The Ultimate Amps Wire Size Chart: NEC 310.16 Copper Ampacity Guide

When sizing conductors for residential and light commercial branch circuits, the most common copper wire sizes are 14 AWG (15A), 12 AWG (20A), 10 AWG (30A), 8 AWG (40A), and 6 AWG (55A/50A breaker). However, simply matching a breaker size to a wire gauge without considering termination temperature ratings and conduit fill can lead to overheated terminals or tripped breakers.

This amps wire size chart is based on NEC Table 310.16 (formerly 310.15(B)(16)) for copper conductors. To read this table correctly, locate your wire gauge (AWG) in the left column, then read across to the temperature column that matches the lowest temperature rating of any connected termination, device, or breaker in your circuit.

The NEC Amps Wire Size Chart (Copper, THHN/THWN-2)

The following data assumes copper conductors with THHN/THWN-2 insulation in an ambient temperature of 30°C (86°F) with no more than three current-carrying conductors in a raceway. Note: NEC-style guidance is provided here; your local AHJ has final authority.

*Asterisk denotes sizes subject to NEC 240.4(D) small conductor overcurrent protection limits (14 AWG = 15A max, 12 AWG = 20A max, 10 AWG = 30A max), regardless of the higher ampacities listed in the 75°C or 90°C columns.

Which Column Applies to Your Installation?

The most common mistake DIYers make is using the 90°C column because THHN wire is rated for 90°C. Under NEC 110.14(C), you must use the "weakest link" rule. The allowable ampacity is determined by the lowest temperature rating of any termination, connector, or device in the circuit.

• Use the 60°C Column: For 14, 12, and 10 AWG circuits (due to NEC 240.4(D) hard limits), or when connecting to older devices, NM-B (Romex) cable assemblies, or equipment explicitly marked 60°C.
• Use the 75°C Column: For most modern residential breakers, receptacles, and hardwired appliances rated 8 AWG and larger. Most THHN in conduit terminating at a modern panel uses this column.
• Use the 90°C Column: Almost never for final ampacity. It is strictly used as the starting point for derating calculations (explained below) before applying the termination temperature limit.

How Derating and Ambient Temperature Modify These Values

The chart above assumes perfect conditions: 30°C ambient air and no more than three current-carrying conductors in a conduit. Real-world jobsites rarely match this. When conditions change, you must derate the 90°C column value, then compare it to the termination column limit, and use the lower of the two.

Worked Example: Bundled Conductors
Imagine you are pulling four current-carrying conductors (two 120V circuits sharing a neutral) through a single conduit to a detached garage. You are using 10 AWG THHN copper.

1. Base Value: 10 AWG in the 90°C column is 45A.
2. Derating Factor: NEC Table 310.15(C)(1) dictates an 80% adjustment factor for 4-6 current-carrying conductors.
3. Calculation: 45A × 0.80 = 36A.
4. Final Limit: While 36A allows you to upsize to a 40A breaker under standard rounding rules (NEC 240.4(B)), NEC 240.4(D) strictly caps 10 AWG copper at a 30A breaker. Therefore, your maximum breaker size remains 30A. If you needed a true 40A circuit after derating, you would have to step up to 8 AWG THHN.

What This Chart Cannot Tell You

An amps wire size chart is only half the equation. It guarantees the wire won't melt under continuous load, but it ignores three critical factors:

• Voltage Drop: NEC 310.15(B) recommends keeping voltage drop under 3% for branch circuits. If you are running a 50A circuit to a detached workshop 150 feet away, 6 AWG copper will suffer a ~4.5% voltage drop under full load. You must upsize to 4 AWG or 3 AWG to maintain equipment efficiency, even though 6 AWG is legally allowed by the ampacity chart.
• Aluminum vs. Copper: This chart is exclusively for copper. Aluminum conductors (like SER or XHHW-2 used for service entrances) have lower ampacity. For example, a 100A aluminum feeder requires 1/0 AWG, not 3 AWG.
• Short-Circuit Let-Through Current: Ampacity charts deal with thermal heating over time. They do not account for the magnetic and thermal stresses of a dead-short fault, which is why proper breaker interrupting ratings (AIC) and equipment bracing are required.

Frequently Asked Questions

What size wire do I need for a 50 amp breaker?

For a standard 50A circuit (like an EV charger or welder outlet), you need 6 AWG copper wire if your terminations are rated 75°C. If you are using NM-B (Romex) cable, which is limited to the 60°C column, 6 AWG is only rated for 55A, which is acceptable for a 50A breaker. However, if the run exceeds 50 feet, you should upsize to 4 AWG copper to mitigate voltage drop.

Can I use 10 AWG wire on a 40 amp breaker?

No. While the 75°C and 90°C columns in the amps wire size chart show 10 AWG copper handling 40A and 45A respectively, NEC 240.4(D) explicitly limits the overcurrent protection for 10 AWG copper to a maximum of 30 amps. You must use a 30A breaker or upsize your wire to 8 AWG copper to use a 40A breaker.

Why does the 90°C column exist if I have to use the 75°C column?

The 90°C column exists primarily for derating calculations. Because THHN wire can physically withstand 90°C, the NEC allows you to use the higher 90°C ampacity as your baseline when applying adjustment factors for conduit bundling or high ambient temperatures. After derating, you simply verify that the final calculated ampacity does not exceed the 75°C (or 60°C) termination limit.

Does this amps wire size chart apply to aluminum wire?

No. Aluminum has a higher electrical resistance than copper, meaning it generates more heat at the same current. If you are using aluminum wire (common for 100A+ subpanel feeders due to cost), you must consult the aluminum section of NEC Table 310.16. As a rule of thumb, aluminum wire typically needs to be two AWG sizes larger than copper for the same ampacity (e.g., 2 AWG aluminum instead of 4 AWG copper for an 85A load).