When sizing conductors for a branch circuit or feeder, the baseline numbers you need are locked inside the wire ampacity rating chart, officially known as NEC Table 310.16. For standard residential copper wiring, the quick answers are: 14 AWG is rated for 15 amps, 12 AWG for 20 amps, and 10 AWG for 30 amps. However, pulling those exact numbers depends entirely on which temperature column your installation falls under and whether your conductors are bundled in a conduit or running free. Below is the complete reference data, followed by the derating math and field realities that dictate your final wire size.
How to Read the NEC Wire Ampacity Rating Chart
Before jumping to the numbers, you must understand the three temperature columns. Wire insulation (like THHN or XHHW-2) is often rated for 90°C, but your terminations (breaker lugs, receptacle screws) are typically only rated for 75°C. Per NEC 110.14(C), you must use the lowest temperature rating of any connected component. Furthermore, standard nonmetallic-sheathed cable (NM-B / Romex) is legally restricted to the 60°C column by NEC 334.80, regardless of the fact that its internal THHN conductors have 90°C insulation.
| AWG / kcmil | Copper 60°C (NM-B) | Copper 75°C (THHN Term) | Copper 90°C (THHN Wire) | Aluminum 60°C | Aluminum 75°C | Aluminum 90°C |
|---|---|---|---|---|---|---|
| 14 | 15 | 20 | 25 | — | — | — |
| 12 | 20 | 25 | 30 | 15 | 20 | 25 |
| 10 | 30 | 35 | 40 | 25 | 30 | 35 |
| 8 | 40 | 50 | 55 | 30 | 40 | 45 |
| 6 | 55 | 65 | 75 | 40 | 50 | 60 |
| 4 | 70 | 85 | 95 | 55 | 65 | 75 |
| 3 | 85 | 100 | 110 | 65 | 75 | 85 |
| 2 | 95 | 115 | 130 | 75 | 90 | 100 |
| 1 | 110 | 130 | 145 | 85 | 100 | 115 |
| 1/0 | 125 | 150 | 170 | 100 | 120 | 135 |
| 2/0 | 145 | 175 | 195 | 115 | 135 | 150 |
| 3/0 | 165 | 200 | 225 | 130 | 155 | 175 |
| 4/0 | 195 | 230 | 260 | 150 | 180 | 205 |
Derating Factors and What the Chart Cannot Tell You
The ampacities listed above assume you are running no more than three current-carrying conductors in a raceway or cable, and that your ambient temperature is exactly 30°C (86°F). When real-world conditions deviate, you must apply derating multipliers.
How Derating Modifies the Base Value
When you pull more than three current-carrying conductors through a single conduit, they heat each other up. Per NEC Table 310.15(C)(1), you must multiply the base ampacity by a correction factor. Crucially, derating is calculated using the 90°C column for THHN/THWN-2 wire, not the 60°C column.
Worked Example: You are pulling 6 current-carrying conductors (three multi-wire branch circuits) through a single 3/4-inch EMT conduit using 12 AWG THHN.
- Base ampacity from 90°C column: 30A.
- Adjustment factor for 4-6 conductors: 80%.
- Derated ampacity: 30A × 0.80 = 24A.
What the Ampacity Table Cannot Tell You
While NEC Table 310.16 is the bible for thermal limits, it is blind to three critical field variables:
- Voltage Drop: The chart does not account for distance. A 10 AWG wire can safely carry 30A thermally, but if that run is 150 feet long to a detached garage, your voltage will drop below the recommended 3% threshold, causing motors to overheat and lights to dim. You must calculate voltage drop using circular mils (NEC Chapter 9, Table 8).
- Ambient Temperature Corrections: If your conduit runs across a 110°F attic or along a hot roof, you must apply the temperature correction factors in NEC Table 310.15(B)(1) before applying the bundling derating factors.
- Continuous Loads: If a load will run for 3 hours or more (like an EV charger or hardwired heating), NEC 210.20(A) requires you to multiply the load by 125%. A continuous 20A load requires a wire and breaker rated for at least 25A.
Wire Ampacity Rating Chart FAQ
Can I use the 90°C column for standard residential breakers?
No. While the wire insulation itself can handle 90°C, the breakers, lugs, and receptacles in standard residential panels are tested and rated for a maximum of 75°C. Furthermore, if you are using standard NM-B (Romex) cable, NEC 334.80 explicitly forces you to use the 60°C column, regardless of the fact that the individual conductors inside the jacket are THHN (90°C). The 90°C column is strictly used as the starting point for calculating derating adjustments and ambient temperature corrections.
Does the ground wire count as a current-carrying conductor for derating?
No. Per NEC 310.15(B)(6), equipment grounding conductors (bare copper or green) are not counted as current-carrying conductors when applying the adjustment factors in Table 310.15(C)(1). However, if you are using a grounded conductor (a neutral/white wire) that only carries the unbalanced return current of a standard single-phase, 2-wire or 3-wire circuit, it is counted. Neutral conductors on 3-phase wye systems where the major portion of the load consists of nonlinear loads (like LED drivers or computers) must also be counted due to harmonic currents.
Why is my 10 AWG wire rated for 30A but I can only put 24A on it continuously?
This is the continuous load rule in action. If you are wiring a dedicated circuit for an EV Level 2 charger that pulls 24 amps and will run for more than 3 hours, it is classified as a continuous load. The NEC requires continuous loads to be derated to 80% of the circuit's rating (or conversely, the circuit must be sized at 125% of the load). 24A × 1.25 = 30A. While a 10 AWG wire and 30A breaker meet the absolute minimum, it leaves zero margin for voltage drop or temperature spikes. Best practice for a 24A continuous EV load is to step up to 8 AWG copper on a 40A breaker.
How do I size aluminum wire for a 100-amp subpanel feeder?
Aluminum is highly cost-effective for large feeders. Because subpanel lugs are generally rated for 75°C, you look at the Aluminum 75°C column in the chart above. For a 100-amp subpanel, you need #1 AWG aluminum (rated for 100A). Do not use #2 AWG aluminum (rated for 90A in the 75°C column) unless your calculated load is under 90 amps and your breaker is sized accordingly. Always ensure your aluminum terminations are torqued to the manufacturer's specifications using an anti-oxidant compound like Noalox to prevent thermal creep and arcing over time.
