The ampacity of wire sizes dictates the maximum continuous current a conductor can carry before its insulation begins to thermally degrade. For standard residential branch circuits, 14 AWG copper is rated for 15 amps, 12 AWG for 20 amps, and 10 AWG for 30 amps. These baseline numbers are derived directly from the 60°C column of the National Electrical Code (NEC). However, simply memorizing three numbers is not enough for subpanel feeders, long conduit runs, or aluminum service entrance cables. To size wire correctly and legally, you must understand how to read and apply the full ampacity tables.
How to Read the NEC 310.16 Ampacity Table
Before pulling wire, you need to understand the architecture of the NFPA 70: National Electrical Code Table 310.16. The table is split into two primary halves: the left side lists values for Copper conductors, and the right side lists values for Aluminum (or Copper-Clad Aluminum). Each half is further divided into three temperature columns: 60°C (140°F), 75°C (167°F), and 90°C (194°F).
These temperature ratings correspond to the thermal rating of the wire's insulation (e.g., TW is 60°C, THHW is 75°C, THHN/XHHW is 90°C). The numbers inside the cells represent the allowable ampacity in amperes. The table assumes an ambient air temperature of 30°C (86°F) and no more than three current-carrying conductors in a single raceway or cable. If your installation deviates from these baseline conditions, the base ampacity must be modified using derating factors found in adjacent NEC tables.
Ampacity of Wire Sizes: Copper and Aluminum Data Chart
Below is the reference data for the most commonly used AWG sizes in residential and light commercial work. This data is sourced directly from NEC Table 310.16. We have added anchor IDs to the most frequently queried residential branch circuit and feeder rows for quick reference.
| AWG Size | 60°C Copper | 75°C Copper | 90°C Copper | 60°C Aluminum | 75°C Aluminum | 90°C Aluminum |
|---|---|---|---|---|---|---|
| 14 AWG | 15 A | 20 A | 25 A | -- | -- | -- |
| 12 AWG | 20 A | 25 A | 30 A | -- | -- | -- |
| 10 AWG | 30 A | 35 A | 40 A | -- | -- | -- |
| 8 AWG | 40 A | 50 A | 55 A | -- | -- | -- |
| 6 AWG | 55 A | 65 A | 75 A | 40 A | 50 A | 60 A |
| 4 AWG | 70 A | 85 A | 95 A | 55 A | 65 A | 75 A |
| 3 AWG | 85 A | 100 A | 115 A | 65 A | 75 A | 90 A |
| 2 AWG | 95 A | 115 A | 130 A | 75 A | 90 A | 100 A |
| 1 AWG | 110 A | 130 A | 145 A | 85 A | 100 A | 120 A |
| 1/0 AWG | 125 A | 150 A | 170 A | 100 A | 120 A | 135 A |
| 2/0 AWG | 145 A | 175 A | 195 A | 115 A | 135 A | 150 A |
| 3/0 AWG | 165 A | 200 A | 225 A | 130 A | 155 A | 175 A |
| 4/0 AWG | 195 A | 230 A | 260 A | 150 A | 180 A | 205 A |
Which Column Applies to Your Installation?
The most common point of failure in DIY and junior-level electrical work is selecting the wrong temperature column. The column you use is not solely determined by the wire's insulation; it is dictated by the weakest link in the circuit, which is almost always the termination point (the breaker lug or receptacle screw).
According to NEC 110.14(C), you must follow these rules to select your column:
- The 60°C Column: Use this column for circuits rated 100 amps or less, or for wire sizes 14 AWG through 1 AWG. This applies to almost all standard residential branch circuits (lighting, receptacles, small appliances). It also universally applies to NM-B cable.
- The 75°C Column: Use this column for circuits rated over 100 amps, or for wire sizes larger than 1 AWG (e.g., 1/0 to 4/0). Modern main lugs, subpanel feed lugs, and larger breakers are typically rated for 75°C terminations.
- The 90°C Column: You can never use the 90°C column to determine the final breaker size for standard terminations, because standard breakers and receptacles are not rated for 90°C. The 90°C column is used exclusively as the starting baseline for calculating derating adjustments.
How Derating Modifies the Base Value
When you pull more than three current-carrying conductors through a single conduit, or when the ambient temperature exceeds 30°C (86°F), the wires cannot dissipate heat effectively. You must apply a derating multiplier to the 90°C column value.
Worked Example: You are pulling four 12 AWG THHN current-carrying conductors in a single EMT conduit for a multi-wire branch circuit.
1. Base 90°C ampacity for 12 AWG Copper = 30A.
2. NEC Table 310.15(C)(1) requires an 80% derating factor for 4-6 conductors.
3. Derated ampacity = 30A × 0.80 = 24A.
4. Because 24A is greater than the 20A termination limit (60°C column), you are legally permitted to protect this circuit with a standard 20A breaker.
However, if you had 10 conductors in the pipe (requiring a 50% derating factor), the math becomes 30A × 0.50 = 15A. You would be forced to downsize the breaker to 15A, despite using 12 AWG wire.
What the Table Cannot Tell You
While Table 310.16 is the bible for thermal limits, it is not a complete wire-sizing tool. Relying on it blindly will lead to code violations and poor system performance in three specific scenarios:
1. Voltage Drop on Long Runs
Table 310.16 assumes the wire run is short enough that voltage drop is negligible. If you are running a 120V circuit 150 feet to a detached garage, 12 AWG copper might have the thermal ampacity to handle 20 amps, but the resistance of the wire will cause the voltage at the receptacle to drop below the acceptable 3% threshold (116.4V). For long runs, you must calculate voltage drop using Chapter 9, Table 8 (conductor properties) and upsize the wire accordingly, even if the breaker size remains the same.
2. Equipment Terminal Ratings
The table assumes your equipment can handle the heat. If you are terminating a 2 AWG aluminum feeder into an older, legacy disconnect switch that only has 60°C rated terminal lugs, you are forced to use the 60°C column (75A) rather than the 75°C column (90A). Always check the manufacturer's spec sheet or the stamped labeling inside the equipment enclosure to verify the termination temperature rating.
3. Equipment Grounding Conductor Sizing
Table 310.16 only applies to current-carrying conductors (hots and neutrals). It does not tell you what size ground wire to pull. Equipment grounding conductors (EGCs) are sized based on the rating of the overcurrent protective device (the breaker), not the load. For that data, you must cross-reference NEC Table 250.122. For example, a 100A breaker requires a minimum 8 AWG copper ground, regardless of whether your ungrounded conductors were upsized to 1 AWG for voltage drop mitigation.
