The Ultimate Wire Size Gauge Chart: NEC Ampacity & AWG Sizing Guide

If you are wiring a standard residential 120V/240V branch circuit using copper wire, here is your immediate answer: use 14 AWG for 15A breakers, 12 AWG for 20A breakers, 10 AWG for 30A breakers, 8 AWG for 40A breakers, and 6 AWG for 50A breakers. These are the hard limits dictated by NEC Article 240.4(D) for small conductors, regardless of what the higher temperature columns in the ampacity tables might suggest.

Sizing wire correctly is not just about preventing melted insulation; it is about ensuring the breaker trips before the terminals at your outlets or panel overheat. This guide provides a decision-forward reference based on the NFPA 70 National Electrical Code (NEC), specifically Article 310.16, to help you pick the exact wire gauge and breaker for your next project.

How to Read the NEC Wire Size Gauge Chart

The most common mistake DIYers make when looking up a wire size gauge chart is reading the wrong temperature column. The NEC ampacity tables are divided into temperature ratings (60°C, 75°C, and 90°C) based on the insulation type and the terminal ratings of the equipment you are connecting to.

• The 60°C Column (NM-B / Romex): If you are using standard non-metallic sheathed cable (NM-B, commonly called Romex), NEC Article 334.80 strictly limits its ampacity to the 60°C column, even though the physical insulation might be rated for 90°C. This is because standard residential receptacles and switches are typically only rated for 60°C terminations.
• The 75°C / 90°C Columns (THHN in Conduit): If you are pulling individual THHN/THWN-2 wires through a conduit, you can use the 75°C or 90°C columns for derating purposes, provided the equipment terminals are explicitly rated for 75°C (common in modern subpanels and large breakers).

The Master AWG Ampacity Table (Copper, THHN/NM-B)

Below is the complete data table derived from NEC Table 310.16 for copper conductors. Use the quick-jump links to find the most queried residential sizes.

Quick Jump: 14 AWG (15A) | 12 AWG (20A) | 10 AWG (30A) | 8 AWG (40A) | 6 AWG (50A)

Derating Factors: When the Base Chart Fails

The ampacity values in the table above assume an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a single raceway or conduit. When real-world conditions change, you must apply derating factors to the 90°C column (even if your final termination is limited to 75°C or 60°C).

1. Ambient Temperature Derating (NEC 310.15(B)(1))

If you are running wire through an attic in the summer where temperatures reach 120°F (49°C), the wire cannot dissipate heat as efficiently. You must multiply the base 90°C ampacity by a correction factor. For 12 AWG THHN (base 30A at 90°C) in a 120°F attic, the correction factor is 0.71. 30A × 0.71 = 21.3A. You would then compare this derated value to the terminal temperature limit to find your final allowable ampacity.

2. Conduit Fill Derating (NEC 310.15(C)(1))

When you bundle more than three current-carrying conductors in a conduit, they heat each other up. If you pull four to six conductors, you must multiply the 90°C base ampacity by 80%. If you pull seven to nine conductors, the multiplier drops to 70%.

Worked Example: You are pulling 4 current-carrying 10 AWG THHN wires through an EMT conduit for a multi-wire branch circuit. The 90°C base is 40A. Applying the 80% derating factor gives you 40A × 0.80 = 32A. Because 32A is still higher than the 60°C column limit (30A), you are legally permitted to protect this circuit with a standard 30A breaker.

Decision Tree: Picking Your Exact Wire and Breaker

Use this decision matrix to lock in your final materials list. Follow the path that matches your installation method.

What This Chart Cannot Tell You

While the NEC wire size gauge chart is the legal baseline for preventing fires, it does not account for performance degradation over distance or alternative materials. Before finalizing your purchase, verify these three edge cases:

1. Voltage Drop: The NEC ampacity tables assume the wire is short enough that resistance won't cause a meaningful voltage drop. For runs exceeding 100 feet, a 12 AWG wire carrying 15A might drop the voltage at the receptacle below 114V (a 5% drop on a 120V circuit), causing motors to overheat or electronics to brownout. Rule of thumb: bump up one wire size (e.g., from 12 AWG to 10 AWG) for every 100 feet of one-way run.
2. Aluminum Conductors: The table above is strictly for copper. If you are using aluminum or copper-clad aluminum (common for large feeders like 2/0 AWG service entrance wire due to cost), you must use the aluminum-specific columns in NEC 310.16, which require a wire roughly two AWG sizes larger than copper for the same ampacity.
3. Short Circuit Available Current (AIC): The chart tells you what the wire can carry continuously, but it doesn't tell you if the wire will survive a dead short before the breaker trips. In commercial settings or near utility transformers, you must ensure your breaker's AIC rating (e.g., 10kA or 22kA) matches the available fault current.

The Default Recommendation: If you are wiring a standard home branch circuit and lack the specific terminal temperature ratings of your devices, always default to the 60°C column and strictly obey the NEC 240.4(D) breaker caps. Buy 12 AWG NM-B for 20A circuits and 14 AWG NM-B for 15A circuits. It is the safest, most code-compliant baseline that will pass inspection in any jurisdiction.