If you are sizing a standard 120V/240V residential branch circuit, use the 60°C column for 14 and 12 AWG copper wire, and the 75°C column for 10 AWG and larger, per NEC 110.14(C). For feeders and high-temperature environments, you will use the 90°C column as your starting point for derating calculations. The chart below provides the exact allowable ampacities for insulated copper conductors based on the National Electrical Code (NEC).

How to Read the Copper Wire Ampacity Chart (NEC Table 310.16)

Before pulling wire, you need to understand how this chart is structured. The rows represent the American Wire Gauge (AWG) size of the copper conductor. The columns represent the maximum temperature rating of the wire’s insulation (e.g., THHN is 90°C, while older NM-B romex is 60°C). The intersection gives you the allowable ampacity—the maximum continuous current the wire can carry without degrading its insulation under standard conditions (ambient temperature of 30°C / 86°F and no more than three current-carrying conductors in a raceway).

Bookmark this section: Quick-jump links are provided for the most commonly used residential and light-commercial wire sizes.

Source Standard: NEC Table 310.16 (formerly 310.15(B)(16)) — Allowable Ampacities of Insulated Copper Conductors, Rated Up to and Including 2000 Volts, 60°C Through 90°C (140°F Through 194°F), Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth, Based on Ambient Temperature of 30°C (86°F).
AWG / kcmil 60°C (140°F)
TW, UF		 75°C (167°F)
RHW, THHW, XHHW		 90°C (194°F)
THHN, THWN-2, XHHW-2
14 AWG 15A 20A 25A
12 AWG 20A 25A 30A
10 AWG 30A 35A 40A
8 AWG 40A 50A 55A
6 AWG 55A 65A 75A
4 AWG 70A 85A 95A
3 AWG 85A 100A 115A
2 AWG 95A 115A 130A
1 AWG 110A 130A 145A
1/0 AWG 125A 150A 170A

Which Temperature Column Applies to Your Installation?

The most common mistake DIYers and junior electricians make is looking at a spool of 90°C THHN wire, reading the 90°C column, and assuming they can push 40 amps through 10 AWG wire. In almost all residential and commercial scenarios, you cannot use the 90°C column for your final breaker sizing.

NEC 110.14(C) dictates that the ampacity of a circuit is limited by the lowest temperature rating of any connected component, including lugs, breakers, and receptacles. Because standard residential breakers and receptacles are typically rated for 75°C, your circuit is capped at the 75°C column. Furthermore, for the smallest wires, NEC 240.4(D) imposes strict overcurrent protection limits regardless of the insulation temperature.

The Small Conductor Rule (NEC 240.4(D)):
Even if your terminations are rated 75°C and your wire is 90°C THHN, the NEC hard-caps the overcurrent protection for small copper wires to prevent nuisance tripping and overheating at the terminal screws:
  • 14 AWG Copper: Maximum 15A breaker (Use 60°C column value)
  • 12 AWG Copper: Maximum 20A breaker (Use 60°C column value)
  • 10 AWG Copper: Maximum 30A breaker (Use 60°C column value)
For 8 AWG and larger, you revert to the 75°C column (assuming 75°C rated equipment) for your final ampacity and breaker sizing.

For a deeper dive into termination temperature rules and how manufacturers test these ratings, refer to the NFPA 70 National Electrical Code development resources or industry analyses on EC&M’s breakdown of NEC 110.14(C).

Derating Factors: When the Base Chart Isn’t Enough

The base ampacity chart assumes ideal conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors bundled together. When you run wire through a hot attic or pull four or more circuits through a single conduit, the wires heat each other up. You must apply derating factors to prevent the insulation from melting.

The Golden Rule of Derating: You always start your derating math using the 90°C column, even if your terminations are only rated for 75°C. After you apply the derating multiplier, you compare the result to the 75°C column, and use the lower of the two numbers to select your breaker.

NEC Table 310.15(C)(1): Adjustment Factors for More Than Three Current-Carrying Conductors
Number of Current-Carrying Conductors Adjustment Factor (Percentage) Multiplier
1 – 3 100% 1.0
4 – 6 80% 0.8
7 – 9 70% 0.7
10 – 20 50% 0.5
21 – 30 45% 0.45
Worked Example: Bundled 12 AWG THHN
You are pulling four 12 AWG THHN (90°C) current-carrying conductors through a single EMT conduit for two 20A multi-wire branch circuits.
1. Base 90°C ampacity for 12 AWG = 30A.
2. Four conductors require an 80% adjustment factor (0.8).
3. Derated ampacity = 30A × 0.8 = 24A.
4. Compare 24A to the 75°C column (25A). The lower value is 24A.
5. However, because it is 12 AWG, NEC 240.4(D) overrides everything and caps the breaker at 20A. The wire is safely protected, and the installation is code-compliant.

What the Ampacity Chart Cannot Tell You

While NEC Table 310.16 is the bible for thermal limits, it is only one piece of the wire sizing puzzle. Relying on it blindly will lead to failures in three specific scenarios:

1. Voltage Drop Over Distance

Ampacity measures heat, not voltage preservation. A 12 AWG copper wire can safely carry 20 amps indefinitely, but if you run it 150 feet to a receptacle powering a 16-amp table saw, you will lose roughly 5 volts (over 4% drop). The motor will run hot, stall, and potentially burn out. As a rule of thumb, size your wire to keep voltage drop under 3% for branch circuits and 5% for the total feeder-plus-branch run. For long runs, you must upsize the wire (e.g., stepping from 12 AWG to 10 AWG or 8 AWG) purely for voltage drop, even if the breaker remains 20A.

2. Conduit Fill Limits (NEC Chapter 9)

The ampacity chart tells you how many wires you can electrically bundle before derating kicks in, but it doesn’t tell you if they will physically fit in the pipe. NEC Chapter 9, Table 1 limits conduit fill to 40% for three or more wires. Stuffing too many wires into a tight conduit makes pulling impossible, damages the insulation, and traps heat. Always cross-reference your wire count with a conduit fill calculator.

3. Short-Circuit Interrupting Capacity

Ampacity assumes normal operating loads. It does not account for the massive magnetic and thermal forces generated during a dead-short fault. If your service panel has an available fault current of 22,000 amps, but your breaker and busbar are only rated for 10,000 amps (AIC rating), the breaker may fail to clear the fault, resulting in an arc flash. Wire sizing must always be paired with properly rated overcurrent protective devices that match the utility’s available fault current at the service entrance.