Wire Gauge Ampacity Chart: NEC Table 310.16 Reference Guide

The ampacity of a copper wire depends strictly on its AWG size, insulation temperature rating, and installation method. For standard residential branch circuits using copper wire, the baseline ampacities are: 14 AWG is 15A, 12 AWG is 20A, 10 AWG is 30A, 8 AWG is 40A, and 6 AWG is 55A. These values assume an ambient temperature of 30°C (86°F), no more than three current-carrying conductors in a raceway, and termination at equipment rated for 60°C, per NEC Article 110.14(C). Sizing your wire and breaker correctly prevents insulation meltdown, voltage drop, and electrical fires.

Copper Wire Gauge Ampacity Chart (NEC Table 310.16)

This chart provides the allowable ampacities for insulated copper conductors rated up to 2000 volts. Before using this table, you must understand how to read the temperature columns, as picking the wrong column is the most common mistake DIYers make.

How to Read the Temperature Columns

• 60°C Column (TW, UF): This is the default column for almost all residential branch circuits rated 100A or less. Even if you use 90°C wire, NEC 110.14(C) requires you to use the 60°C column because standard breakers, receptacles, and switches are typically only tested and rated for 60°C terminations. Furthermore, if you are using NM-B cable (commonly known as Romex), NEC 334.80 legally restricts you to the 60°C column regardless of the internal wire's 90°C rating.
• 75°C Column (THHW, THWN): Used for circuits over 100A, or when terminating into industrial equipment and heavy-duty panels explicitly marked for 75°C terminations.
• 90°C Column (THHN, THWN-2, XHHW): You can only use this column as your starting point for derating calculations (adjusting for heat or bundling). You cannot use the 90°C ampacity as your final breaker size for standard terminations.

How Derating and Installation Conditions Modify Base Ampacity

The values in the chart above assume ideal conditions: an ambient temperature of 30°C (86°F) and a maximum of three current-carrying conductors in a single conduit. When your installation deviates from these baselines, you must apply adjustment factors found in NEC Article 310.15. Derating protects the wire from trapping its own heat, which degrades the insulation over time.

1. Bundling (More Than Three Current-Carrying Conductors)

When you pull four to six current-carrying conductors through a single conduit, the wires heat each other up. You must multiply the base ampacity by 80%. For seven to nine conductors, the factor drops to 70%.

Worked Example: You are running a 240V circuit with a neutral and a ground wire in a conduit (four current-carrying conductors: two hots, one neutral). You are using 8 AWG THHN copper wire.

• Base ampacity from the 90°C column for 8 AWG THHN = 55A.
• Derating factor for 4-6 conductors = 80% (0.80).
• Calculation: 55A × 0.80 = 44A.
• Final Step: You must size the breaker to protect the wire at 44A. The next standard breaker size down is 40A. Therefore, you can use a 40A breaker. (Note: You cannot round up to 45A or 50A because 44A does not correspond to a standard breaker size in NEC 240.6, and the termination limits still apply).

2. Ambient Temperature Corrections

If your conduit runs through an attic in a hot climate or near a heat source where the ambient temperature exceeds 30°C, the wire's ability to dissipate heat drops. If the attic reaches 45°C (113°F), you must multiply the 90°C column value by 0.87 for THHN wire. Always use the 90°C column for temperature correction math, then verify the final derated number against your termination limits.

What the Ampacity Table Cannot Tell You (and When to Call an Electrician)

While NEC Table 310.16 is the definitive guide for wire gauge ampacity, it is a thermal limit chart, not a comprehensive design tool. Relying on it blindly will lead to failed inspections or poorly performing circuits in specific scenarios.

Voltage Drop on Long Runs

The ampacity table assumes the wire is short enough that resistance doesn't cause a significant voltage drop. If you are running a 120V circuit to a detached garage 150 feet away, 12 AWG wire might be thermally safe for a 20A load, but the voltage drop will exceed the recommended 3% threshold. At 150 feet, a 16A load on 12 AWG copper will drop roughly 6.4 volts (over 5%). You must upsize to 10 AWG or 8 AWG to maintain voltage stability, even though the breaker remains 20A. The National Electrical Code informational notes recommend a maximum 3% drop on branch circuits and 5% total drop from the utility feed to the furthest outlet.

Aluminum vs. Copper Conductors

The table provided above is exclusively for copper. If you are sizing aluminum wire (common for service entrance cables, feeders to subpanels, and heavy appliances like ranges), you must use the aluminum columns in NEC Table 310.16. Aluminum has higher resistance and expands/contracts differently than copper. For example, to carry 100A, you need 3 AWG copper, but you must step up to 1 AWG aluminum. Furthermore, aluminum terminations require specific anti-oxidant compounds and torque settings to prevent arcing and fires.

Service Entrance and Utility Rules

This chart applies to branch circuits and feeders on the load side of the main disconnect. Sizing service entrance conductors (the wires from the utility meter to your main panel) involves utility company specifications, local AHJ (Authority Having Jurisdiction) amendments, and load calculations per NEC Article 220. Modifying or upgrading your service entrance is not a DIY task; it requires a licensed electrician and utility coordination to avoid lethal arc flash hazards and grid-tie violations.

For the most current updates to wiring standards and safety practices, always refer directly to the National Fire Protection Association (NFPA) NEC resources. Understanding how to read the wire gauge ampacity chart—and knowing its limitations—is the difference between a safe, code-compliant installation and a hidden fire hazard behind your drywall.