and electronics to brown out. NEC 210.19(A) recommends keeping branch circuit voltage drop under 3%. For long runs, use the Southwire Voltage Drop Calculator to size up your wire, often jumping 2 or 3 AWG sizes larger than the thermal chart requires.
2. Conduit Fill Capacity: Just because six 12 AWG wires fit the thermal derating requirements doesn't mean they will physically fit inside a 1/2-inch EMT conduit. You must cross-reference NEC Chapter 9, Table 1 to ensure your conduit is not overfilled, which prevents heat dissipation and makes pulling the wire impossible without damaging the insulation.
3. Short-Circuit Let-Through Current: The chart tells you what the wire can handle continuously. It does not tell you if the wire can survive the massive magnetic and thermal forces of a 10,000A short circuit before the breaker trips. For high-available fault current environments (like commercial services near the utility transformer), equipment AIC (Ampere Interrupting Capacity) ratings and wire bracing become the limiting factors.

Always start with the wire size to amp load chart to satisfy the thermal and fire safety requirements of the NEC, then verify your choice against voltage drop and physical conduit fill. When in doubt, stepping up one AWG size costs slightly more in copper but buys massive margins in safety and performance.