National Electrical Code Wire Size Chart: NEC Table 310.16 Explained

The official National Electrical Code wire size chart is designated as NEC Table 310.16 (formerly 310.15(B)(16)). This table dictates the allowable ampacity—the maximum continuous current a conductor can carry without exceeding its temperature rating—for insulated conductors rated up to 2000 volts. Sizing wire correctly is not just about preventing a breaker trip; it is about preventing the insulation from melting and starting a fire inside your walls.

How to Read the NEC Table 310.16 Wire Size Chart

Before looking at the numbers, you must understand how the table is structured. The chart is divided into two primary material sections: Copper and Aluminum (or Copper-Clad Aluminum). Within each material section, there are three temperature columns: 60°C (140°F), 75°C (167°F), and 90°C (194°F).

The temperature columns correspond to the insulation type of the wire you are pulling. For example, standard NM-B (Romex) cable is rated for 60°C. THHN/THWN-2 wire pulled in conduit is rated for 90°C. However, the insulation rating of the wire does not automatically mean you get to use the highest ampacity number. The allowable ampacity is ultimately bottlenecked by the temperature rating of the terminals (lugs) on your breakers, receptacles, and disconnects, which brings us to the most misunderstood rule in residential wiring.

The Standard Copper and Aluminum Ampacity Table

Below is the complete data table for standard residential and light-commercial wire sizes, sourced directly from NFPA 70 (NEC 2023 Edition), Table 310.16. These values assume an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway or cable.

Derating, Temperature Columns, and What the Chart Hides

Reading the base numbers is only step one. To legally and safely size a circuit, you must apply NEC rules that modify these base values based on your specific installation environment.

Which Column Applies to Your Installation?

This is where most DIYers and junior apprentices fail. You might pull 90°C THHN wire in conduit and look at the 90°C column, assuming a 10 AWG wire can handle 40A. It cannot.

Under NEC 110.14(C)(1)(a), for circuits rated 100 amps or less (or wire sizes 14 AWG through 1 AWG), you must size the wire based on the 60°C column unless the equipment terminals are explicitly marked and listed for 75°C. Standard residential breakers and receptacles are typically rated 75°C, but the conservative 60°C rule applies to the final overcurrent protection sizing. You are permitted to use the 90°C column only as a starting point for calculating derating factors, but the final ampacity after derating cannot exceed the 60°C (or 75°C, if >100A) column value.

How Derating Modifies the Base Value

When you bundle more than three current-carrying conductors in a single conduit, the wires heat each other up. You must apply adjustment factors from NEC Table 310.15(C)(1).

Worked Example: You are pulling four 8 AWG THHN copper wires (two hots, one neutral, one ground; the ground does not count as current-carrying, leaving three current-carrying conductors—wait, if it's a multi-wire branch circuit or 240V with no neutral, you have two. Let's assume you are pulling four current-carrying conductors for two separate 240V circuits).

• Four current-carrying conductors require an 80% derating multiplier.
• Base 90°C ampacity for 8 AWG Copper = 55A.
• 55A × 0.80 = 44A derated ampacity.
• Because 44A exceeds the 75°C column limit (50A) and 60°C limit (40A), and standard breakers don't come in 44A, you must protect this wire with a 40A breaker.

What the Table Cannot Tell You

Table 310.16 assumes two things that are rarely true in large homes or outbuildings: an ambient temperature of exactly 30°C (86°F), and an acceptably short wire run.

1. Ambient Temperature: If your conduit runs through an attic in Arizona where temperatures hit 120°F (49°C), you must apply an ambient temperature correction factor (NEC Table 310.15(B)(1)). The base ampacity drops significantly.
2. Voltage Drop: The NEC does not strictly enforce voltage drop for most residential branch circuits, but it highly recommends a maximum 3% drop on branch circuits and 5% total from service to appliance. Table 310.16 will tell you that 12 AWG wire can safely carry 20A without melting, but if that wire is 150 feet long to a shed, your 120V tools will only see 112V, causing motors to overheat and fail. For runs over 100 feet, always calculate voltage drop and upsize your wire by at least one AWG size.

Quick-Jump Reference for Common Home Branch Circuits

Bookmark this section for fast lookups. This matrix translates the complex NEC tables into direct, actionable wire and breaker pairings for standard residential 120V/240V applications using copper wire in standard 30°C ambient conditions.

*Note: Using the 75°C column for THHN wire requires equipment explicitly rated and marked for 75°C terminations. When in doubt, or when using standard NM-B cable, default to the 60°C column requirements.

For further reading on terminal temperature limitations and inspector expectations, refer to the International Association of Electrical Inspectors (IAEI) guidelines on the 60°C rule, and always consult your local building department before pulling permits for feeder or service work.