The ampacity of 10 AWG copper wire is 30 amps in the 60°C column and 35 amps in the 75°C and 90°C columns, per NEC Table 310.16. For 10 AWG aluminum, the base ampacity is 25 amps (60°C) and 30 amps (75°C/90°C). However, the breaker size and insulation type dictate which number you actually use on the jobsite. Sizing a breaker based solely on the highest temperature column is a common mistake that leads to failed inspections and melted terminals.
The 10 AWG Ampacity Reference Table (NEC 310.16)
Before pulling wire, you need to know how to read the ampacity tables. The values below are sourced directly from NFPA 70 (National Electrical Code) Table 310.16. The table assumes an ambient air temperature of 30°C (86°F) and not more than three current-carrying conductors bundled together. The columns represent the maximum temperature rating of the wire's insulation. You must match the column to your specific cable type and termination ratings.
| Conductor Material | 60°C Column (140°F) | 75°C Column (167°F) | 90°C Column (194°F) |
|---|---|---|---|
| Copper | 30 Amps | 35 Amps | 40 Amps |
| Aluminum / CCA | 25 Amps | 30 Amps | 35 Amps |
- NM-B (Romex): Always use the 60°C column. Max ampacity = 30A.
- UF-B (Underground Feeder): Always use the 60°C column. Max ampacity = 30A.
- THHN / THWN-2 (in conduit): Wire is rated 90°C (40A base), but termination limits usually force you to use the 75°C column (35A) or 60°C column (30A).
Which Temperature Column Applies to Your Installation?
The most critical rule in wire sizing is the 'weakest link' principle, codified in NEC 110.14(C). You cannot use the 90°C ampacity column just because your THHN wire insulation is rated for 90°C. The allowable ampacity is limited by the lowest temperature rating of any connected device, terminal, or splice in the circuit.
Most modern residential circuit breakers and receptacles are rated for 75°C terminations. However, if you are using NM-B (Romex) cable, the NEC explicitly restricts the ampacity to the 60°C column, regardless of the breaker's terminal rating. Therefore, a 10 AWG NM-B circuit is strictly limited to a 30-amp breaker. If you run individual 10 AWG THHN wires in a conduit to a 75°C-rated breaker panel and a 75°C-rated disconnect, you may use the 75°C column (35 amps).
Derating Factors: When 30 Amps Becomes Less Than 30 Amps
Base ampacity assumes ideal conditions: 30°C ambient temperature and no more than three current-carrying conductors in a raceway. When you bundle wires together, they trap heat. NEC 310.14 requires you to apply adjustment factors (derating) to the wire's base ampacity to prevent the insulation from melting inside the conduit.
| Number of Current-Carrying Conductors | Adjustment Factor (Multiplier) |
|---|---|
| 1 through 3 | 100% (No derating) |
| 4 through 6 | 80% |
| 7 through 9 | 70% |
| 10 through 20 | 50% |
Worked Derating Example: You are pulling four 10 AWG THHN copper wires (two hot, one neutral, one ground) through a single conduit for a multi-wire branch circuit. Only the two hots and the neutral count as current-carrying conductors (the ground does not). That gives you 3 current-carrying conductors, meaning no derating is required.
However, if you pull two separate 120V circuits (four hots, two neutrals = 6 current-carrying conductors) using 10 AWG THHN, you must apply the 80% derating factor. You start with the 90°C base ampacity of 40 amps.
Calculation: 40A × 0.80 = 32 Amps.
Because the derated ampacity (32A) is still higher than your 30A load, the wire is safe from overheating. You then check NEC 110.14(C): since your breaker terminals are rated 75°C, and 32A is less than the 75°C column limit of 35A, you can legally protect this circuit with a standard 30-amp breaker.
What This Table Cannot Tell You (Voltage Drop & Local Code)
Ampacity tables only measure a wire's ability to dissipate heat. They tell you nothing about efficiency or voltage drop. A 10 AWG wire carrying 30 amps over a long distance will not catch fire, but the voltage at the far end may drop low enough to damage motors or cause lighting flicker. The Copper Development Association and NEC informational notes recommend keeping voltage drop under 3% for branch circuits.
Voltage Drop Calculation: Imagine a 120V circuit using 10 AWG copper wire, carrying a full 30-amp load, with a one-way distance of 100 feet. The resistance of 10 AWG copper is roughly 1.24 ohms per 1,000 feet at 75°C.
- Round-trip wire length = 200 feet.
- Total resistance = 1.24 × (200 / 1000) = 0.248 ohms.
- Voltage drop (V = I × R) = 30A × 0.248 ohms = 7.44 volts.
- Percentage drop = (7.44 / 120) × 100 = 6.2%.
A 6.2% drop exceeds the recommended 3% maximum. Even though 10 AWG wire has the thermal ampacity to handle the 30 amps safely, you must upsize to 8 AWG wire for a 100-foot run to maintain proper voltage delivery.
Finally, always remember that NEC-style guidance provides the minimum safety baseline. Your local Authority Having Jurisdiction (AHJ) or municipal inspector has the final say on code compliance, and local amendments may require larger wire sizes or stricter derating rules in high-ambient-temperature environments like attics.
