The maximum standard overcurrent protection for 10 AWG copper wire is 30 amps. While the physical insulation on modern 10 AWG wire can handle significantly more heat, NEC Article 240.4(D) strictly caps the breaker size for 10 AWG copper at 30A for standard branch circuits. If your calculated load exceeds 30A, or if derating factors drop your adjusted ampacity below 30A, you must upsize to 8 AWG wire.
The Quick Answer: 10 Gauge Ampacity at a Glance
Bookmark this section for quick jobsite reference. The values below apply to single copper conductors in a raceway or cable, assuming an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors bundled together.
| Temperature Rating | Common Insulation Types | Base Ampacity | Max Standard Breaker Size* |
|---|---|---|---|
| 60°C (140°F) | TW, UF-B | 30 Amps | 30A |
| 75°C (167°F) | RHW, THHW, XHHW | 35 Amps | 30A |
| 90°C (194°F) | THHN, THWN-2, XHHW-2 | 40 Amps | 30A |
*Per NEC 240.4(D), the overcurrent device for 10 AWG copper cannot exceed 30A, regardless of the 75°C or 90°C column values, unless specific motor or capacitor exceptions apply.
How to Read the NEC 310.16 Ampacity Table
The National Electrical Code (NEC) publishes ampacity tables that list wire capacities across three temperature columns: 60°C, 75°C, and 90°C. The most common mistake DIYers and junior apprentices make is blindly using the 90°C column because modern THHN wire is rated for it.
Which column applies to your installation?
Per NEC 110.14(C), the ampacity of a circuit is determined by the lowest temperature rating of any connected device, termination, or conductor in the circuit. Most standard residential breakers, receptacles, and switches are rated for 75°C terminations. However, for circuits rated 100A or less, and for wire sizes 14 AWG through 1 AWG, the NEC defaults to the 60°C column for final ampacity and breaker sizing unless the equipment is explicitly listed and identified for 75°C terminations.
The 90°C column is not useless; it is your starting point for derating calculations. You calculate your derated ampacity using the 90°C base, but your final adjusted number cannot exceed the 60°C base ampacity (30A for 10 AWG) when selecting your breaker.
Derating 10 AWG Wire: When 30 Amps Isn't 30 Amps
Ampacity tables assume ideal conditions: 30°C ambient temperature and no more than three current-carrying conductors in a raceway. When you bundle wires or run them through hot spaces, the wire cannot dissipate heat as efficiently, and you must apply derating factors.
If you pull four 10 AWG THHN circuits (8 current-carrying conductors) through a single conduit in a 40°C (104°F) attic, you must derate.
- Base (90°C column): 40A
- Ambient Temp Factor (40°C): 0.88
- Bundling Factor (7-9 conductors): 0.70
- Adjusted Ampacity: 40A × 0.88 × 0.70 = 24.64 Amps
Decision Tree: Sizing Your 10 AWG Breaker and Insulation
Use this decision path to select the exact breaker and wire type for your project. Do not guess; follow the logic to the concrete pick.
| Application Scenario | Load Type & Conditions | Concrete Pick (Wire & Breaker) |
|---|---|---|
| Standard 240V Electric Baseboard Heater | Non-continuous load, standard conduit run, max 30A. | Wire: 10 AWG THHN/THWN-2 Breaker: 30A 2-Pole (e.g., Square D HOM230) |
| 30-Amp RV Receptacle (TT-30R) | 120V continuous-capable, buried or indoor NM cable. | Wire: 10/2 NM-B (Romex) Breaker: 30A 1-Pole (e.g., Square D HOM130) |
| Window AC Unit (Nameplate 28A) | Continuous motor load, short run. | Wire: 10 AWG THHN Breaker: 30A 2-Pole (Motor rules allow 10 AWG on 30A if nameplate dictates) |
| Calculated Load is 32 Amps | Exceeds the 240.4(D) 30A hard cap for 10 AWG. | STOP. Upsize to 8 AWG Copper and use a 40A breaker. |
| Conduit contains 6 current-carrying 10 AWG wires | Bundling derating drops adjusted ampacity below 30A. | STOP. Upsize to 8 AWG THHN or run a second conduit. |
What the Ampacity Table Cannot Tell You
The NEC 310.16 table only tells you the thermal limit of the wire's insulation before it melts or degrades. It does not account for voltage drop or continuous load rules, both of which will force you to upsize your wire even if the breaker is sized correctly.
1. Voltage Drop Limitations
The NEC recommends a maximum 3% voltage drop on branch circuits for reasonable efficiency. If you push a full 30A through 10 AWG copper on a 240V circuit, you will hit that 3% drop threshold (7.2 volts) at approximately 95 feet. If your 30A electric heater is 120 feet away from the panel, 10 AWG wire will result in a 3.8% voltage drop, causing the heater to run hotter, draw more current, and underperform. For a 120-foot run at 30A, you must upsize to 8 AWG copper to maintain voltage stability. You can verify exact runs using the Southwire Voltage Drop Calculator.
2. Continuous Load Derating (The 125% Rule)
If your load will run for 3 hours or more continuously (like an EV charger, a large aquarium heater, or commercial lighting), NEC Article 210.20 requires the branch circuit to be rated at 125% of the continuous load. Conversely, this means a 30A breaker can only safely carry 24 Amps of continuous load (30A ÷ 1.25 = 24A). If your continuous load nameplate reads 25A, a 30A breaker and 10 AWG wire are illegal; you must step up to a 40A breaker and 8 AWG wire.
