Wiring Thickness Chart: AWG to Ampacity Reference Guide

When sizing conductors for a new circuit, the wiring thickness chart (formally NEC Table 310.16) is your single source of truth for matching American Wire Gauge (AWG) to maximum allowable ampacity. For standard 15A and 20A residential branch circuits, you will use 14 AWG and 12 AWG copper wire, respectively, referencing the 60°C column. This guide provides the exact copper ampacity values, explains which temperature column applies to your specific installation, and covers the derating math that modifies these base numbers in real-world conditions.

The NEC 310.16 Wiring Thickness Chart (Copper)

Before using the table below, you need to understand how to read it. The chart is divided into temperature columns based on the insulation type of your wire. The 60°C column applies to older or basic insulation like TW and UF-B. The 75°C column covers THHW and THWN. The 90°C column applies to modern, high-heat thermoplastics like THHN and XHHW-2, which are the standard for individual conductors pulled through conduit today. The values below assume an ambient temperature of 86°F (30°C) and no more than three current-carrying conductors bundled together.

Table 310.16 Allowable Ampacities of Insulated Copper Conductors (Up to 3 Current-Carrying)

AWG / kcmil	60°C (140°F) TW, UF	75°C (167°F) THHW, THWN	90°C (194°F) THHN, XHHW	Typical Max Breaker*
14	15A	20A	25A	15A
12	20A	25A	30A	20A
10	30A	35A	40A	30A
8	40A	50A	55A	40A
6	55A	65A	75A	60A
4	70A	85A	95A	80A
3	85A	100A	110A	100A
2	95A	115A	130A	115A
1	110A	130A	145A	130A
1/0	125A	150A	170A	150A
2/0	145A	175A	195A	175A
3/0	165A	200A	225A	200A
4/0	195A	230A	260A	230A

*Note: Breaker sizes are limited by NEC 240.4(D) for small conductors (14, 12, and 10 AWG) and standard breaker availability for larger sizes. Source: NFPA 70 National Electrical Code.

Which Temperature Column Applies to Your Installation?

The most common mistake DIYers make is looking at the 90°C column because they bought THHN wire, and assuming they can push 40A through 8 AWG. You cannot. The NEC enforces a 'weakest link' rule under Article 110.14(C), which dictates that your ampacity is limited by the lowest temperature rating of any component in the circuit—including the wire, the breaker terminals, and the receptacles.

For circuits 100A or less (14 AWG through 1 AWG):
You must use the 60°C column for your final ampacity sizing. Even though your THHN wire is rated for 90°C, standard residential breakers and receptacles are typically tested and rated for 60°C or 75°C terminations. The NEC mandates the 60°C column for these smaller conductor sizes to protect the termination points from melting. Therefore, 12 AWG is strictly limited to 20A, and 10 AWG is strictly limited to 30A, regardless of the insulation jacket.

For circuits over 100A (1/0 AWG and larger):
You are permitted to use the 75°C column. This is why a 4/0 AWG copper feeder can safely be placed on a 200A main breaker (using the 75°C value of 230A, which is greater than 200A).

When do you use the 90°C column?
The 90°C column is almost exclusively used as your starting point for derating calculations. You use the 90°C ampacity to apply correction factors for high ambient temperatures or conduit bundling, but the final derated ampacity must still be equal to or greater than the load, and you must still respect the 60°C/75°C termination limits. For a deeper look at insulation types, refer to the Southwire Ampacity Tool and insulation specifications.

Derating and What the Chart Cannot Tell You

The wiring thickness chart provides ideal-world base values. In practice, two major factors modify these numbers: derating and voltage drop.

How Derating Modifies the Base Value

When you pull more than three current-carrying conductors through a single raceway (conduit), the wires heat each other up. NEC Table 310.15(C)(1) requires you to multiply the base ampacity by a derating factor. Furthermore, if your attic or rooftop ambient temperature exceeds 86°F (30°C), you must apply a temperature correction factor from Table 310.15(B)(1).

Worked Example: You are pulling four current-carrying 12 AWG THHN conductors through a conduit in a standard 86°F environment to feed two separate 20A circuits.
1. Start with the 90°C column base value for 12 AWG: 30A.
2. Apply the bundling derating factor for 4-6 conductors: 80%.
3. Calculate derated ampacity: 30A × 0.80 = 24A.
Because 24A is still greater than your 20A breaker size, 12 AWG THHN is perfectly legal and safe for this conduit run. If you had pulled 9 conductors (derating factor 70%), the math would be 30A × 0.70 = 21A. Still acceptable, but you are getting close to the limit.

What the Table Cannot Tell You: Voltage Drop

The NEC 310.16 chart assumes a short, ideal run. It tells you what size wire will prevent the insulation from melting, but it does not tell you if the wire is thick enough to deliver adequate voltage to the load at the end of a long run. As wire length increases, resistance increases, causing voltage to drop.

The NEC recommends (in Informational Note to 210.19) a maximum 3% voltage drop for branch circuits and 5% for the total feeder + branch combined. If you are running a 120V, 20A circuit to a detached workshop 150 feet away using 12 AWG copper, the wire will not overheat, but the voltage drop will be roughly 6.4% (7.7 volts). Your 120V tools will only see 112.3V, which can cause motors to overheat and trip their internal thermal protectors. To fix this, you must upsize to 8 AWG or 6 AWG purely for voltage drop mitigation, even though the ampacity chart says 12 AWG is sufficient for 20A. Always calculate voltage drop for any run exceeding 50 feet. For comprehensive voltage drop formulas and code interpretations, consult EC&M's National Electrical Code resources.