Yes, 10 AWG copper wire is the correct and minimum size for a 30 amp breaker. Under NEC Table 310.16, 10 AWG copper with 75°C insulation is rated for 35 amps, safely handling a 30A overcurrent device. For standard 60°C NM-B cable, the ampacity is exactly 30 amps.
- Material: Copper (Aluminum rules differ significantly; see Section 4)
- Temperature Column: 75°C for THHN/THWN-2 in conduit; 60°C for NM-B (Romex)
- Ambient Temperature: 30°C (86°F) or lower
- Conduit Fill: Not more than 3 current-carrying conductors in a single raceway
The Baseline: Why 10 AWG Copper is the Minimum for 30 Amps
When sizing a branch circuit, the overcurrent protective device (the breaker) must be sized to protect the wire from overheating, not just to supply the load. According to the National Fire Protection Association (NFPA) and NEC Article 240.4, conductors must be protected at their ampacity ratings.
Why not use a smaller wire, like 12 AWG? A 12 AWG copper wire is rated for 20 amps (in the 60°C column) or 25 amps (in the 75°C column). If you connect 12 AWG wire to a 30 amp breaker and the circuit draws 28 amps, the wire will overheat and its insulation will melt long before the breaker trips. The 30 amp breaker simply cannot protect a 12 AWG conductor. Therefore, 10 AWG is the absolute minimum physical size required to safely pair with a 30A breaker.
Ampacity, Insulation, and the NM-B (Romex) Catch
Not all 10/2 wire is created equal. The ampacity of your 10 AWG wire changes depending on the insulation type and where it is installed. While the 90°C column in NEC Table 310.16 lists 10 AWG copper at 40 amps, you are almost never allowed to use the 90°C column for final breaker sizing because standard breaker terminals are only rated for 75°C or 60°C.
| Wire / Insulation Type | NEC Temp Column Used | Base Ampacity | Max Breaker Size | Application Notes |
|---|---|---|---|---|
| 10 AWG THHN (Copper) | 75°C (Termination limit) | 35A | 35A (Use 30A) | Standard in conduit; allows 30A breaker |
| 10 AWG NM-B (Copper) | 60°C (NEC 334.80) | 30A | 30A | Romex; dry indoor only; exact 30A limit |
| 10 AWG THHN (Copper) | 90°C (Derating base only) | 40A | N/A | Used only as a starting point for derating math |
| 10 AWG XHHW (Aluminum) | 75°C | 25A | 25A | Fails 30A requirement; must upsize to 8 AWG |
The most common point of confusion for DIYers is NM-B cable (commonly known by the brand name Romex). Under NEC 334.80, the ampacity of NM-B cable is strictly limited to the 60°C column of Table 310.16, regardless of the fact that the internal THHN wires might technically have 90°C insulation. In the 60°C column, 10 AWG copper is rated for exactly 30 amps. This means 10/2 NM-B is perfectly legal on a 30 amp breaker, but it leaves zero mathematical headroom.
Voltage Drop: When 10/2 Wire Fails a Long Run
Ampacity tells you if the wire will catch fire. Voltage drop tells you if your equipment will actually run. The NEC recommends (in Informational Note to 210.19) that branch circuit voltage drop be limited to 3% for maximum efficiency. For a 240V circuit, a 3% drop is 7.2 volts. For a 120V circuit, it is 3.6 volts.
According to Southwire's engineering resources, 10 AWG copper has a resistance that causes voltage to degrade over distance. If you are pulling a full 30 amps through 10/2 wire, here is the maximum distance you can run before exceeding the 3% threshold:
| Circuit Voltage | Max Load (Amps) | 3% Drop Limit (Volts) | Max Run Length on 10 AWG | Required Action if Exceeded |
|---|---|---|---|---|
| 240V (e.g., Dryer, RV) | 30A | 7.2V | 96 feet | Upsize to 8 AWG copper |
| 120V (e.g., Heavy Tool) | 30A | 3.6V | 48 feet | Upsize to 8 AWG copper |
What Changes the Sizing Answer (Derating, Bundling, and Continuous Loads)
While 10 AWG copper is the baseline answer, three specific jobsite conditions will force you to abandon 10/2 and pull 8 AWG instead.
1. Conductor Bundling (The Derating Trap)
NEC Table 310.15(C)(1) requires you to reduce (derate) the ampacity of conductors when you bundle more than three current-carrying conductors in a single conduit. If you pull three separate 10/2 cables through one piece of EMT conduit, you have six current-carrying conductors. This triggers an 80% derating factor.
Here is where insulation type matters immensely:
- THHN in Conduit: You start with the 90°C column (40A). 40A x 0.80 = 32A. Because 32A is still above 30A, 10 AWG THHN passes the derating check.
- NM-B (Romex): You are already capped at the 60°C column (30A). You cannot use the 90°C column for derating NM-B because the final ampacity cannot exceed the 60°C rating. 30A x 0.80 = 24A. 10 AWG NM-B fails when bundled. You must upsize to 8 AWG.
2. Aluminum Conductors
Aluminum has higher resistance and lower thermal conductivity than copper. Under the 75°C column, 10 AWG aluminum is only rated for 25 amps. You cannot use 10 AWG aluminum for a 30 amp breaker under any circumstance. If you are using aluminum feeder wire (like SER cable) for a 30A subpanel or disconnect, you must use a minimum of 8 AWG aluminum (rated 40A at 75°C).
3. Continuous Loads and AHJ Approval
NEC Article 100 defines a continuous load as one where the maximum current is expected to continue for 3 hours or more. Examples include baseboard heaters, EV chargers, and commercial lighting. Under NEC 210.20(A), a branch circuit breaker must be rated at 125% of the continuous load.
If you are installing a 24-amp continuous EV charger, the math dictates a 30-amp breaker (24 x 1.25 = 30). The wire must also be sized at 125% (30A). While 10 AWG NM-B is technically rated for 30A, many local Authorities Having Jurisdiction (AHJs) and electrical inspectors will reject 10 AWG for continuous 30A loads because the wire will be operating at 100% of its thermal capacity indefinitely. In these scenarios, it is standard professional practice to upsize to 8 AWG copper to provide a thermal buffer and ensure the inspector signs off on the work without dispute. Always consult your local electrical inspector before finalizing continuous load circuits.
