The Direct Answer: Ampacity of 10 AWG Wire

The base ampacity of 10 AWG copper wire is 30 amps (60°C column), 35 amps (75°C column), and 40 amps (90°C column) per NEC Table 310.16. For aluminum 10 AWG, the values are 25A, 30A, and 30A respectively.

However, the number you actually use to size your breaker is almost always 30 amps. According to NEC Section 110.14(C), for circuits rated 100 amps or less, you must use the 60°C ampacity column to size your overcurrent protection, regardless of the fact that your THHN wire insulation is rated for 90°C. The 90°C column is strictly used as your starting baseline before applying derating factors for heat and conduit bundling.

Safety & Code Caveat: Never put 10 AWG wire on a 40-amp breaker just because the 90°C column says "40A." The breaker protects the weakest link in the circuit, which is usually the 60°C-rated termination lug on your receptacle or panel. Sizing a breaker to the 90°C column will result in a failed inspection and a severe fire hazard.

How to Read the NEC 310.16 Ampacity Table

Before pulling wire, you need to understand how the National Fire Protection Association (NFPA) structures ampacity data. The table below assumes an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway.

How to read the columns:

  • 60°C Column: Used for sizing breakers on standard residential branch circuits (100A or less) because standard receptacles, switches, and panel lugs are typically rated for 60°C.
  • 75°C Column: Used for sizing breakers on larger commercial/industrial equipment or specific high-temp rated terminations.
  • 90°C Column: Used only as the starting point for derating calculations (adjusting for high ambient heat or bundling more than 3 wires in a conduit). You cannot use this column to size the breaker directly.
NEC Table 310.16 Allowable Ampacities (Copper & Aluminum)
AWG Size Copper 60°C Copper 75°C Copper 90°C Aluminum 60°C Aluminum 75°C
14 AWG 15A 20A 25A — —
12 AWG 20A 25A 30A 15A 20A
10 AWG (Target) 30A 35A 40A 25A 30A
8 AWG 40A 50A 55A 30A 40A

Source: NFPA 70 (National Electrical Code) Table 310.16. Assumes THHN/THWN-2 or XHHW insulation.

Derating 10 AWG Wire: When Your Base Ampacity Drops

The 40A rating in the 90°C column is not a free pass to run 40 amps; it is your mathematical starting point for derating. According to Mike Holt Enterprises' NEC breakdowns, you must apply adjustment factors when you bundle more than three current-carrying conductors in a single conduit, or when the ambient temperature exceeds 30°C (86°F).

Worked Numeric Example: Bundled Conductors
Imagine you are pulling wire for a subpanel and a multi-way switch circuit through the same 3/4-inch EMT conduit. You have 5 current-carrying conductors (the neutral in a multi-wire branch circuit counts, grounds do not).

  1. Base Ampacity: Look at the 90°C column for 10 AWG copper = 40A.
  2. Adjustment Factor: NEC Table 310.15(C)(1) states that 4-6 current-carrying conductors require an 80% adjustment factor.
  3. Derated Ampacity: 40A × 0.80 = 32A.
The Termination Trap: Your derated wire can now safely carry 32A. However, because your breaker terminations are rated for 60°C, NEC 110.14(C) still forces you to cap the overcurrent protection at the 60°C column value: 30 amps. The derating math proved your wire won't melt in the conduit, but the breaker size remains 30A. If your derating math had dropped the 90°C adjusted value below 30A (e.g., 28A), you would be forced to upsize to 8 AWG wire.

Decision Tree: Which 10 AWG Insulation and Breaker to Choose

Use this decision path to terminate your planning and pick the exact materials for your cart.

Installation Scenario Condition / Constraint Concrete Material Pick
Standard Residential Dryer / RV Outlet Run inside finished walls, max 3 current-carrying conductors, run under 50 ft. 10/2 NM-B (Romex) with a 30A double-pole breaker.
Conduit Run to Detached Garage / AC Disconnect Pulled through PVC/EMT conduit, exposed to potential moisture underground or outdoors. 10 AWG THHN/THWN-2 (individual conductors) with a 30A breaker.
High Heat Attic Run (Summer) Ambient temp reaches 110°F (43°C). Requires 87% temp correction on 90°C column. 10 AWG THHN (40A x 0.87 = 34.8A). Still safe for a 30A breaker.
Long Distance Run (> 75 feet) 240V circuit pulling 28A continuous over 80 feet. Voltage drop exceeds 3%. REJECT 10 AWG. Upsize to 8 AWG Copper to mitigate voltage drop.

What the Ampacity Table Cannot Tell You

While NEC Table 310.16 dictates thermal limits (preventing the wire insulation from melting), it is blind to three critical field realities that will ruin your installation if ignored:

  1. Voltage Drop: The NEC ampacity tables assume a short run. If you are feeding a 30A, 240V welder outlet 120 feet away from your main panel, 10 AWG wire will experience a voltage drop of roughly 5.2%. This exceeds the NEC's recommended 3% limit for branch circuits (Informational Note to 310.15(B)). You must upsize to 8 AWG or 6 AWG purely for voltage retention, even though 10 AWG won't overheat.
  2. Conduit Fill Capacity: Chapter 9, Table 1 of the NEC limits conduit fill to 40% for three or more wires. While 10 AWG THHN is relatively thin, if you are pulling four 10/2 circuits (8 current-carrying conductors plus grounds) through a single 1/2-inch EMT, you will physically jam the wires and strip the insulation. Always run a conduit fill calculation before buying pipe.
  3. Short-Circuit Withstand: Ampacity handles continuous thermal loads. It does not tell you if the wire can survive the magnetic and thermal shock of a dead short before the breaker trips. Fortunately, standard 10 AWG copper easily withstands the let-through current of a standard 30A residential thermal-magnetic breaker, but this becomes a critical engineering factor in industrial settings with high available fault currents.

Final Recommendation: For 95% of DIY and residential jobsite applications requiring 10 AWG wire (water heaters, window AC units, 30A RV receptacles), purchase 10 AWG THHN/THWN-2 copper for conduit runs or 10/2 NM-B for indoor stud bays, and terminate them exclusively on a 30-amp breaker. Do not attempt to leverage the 90°C column for breaker sizing.