The direct answer for 10 AWG copper wire ampacity is 30 amps in the 60°C column, 35 amps in the 75°C column, and 40 amps in the 90°C column, according to NEC Table 310.16. However, simply picking the highest number on the chart is a fast track to a failed inspection or a melted terminal lug. The actual allowable ampacity for your specific installation depends entirely on your insulation type, the temperature rating of your breaker terminals, and how many current-carrying conductors are sharing the same raceway.
Below is the complete reference data you need to size your overcurrent protective device (OCPD) correctly, followed by the derating math that modifies these base numbers in real-world jobsite conditions.
The Master 10 AWG Wire Ampacity Table (NEC 310.16)
How to read this table: This data is pulled directly from the NFPA 70 National Electrical Code, specifically Table 310.16. The columns represent the maximum allowable temperature rating of the wire insulation and the termination points. To find your base ampacity, locate your wire material and trace across to the column that matches the lowest temperature rating in your entire circuit (wire, connector, or breaker terminal). Ambient temperature is assumed to be 30°C (86°F) with no more than three current-carrying conductors in the raceway.
| Material | Size (AWG) | Area (Circular Mils) | 60°C (140°F) Column | 75°C (167°F) Column | 90°C (194°F) Column |
|---|---|---|---|---|---|
| Copper | 12 | 6,530 | 20A | 25A | 30A |
| Copper | 10 | 10,380 | 30A | 35A | 40A |
| Copper | 8 | 16,510 | 40A | 50A | 55A |
| Aluminum | 10 | 10,380 | 25A | 30A | 35A |
- NM-B (Romex) Cable: Legally limited to the 60°C column per NEC 334.80. Maximum ampacity is 30A, even though the internal THHN wires are rated for 90°C.
- THHN/THWN-2 in Conduit: The wire insulation is 90°C (40A), but standard residential breakers (like Square D QO or Eaton BR) have 75°C terminals. Your base ampacity is capped at 35A unless the termination is explicitly rated for 90°C.
Which Temperature Column Applies to Your Installation?
The most common mistake DIYers and junior apprentices make is using the 90°C column for THHN wire simply because the jacket is printed with "90°C". The NEC enforces a strict "weakest link" rule (NEC 110.14(C)). You must use the temperature column that matches the lowest-rated component in your entire circuit path.
In residential and light commercial panels, the lugs on circuit breakers and bus bars are almost universally rated for 75°C. Therefore, if you are pulling 10 AWG THHN through EMT conduit to a standard 75°C breaker, your starting ampacity is 35A. You cannot protect this wire with a 40A breaker, because 40A requires a 90°C termination rating.
If you are using NM-B cable (commonly known by the brand name Romex), NEC Article 334.80 explicitly states that the ampacity of NM cable must be determined using the 60°C column, regardless of the actual temperature rating of the individual insulated conductors inside the sheath. For 10 AWG NM-B, your absolute maximum ampacity is 30A, and it must be protected by a 30A breaker.
How Derating Modifies the Base 10 AWG Ampacity
The base numbers in Table 310.16 assume an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway. When you exceed these parameters, you must apply derating factors. Derating is calculated using the 90°C column as your starting baseline, but the final derated number must still be compared against your terminal temperature limits.
Scenario: Bundling in Conduit
Imagine you are pulling four current-carrying 10 AWG THHN conductors (two 240V circuits sharing a neutral, or a multi-wire branch circuit setup) through a single 3/4-inch EMT conduit. According to NEC Table 310.15(C)(1), 4 to 6 current-carrying conductors require an 80% adjustment factor.
- Start with the 90°C base ampacity for 10 AWG copper: 40A.
- Apply the 80% bundling derating factor: 40A × 0.80 = 32A.
- Compare the derated ampacity (32A) to the terminal rating (75°C column = 35A).
- The lower number wins. Your final allowable ampacity is 32A.
- Select the next standard OCPD size down (or exact match). Since 32A is not a standard breaker size (NEC 240.6), you must protect this circuit with a 30A breaker.
Scenario: High Ambient Temperature
If that same conduit is routed through an attic where the ambient temperature reaches 45°C (113°F), you must apply the temperature correction factor from Table 310.15(B)(1). For 90°C wire at 41-45°C ambient, the correction factor is 0.87. Your new math: 40A × 0.87 = 34.8A. If you combine both high heat and bundling, you multiply both factors: 40A × 0.80 × 0.87 = 27.8A, forcing you to upsize to 8 AWG wire to maintain a 30A circuit.
What the Ampacity Table Cannot Tell You
While ECMWeb NEC Code Basics and the NEC handbook provide the thermal limits for wire insulation, ampacity tables completely ignore three critical physical realities of electrical installations.
1. Voltage Drop Over Distance
Ampacity only tells you how much current the wire can carry before the insulation melts. It does not tell you if the voltage at the load will be sufficient to operate the equipment. 10 AWG copper has a resistance of approximately 1.24 ohms per 1,000 feet. If you are wiring a 240V, 20A well pump located 250 feet from the panel, the round-trip distance is 500 feet. The voltage drop will be roughly 12.4V (over 5%). While the wire won't catch fire, the pump motor will run hot, draw excess current, and fail prematurely. For long runs, you must upsize to 8 AWG or 6 AWG strictly for voltage drop mitigation, even if 10 AWG meets the thermal ampacity requirements.
2. Terminal Physical Limitations
Not all breaker lugs are physically designed to accept 10 AWG wire. While a 30A breaker is designed for it, attempting to land a 10 AWG solid wire under the pressure plate of a 15A or 20A GFCI/AFCI breaker can result in the wire slipping out, the pressure plate cracking, or the wire bending so severely that it damages the insulation jacket. Always check the manufacturer's termination specifications on the breaker label.
3. Short-Circuit Withstand Ratings
Ampacity is about continuous thermal loading. It does not account for the magnetic and thermal forces of a dead short. If a 10 AWG wire is protected by a breaker with a high let-through current, the wire could physically vaporize before the breaker trips during a massive fault condition. This is why matching the wire size to the breaker's interrupting rating and specific trip curve is a necessary engineering step in industrial environments, even if it is rarely a concern in standard residential branch circuits.
