The Direct Answer: Ampacity of #10 Copper Wire

The base ampacity of #10 AWG copper wire is 30 Amps in the 60°C column and 40 Amps in the 90°C column, per NEC Table 310.16. However, for standard residential and commercial branch circuits, the maximum overcurrent protection (breaker size) is strictly capped at 30 Amps by NEC 240.4(D), regardless of the wire's insulation temperature rating.

Bookmark Quick-Jump Rows:
• NM-B (Romex) in walls: 30A max (60°C column governs).
• THHN in conduit (3 wires or less): 30A breaker max, though wire is rated 40A for derating.
• THHN in conduit (4-6 current-carrying wires): 32A derated capacity (still safe for 30A breaker).
• THHN in conduit (7-9 current-carrying wires): 28A derated capacity (must downsize breaker to 25A or upsize wire to #8).

NEC Table 310.16: #10 AWG Copper Ampacity Chart

The following data is extracted directly from the NFPA 70 National Electrical Code (NEC), specifically Table 310.16. This table assumes an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway.

Insulation Type Temp Rating Ampacity (Copper) Common Use Case
TW, UF 60°C (140°F) 30 Amps Older NM cable, underground feeder
THW, THWN, RHW 75°C (167°F) 35 Amps Wet locations, some panel feeders
THHN, XHHW, THWN-2 90°C (194°F) 40 Amps Modern conduit pulls, THHN/THWN-2

How to Read This Table (The Termination Rule)

A common jobsite mistake is using the 90°C column (40A) to size the breaker. NEC 110.14(C) dictates that the ampacity of a circuit is limited by the lowest temperature rating of any connected device, lug, or splice. Because most standard residential breakers and receptacles are rated for 75°C (and older ones for 60°C), you must use the 60°C or 75°C column to determine your final termination ampacity. The 90°C column is strictly reserved as a starting point for derating calculations before you apply the termination limit.

Derating Factors: When 40 Amps Becomes Less

The 40A rating for 90°C THHN copper assumes ideal conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors (CCCs) bundled together. When you exceed these parameters, the wire cannot dissipate heat as efficiently, and you must apply derating factors from NEC Table 310.15(C)(1).

Worked Example: Bundling in Conduit

Imagine you are pulling two 120V multi-wire branch circuits (MWBCs) and a dedicated 240V circuit through a single 3/4-inch EMT conduit. You have 6 current-carrying conductors (the neutral on the MWBCs counts as current-carrying if they share a neutral, plus the two 240V hots).

  1. Base Ampacity: #10 THHN at 90°C = 40 Amps.
  2. Derating Factor: NEC Table 310.15(C)(1) for 4-6 CCCs is 80%.
  3. Calculation: 40A × 0.80 = 32 Amps.
  4. Termination Check: 32A is greater than the 60°C termination limit (30A). Therefore, you are legally permitted to protect this wire with a standard 30A breaker.

The Edge Case (7-9 Conductors): If you add one more circuit, pushing the bundle to 8 CCCs, the derating factor drops to 70%. 40A × 0.70 = 28 Amps. Because 28A is now below the 30A termination limit, the wire's effective ampacity drops to 28A. You must either downgrade to a 25A breaker (rare in residential) or upsize your wire to #8 AWG. For a complete breakdown of bundling adjustments, reference the termination and derating guidelines at EC&M.

Decision Tree: Sizing Your Breaker and Insulation

Use this decision path to select the correct wire and breaker combination for your #10 copper installation. Follow the steps sequentially.

Condition / Question If YES If NO
Is the wire NM-B (Romex) inside a wall cavity? Use 60°C column. Max breaker: 30A. Proceed to next step.
Are there 3 or fewer current-carrying conductors in the conduit? Base ampacity is 40A (90°C). Max breaker: 30A. Proceed to next step.
Are there 4 to 6 current-carrying conductors? Derate to 32A. Termination limit applies. Max breaker: 30A. Proceed to next step.
Are there 7 to 9 current-carrying conductors? Derate to 28A. STOP. #10 wire is too small for a 30A breaker. Upsize to #8 AWG. Consult NEC Table 310.15(C)(1) for higher bundling factors.
Is the ambient temperature above 30°C (86°F)? Apply temperature correction factors from Table 310.16 bottom rows before bundling. No temperature correction needed.
The Default Pick: If you are wiring a standard 30A appliance (like a dryer, RV outlet, or heavy-duty window AC) and want a foolproof setup that passes inspection everywhere, use #10 AWG THHN/THWN-2 pulled in 1/2-inch or 3/4-inch conduit, protected by a 30A double-pole breaker. This gives you the 90°C derating buffer while strictly obeying the 240.4(D) breaker cap.

What the Ampacity Table Cannot Tell You

While NEC Table 310.16 governs thermal limits (preventing the wire from melting or starting a fire), it completely ignores electrical efficiency and physical constraints. Before finalizing your #10 copper run, evaluate these three factors:

1. Voltage Drop Over Distance

Ampacity assumes the wire can handle the heat, but it doesn't guarantee the voltage will reach the load. #10 copper has a resistance of approximately 1.24 ohms per 1,000 feet. If you are running a 30A, 240V load (like a water heater) over a long distance, voltage drop becomes critical.

  • At 50 feet: Voltage drop is roughly 3.7V (1.5%). Perfectly acceptable.
  • At 100 feet: Voltage drop is roughly 7.4V (3.1%). This crosses the NEC recommended 3% threshold for branch circuits.
  • The Fix: If your one-way wire run exceeds 90 feet on a 30A, 240V circuit, upsize to #8 AWG copper to maintain voltage regulation, even though #10 is thermally rated for the breaker.

2. Physical Lug Fit and Torque

#10 AWG solid copper wire is stiff and has a diameter of roughly 0.1019 inches. While it fits easily into standard 30A breaker lugs, attempting to terminate #10 solid wire into a 15A or 20A receptacle (which is sometimes done by amateurs trying to adapt a 30A circuit) can result in poor contact or stripped terminal screws. Always use a properly rated 30A receptacle (like a NEMA 10-30 or 14-30) and torque the lugs to the manufacturer's specification (usually 20-25 in-lbs for residential breakers) to prevent high-resistance hot spots.

3. Local AHJ Amendments

The NEC is a model code. Your local Authority Having Jurisdiction (AHJ) or municipal inspector has the final legal say. Some jurisdictions in high-ambient-heat regions (like parts of Arizona or Nevada) mandate that the 75°C column be used as the absolute maximum for all attic conduit runs, effectively dropping your #10 THHN baseline before bundling derating is even applied. Always verify local amendments before pulling wire in extreme climates.