If you are sizing a branch circuit, feeder, or subpanel, the wire amp table you need is based on NEC Table 310.16. For standard residential copper wiring, the baseline ampacities you will reference most often are: 14 AWG = 15A, 12 AWG = 20A, 10 AWG = 30A, 8 AWG = 40A, 6 AWG = 55A, 4 AWG = 70A, and 2 AWG = 95A. However, pulling a number straight from the chart without verifying the temperature column and derating factors is the most common cause of failed inspections and melted terminal lugs.

Below is the complete reference chart for copper conductors, followed by the exact rules for applying it to your specific installation.

How to Read the Standard Wire Amp Table (NEC 310.16)

The table below is extracted from the NFPA 70 National Electrical Code (NEC), specifically Table 310.16. It lists the allowable ampacities for insulated copper conductors rated up to 2000 volts.

How to read the columns: The table is divided into three temperature columns (60°C, 75°C, and 90°C). These columns represent the thermal rating of the wire's insulation (e.g., TW is 60°C, THWN is 75°C, THHN is 90°C). The row you select depends on your wire gauge, but the column you are legally allowed to use depends on the temperature rating of the terminations (breakers, lugs, and receptacles) at both ends of the circuit.

Table 310.16: Allowable Ampacities for Copper Conductors (Source: NEC 2023, Table 310.16. Assumes ambient temperature of 30°C / 86°F and not more than three current-carrying conductors in a raceway.)
Wire Size (AWG/kcmil) 60°C Column (140°F)
TW, UF-B		 75°C Column (167°F)
THWN, RHW		 90°C Column (194°F)
THHN, XHHW-2
14 AWG (Most queried)15A20A25A
12 AWG (Most queried)20A25A30A
10 AWG (Most queried)30A35A40A
8 AWG40A50A55A
6 AWG55A65A75A
4 AWG70A85A95A
3 AWG85A100A110A
2 AWG95A115A130A
1 AWG110A130A145A
1/0 AWG125A150A170A
2/0 AWG145A175A195A
3/0 AWG165A200A225A
4/0 AWG195A230A260A
Which column applies to your installation?
According to NEC 110.14(C) Termination Rules, you must use the "weakest link" temperature rating. Almost all standard residential breakers and receptacles are rated for 75°C terminations. However, for wire sizes 14, 12, and 10 AWG, the NEC explicitly mandates that you must use the 60°C column to determine final ampacity, regardless of the wire's 90°C THHN insulation. For 8 AWG and larger, you may use the 75°C column if both terminations are rated for 75°C.

Derating Factors: When the Base Table Lies

The numbers in the wire amp table above assume two ideal conditions: an ambient air temperature of 30°C (86°F) and no more than three current-carrying conductors bundled in a single raceway or cable. If your installation deviates from either of these, you must apply derating multipliers to the base ampacity.

1. Ambient Temperature Correction: If your conduit runs through a hot attic (e.g., 45°C / 113°F), the wire cannot dissipate heat as efficiently. You must multiply the base ampacity by a correction factor. For 90°C THHN wire at 45°C ambient, the multiplier is 0.82.

2. Bundling Adjustment (More than 3 conductors): When you pull multiple circuits through the same conduit, the wires heat each other up. If you have 4 to 6 current-carrying conductors, you multiply the base ampacity by 80%. If you have 7 to 9 conductors, the multiplier drops to 70%.

Real-World Numeric Example:
You are pulling four 12 AWG THHN current-carrying conductors through a conduit in an attic with an ambient temperature of 40°C (104°F). You are protecting the circuit with a 20A breaker.

  • Base Ampacity: 12 AWG THHN in the 90°C column = 30A. (We use the 90°C column for derating math, even if we terminate at 60°C/75°C).
  • Bundling Derating (4 conductors): 30A × 0.80 = 24A.
  • Ambient Derating (40°C for 90°C wire): 24A × 0.91 = 21.84A.
  • Final Result: The derated ampacity is 21.84A. Because 21.84A is greater than the 20A breaker size, this installation is safe and code-compliant.
Pro-Tip for Multi-Wire Branch Circuits (MWBC): The neutral in a standard 120V/240V single-phase MWBC (like a 12-3 NM-B cable feeding two 20A receptacle circuits) is not counted as a current-carrying conductor for bundling derating, because it only carries the unbalanced load. However, if you are running a 3-phase wye circuit with significant harmonic loads (like LED drivers or computers), the neutral must be counted.

What This Wire Amp Table Cannot Tell You

While NEC Table 310.16 is the gold standard for thermal limits, it is not a comprehensive design tool. Relying solely on this wire amp table will lead to failures in three specific scenarios:

1. Voltage Drop Over Long Distances

The ampacity table only tells you the maximum current the wire can carry before the insulation melts. It does not account for voltage drop. If you run 12 AWG copper wire 150 feet to a 20A window AC unit, the wire won't overheat (20A is within the 20A limit), but the voltage at the receptacle will drop by roughly 9.6V (8%). The compressor will struggle, draw higher locked-rotor amperage, and eventually burn out.

The Fix: For branch circuits over 50 feet, use a voltage drop calculator. The general rule of thumb is to keep voltage drop under 3% for branch circuits and 5% total from the service entrance to the furthest outlet. In the 150-foot scenario above, you would need to upsize to 8 AWG or 6 AWG copper to maintain safe voltage, even though 12 AWG satisfies the thermal ampacity table.

2. Physical Lug Limitations

You might calculate that a 40A circuit requires 8 AWG copper (based on the 60°C column). However, if you are terminating into a specific brand of smart breaker or a compact disconnect switch, the physical terminal lugs might only be rated to accept a maximum of 10 AWG wire. You cannot physically torque a larger wire into an undersized lug without damaging the terminal, which creates a high-resistance hot spot. Always check the manufacturer's spec sheet for "wire range" or "lug capacity" before pulling the wire.

3. Aluminum vs. Copper Confusion

The table provided above is strictly for copper. If you are running heavy feeders (like 2 AWG or larger for a subpanel) and choose to use aluminum (SER or XHHW-2) to save money, you must use the aluminum section of NEC 310.16. Aluminum has a higher resistance and lower ampacity per AWG size. For example, while 4 AWG copper is rated for 70A (60°C column), 4 AWG aluminum is only rated for 55A. Mixing up these tables is a frequent cause of overheated main lugs in subpanel installations.

Always verify your final wire size against the specific breaker manufacturer's termination torque specifications (usually printed on the breaker label in inch-pounds). A correctly sized wire that is under-torqued will arc and fail just as surely as an undersized wire.