The Ultimate Wire and Amperage Chart: AWG to Breaker Sizing Guide

For standard residential copper branch circuits, match your breaker to the 60°C column of the NEC ampacity tables: 15A requires 14 AWG, 20A requires 12 AWG, and 30A requires 10 AWG. For feeders, subpanels, and THHN wire in conduit with 75°C-rated terminations, you can step up to the 75°C column (e.g., 4 AWG for 85A, 3 AWG for 100A).

How to Read This Wire and Amperage Chart

Before pulling wire, you need to know which temperature column applies to your specific installation. The NFPA 70 National Electrical Code publishes these baseline values in Table 310.16. Here is how to decode the columns based on your materials:

• 60°C Column: Use this for standard NM-B (Romex) cable. Even though the internal THHN conductors are rated for 90°C, NEC 334.80 restricts NM-B ampacity to the 60°C column. Also use this column if your breaker or device terminations are unmarked or explicitly rated for 60°C (common in older homes and standard 15A/20A residential breakers).
• 75°C Column: Use this for THHN/THWN wires in conduit, or SER/USE-2 feeder cables, provided the terminations at both ends (breaker and lugs) are rated for 75°C. This is the standard for modern load centers, subpanel feeders, and commercial wiring.
• 90°C Column: You almost never use this column to directly size your breaker. It is reserved exclusively for calculating derating adjustments (like bundling wires in a conduit) or ambient temperature corrections before applying the final termination limits.

Derating, Bundling, and What the Chart Cannot Tell You

The wire and amperage chart above assumes ideal conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a single raceway or conduit. When you deviate from this baseline, you must apply derating factors to prevent the wires from cooking each other inside the jacket.

How Derating Modifies the Base Value

According to NEC 310.15(C)(1), if you pull 4 to 6 current-carrying conductors through a single conduit, you must multiply the wire's ampacity by 80%. For 7 to 9 conductors, the multiplier drops to 70%.

Worked Example: You are pulling four 12 AWG THHN wires (two hots, one neutral, one ground) through an EMT conduit to feed a multi-wire branch circuit. The ground does not count. You have 3 current-carrying conductors, so no derating applies. But if you add a second circuit (now 4 current-carrying conductors: two hots, two neutrals), you must derate. Using the 90°C column for THHN (30A) × 80% = 24A. Since 24A is still greater than the 20A breaker protecting the circuit, 12 AWG remains perfectly legal. If you pulled 7 current-carrying wires, 30A × 70% = 21A (still barely fine for a 20A breaker, but you are pushing thermal limits and should consider upsizing to 10 AWG).

What This Table Cannot Tell You

• Voltage Drop: This chart only addresses thermal limits (fire safety), not performance. A 12 AWG wire might safely carry 20A for 200 feet without melting, but the voltage at the far end will drop below the acceptable 3% threshold (114V on a 120V nominal circuit). For runs over 50 feet, always calculate voltage drop using Chapter 9, Table 8 resistance values.
• Ambient Temperature: If your conduit runs across a hot attic in Arizona or along a boiler wall, the 30°C baseline is invalid. You must apply the correction factors in Table 310.15(B)(1). Always cross-reference with OSHA Electrical Safety guidelines when working in commercial environments where ambient temperatures exceed residential norms.
• Small Conductor Rules: NEC 240.4(D) overrides this chart for small wires. Even if your 12 AWG wire mathematically derates to 18A, you are strictly capped at a 20A breaker for 12 AWG, 30A for 10 AWG, etc., unless specific motor or welding exceptions apply.

Wire and Amperage Chart FAQ