The cable size ampacity chart is the foundational reference for determining the maximum continuous current a conductor can safely carry before its insulation begins to thermally degrade. In the United States, this data is governed by the National Electrical Code (NEC), specifically NFPA 70 Table 310.16. Sizing wire isn't just about preventing a fire; it's about matching the conductor's thermal limits to the overcurrent protective device and the termination ratings of your equipment.
The Core Cable Size Ampacity Chart (NEC Table 310.16)
How to read this table: This chart applies specifically to copper conductors in a standard ambient temperature of 30°C (86°F). The columns represent the temperature rating of the wire's insulation. You will cross-reference the American Wire Gauge (AWG) size on the left with the insulation type you are using (e.g., NM-B cable uses the 60°C column, while individual THHN wires in conduit use the 90°C column). Bookmark the specific rows below for the most frequently pulled residential and light-commercial wire sizes.
| AWG / kcmil | 60°C (140°F) NM-B, TW, UF |
75°C (167°F) THWN, RHW, USE |
90°C (194°F) THHN, XHHW-2 |
|---|---|---|---|
| 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 |
| 4 AWG | 70 A | 85 A | 95 A |
| 3 AWG | 85 A | 100 A | 115 A |
| 2 AWG | 95 A | 115 A | 130 A |
| 1 AWG | 110 A | 130 A | 145 A |
| 1/0 AWG | 125 A | 150 A | 170 A |
| 2/0 AWG | 145 A | 175 A | 195 A |
| 3/0 AWG | 165 A | 200 A | 225 A |
| 4/0 AWG | 195 A | 230 A | 260 A |
Which Temperature Column Applies to Your Installation?
A common mistake on the jobsite is looking at a spool of 10 AWG THHN wire, seeing "40 Amps" in the 90°C column, and assuming it can be placed on a 40A breaker. This is a code violation and a fire hazard. The column you must use for overcurrent protection sizing is almost always dictated by the temperature rating of the equipment terminations, not the wire's insulation.
Under NEC 110.14(C), the rules for termination temperature ratings are strict:
- Circuits rated 100A or less (or 14 AWG through 1 AWG): You must use the 60°C column, unless the equipment is specifically marked and listed for 75°C. Most standard residential breakers and receptacles fall into this category, meaning a 12 AWG wire is strictly limited to 20A, even if you use 90°C THHN.
- Circuits rated over 100A (or larger than 1 AWG): You must use the 75°C column, unless marked otherwise.
| Scenario | Column Used for Breaker Sizing | Column Used for Derating Calculations |
|---|---|---|
| Standard NM-B (Romex) in residential walls | 60°C (NM-B is inherently 60°C rated) | N/A (NM-B is rarely bundled in conduit requiring derating) |
| THHN in conduit to a standard 20A breaker | 60°C (per 110.14(C)(1)(a) for ≤100A) | 90°C (Start here, then apply derating factors) |
| THWN-2 in conduit to a 200A main panel lug | 75°C (per 110.14(C)(1)(b) for >100A) | 90°C (Start here, then apply derating factors) |
The 90°C column is incredibly useful, but only as a starting point for derating calculations. The final ampacity after derating cannot exceed the ampacity listed in the 60°C or 75°C column that applies to your terminations.
Derating Factors and What the Chart Cannot Tell You
The base cable size ampacity chart assumes ideal conditions: an ambient air temperature of 30°C (86°F) and no more than three current-carrying conductors bundled together in a raceway or conduit. When real-world conditions deviate from these assumptions, the base values in the chart must be modified.
How Derating Rows Modify the Base Value
When you pull more than three current-carrying conductors through a single conduit, the heat generated by each wire cannot dissipate efficiently. NEC Table 310.15(C)(1) requires you to multiply the base ampacity by a derating factor. Crucially, you always start your derating math using the 90°C column (assuming you are using 90°C rated wire like THHN), because the insulation can handle the heat internally, even if the terminations cannot.
1. Base Value: 8 AWG in the 90°C column is 55A.
2. Derating Factor: 4 to 6 conductors requires an 80% multiplier.
3. Derated Ampacity: 55A × 0.80 = 44A.
4. Termination Check: Because the circuit is under 100A, we must check the 60°C column. 8 AWG in the 60°C column is 40A.
5. Final Result: Since 40A is lower than 44A, your final allowable ampacity is 40A. You must protect this wire with a 40A breaker.
If the ambient temperature in your attic or rooftop conduit exceeds 30°C, you must apply a second temperature correction factor from NEC Table 310.15(B)(1) alongside the bundling factor. For instance, in a 46°C (115°F) attic, you multiply the 90°C base ampacity by 0.82 before applying the bundling derating.
What the Ampacity Chart Cannot Tell You
While the Department of Energy and NEC emphasize ampacity for thermal safety, the chart is blind to several critical engineering constraints:
- Voltage Drop: The chart will tell you that 10 AWG copper is safe for 30A. It will not tell you that running 30A through 10 AWG copper for 150 feet will result in a 5.8% voltage drop, starving your equipment and causing motors to overheat. For runs over 100 feet, you must calculate voltage drop (aiming for ≤3% for branch circuits) and often upsize the wire by one or two AWG sizes beyond what the ampacity chart demands.
- Physical Lug Fitment: You might calculate that you need 250 kcmil wire for a 250A feeder, but the physical lugs on your specific brand of disconnect switch might only accept up to 4/0 AWG. Always check the manufacturer's spec sheet for maximum wire termination sizes.
- Short-Circuit Withstand Ratings: Ampacity measures continuous thermal loading. It does not indicate how long the wire can survive the massive magnetic and thermal forces of a dead short before the breaker trips. This is governed by the breaker's let-through current and the wire's circular mil area, requiring separate engineering calculations for high-fault-current service entrances.
