When makers, DIYers, and apprentice electricians search for 10 ampacity, they are typically looking for one of two distinct data points: the maximum current-carrying capacity (ampacity) of 10 AWG wire, or the correct wire size to safely pair with a 10-Amp breaker.
Here is the direct answer: 10 AWG copper wire has a base ampacity of 30 Amps when used in standard residential cable assemblies (NM-B/UF-B, 60°C column) and 40 Amps when pulled as individual conductors in conduit (THHN/THWN-2, 90°C column) before any derating is applied. If you are sizing wire for a 10-Amp breaker, 14 AWG is the NEC minimum, but 10 AWG is strictly required if your circuit run exceeds 115 feet to prevent excessive voltage drop.
10 AWG Wire Ampacity Chart (NEC Table 310.16)
The following table is extracted from NFPA 70 (National Electrical Code) Table 310.16. This is your master reference for 10 AWG ampacity. Bookmark this section for quick jobsite lookups.
| Wire Material | Insulation Type (Common Examples) | 60°C (140°F) Column | 75°C (167°F) Column | 90°C (194°F) Column |
|---|---|---|---|---|
| Copper | NM-B, UF-B, TW, UF (Romex) | 30 Amps | — | — |
| Copper | THHN, THWN-2, XHHW (in conduit) | 30 Amps | 35 Amps | 40 Amps |
| Copper | FEP, FEPB, PTFE (High-temp specialty) | — | — | 40 Amps |
| Aluminum | XHHW, THWN-2 (in conduit) | 25 Amps | 30 Amps | 35 Amps |
| Aluminum | NM-B, UF-B (Rare for 10 AWG) | 25 Amps | — | — |
Quick-Jump Rules for the Most Queried Values
- Standard 120V/240V Romex (NM-B): Always use the 30A (60°C) value. You cannot use the 90°C rating just because the individual wires inside the sheath are marked THHN.
- THHN in EMT Conduit: You may use the 40A (90°C) value only as a starting point for derating calculations. Your final overcurrent protection device (breaker) cannot exceed 30A for standard terminations.
- Aluminum 10 AWG: Rarely used in branch circuits, but if you are terminating 10 AWG aluminum SER cable to a 75°C rated lug, the ampacity is 30A.
Derating Factors: When 40A Becomes Less Than 30A
The ampacity table above assumes a single isolated circuit in an ambient temperature of 30°C (86°F). In the real world, wires heat each other up when bundled together. NEC Article 310.15(C)(1) mandates that when you pull more than three current-carrying conductors through a single raceway or conduit, you must apply a derating factor to the wire's base ampacity.
Crucially, derating is always calculated from the 90°C column for THHN/THWN-2 wire, even if your final termination limits you to the 60°C or 75°C column. This is a massive advantage of using individual conductors in conduit over NM-B cable.
Worked Numeric Example: Bundled Conduit Derating
Imagine you are pulling three separate 120V circuits (3 hot wires, 3 neutral wires) through a single 3/4-inch EMT conduit. You have 6 current-carrying conductors. (Note: Ground wires do not count as current-carrying for derating purposes).
- Identify Base Ampacity: 10 AWG Copper THHN in the 90°C column = 40 Amps.
- Find Derating Factor: NEC Table 310.15(C)(1) states that for 4 to 6 conductors, the adjustment factor is 80%.
- Calculate Derated Ampacity: 40A × 0.80 = 32 Amps.
- Apply Termination Limits: Your derated ampacity is 32A. However, your breaker terminals are rated for 75°C (35A limit for 10 AWG) or 60°C (30A limit). Because 32A exceeds the standard 60°C termination limit of 30A, you must protect this wire with a 30-Amp breaker, not a 35A or 40A breaker.
Sizing for a 10-Amp Breaker and Hidden Limitations
If your search for '10 ampacity' was actually about sizing wire for a 10-Amp breaker (common for dedicated HVAC controls, garage door openers, or low-draw lighting circuits), the NEC minimum wire size is 14 AWG copper (rated for 15A). However, ampacity tables only tell you what the wire can handle before the insulation melts. They completely ignore voltage drop, which is where 10 AWG becomes mandatory for 10A circuits.
What the Ampacity Table Cannot Tell You
According to industry best practices and NEC informational notes, voltage drop should not exceed 3% for branch circuits. The ampacity table assumes a short run. Here is why you must upsize to 10 AWG for long 10-Amp runs:
- The Scenario: A 120V circuit powering a 10-Amp continuous load (like a block heater or remote pump) located 150 feet from the panel.
- Using 14 AWG (Minimum Code): The voltage drop calculates to roughly 9.4 Volts (a 7.8% drop). Your equipment will only see 110.6V, which can cause motors to overheat, draw excess current, and fail prematurely.
- Using 10 AWG (Upsized): The voltage drop falls to 3.7 Volts (a 3.1% drop). While slightly above the ideal 3%, it is vastly superior and keeps the equipment operating safely within its designed voltage envelope.
Ambient Temperature Corrections
The tables above assume your attic, crawlspace, or conduit is at 30°C (86°F). If you are routing 10 AWG THHN through an attic that reaches 50°C (122°F) in the summer, you must apply an ambient temperature correction factor. For 90°C insulation at 50°C ambient, the correction factor is 0.82.
40A (base 90°C) × 0.82 = 32.8 Amps. You then compare this to your termination limits. If you are wiring a subpanel feeder in a hot environment, always check the ambient correction tables in NEC 310.15(B)(1) before finalizing your wire size.
Final Verification Step
Before energizing any circuit, verify your wire size against the breaker rating using a physical wire gauge tool. Do not rely on the printed text on the jacket of old or sun-baked NM-B cable, as the ink often fades. Strip a small section of the insulation and measure the bare copper diameter with calipers; 10 AWG solid copper should measure exactly 0.1019 inches (2.588 mm). Torque all terminal lugs to the manufacturer's specified inch-pound rating using a calibrated torque screwdriver to prevent high-resistance connections that bypass breaker thermal protection.
