You need 10 AWG copper wire for a standard 30 amp breaker. If you are using aluminum wire, you must step up to 8 AWG. This assumes a standard residential branch circuit, 75°C rated terminations, and an ambient temperature of 30°C (86°F).
- Conductor Material: Copper (unless aluminum is explicitly stated)
- Termination Temperature: 75°C column (standard for modern breakers and panels)
- Ambient Temperature: 30°C (86°F) or lower
- Conduit Fill: Not more than 3 current-carrying conductors in a single raceway
- Load Type: Non-continuous (operates for less than 3 hours at a time)
Ampacity and Wire Sizing Reference Table
Before pulling wire, you need to understand how your specific breaker size maps to conductor gauges across different materials. The table below provides the minimum wire sizes for common branch circuits, referencing the 75°C column of the NEC ampacity tables (Table 310.16). It also includes maximum one-way run lengths to maintain a voltage drop of less than 3%.
| Breaker Size | Copper Wire (75°C) | Aluminum Wire (75°C) | Max Length for <3% Drop (120V) | Max Length for <3% Drop (240V) |
|---|---|---|---|---|
| 20 Amp | 12 AWG | 10 AWG | 64 feet | 128 feet |
| 30 Amp | 10 AWG | 8 AWG | 48 feet | 96 feet |
| 40 Amp | 8 AWG | 6 AWG | 80 feet | 160 feet |
| 50 Amp | 6 AWG | 4 AWG | 64 feet | 128 feet |
| 60 Amp | 6 AWG | 4 AWG | 53 feet | 106 feet |
Note: Voltage drop calculations assume a full load on the breaker (e.g., 30A on a 30A breaker) using standard DC resistance values for copper at 75°C (1.24 ohms/kft for 10 AWG). For exact calculations on your specific run, use a dedicated voltage drop calculator or consult the manufacturer's data.
Why 10 AWG and Not One Size Smaller?
A common question on the bench is why we cannot use 12 AWG wire if the actual connected load only draws 25 amps. The answer lies in the intersection of wire ampacity and the NEC small conductor rule.
Looking at the standard ampacity tables, 10 AWG copper wire is rated for 35 amps in the 75°C column, and 30 amps in the 60°C column. However, NEC Article 240.4(D) explicitly caps the overcurrent protection for small conductors. It states that the overcurrent device (breaker) for 10 AWG copper shall not exceed 30 amps.
If you were to use 12 AWG wire (rated 20A at 60°C and 25A at 75°C), a 30 amp breaker would fail to trip before the wire's insulation begins to melt and degrade under a sustained 28-amp load. The breaker must protect the wire, not just the appliance. Therefore, 10 AWG is the absolute minimum safe size, and because of the 240.4(D) cap, it is also the exact intended match for a 30A breaker.
When to Upsize: Voltage Drop, Bundling, and Continuous Loads
The 10 AWG baseline assumes ideal conditions. In real-world jobsite scenarios, you will frequently need to upsize to 8 AWG copper. Here is the decision framework for when 10 AWG is no longer sufficient.
| Condition | Technical Trigger | Required Action |
|---|---|---|
| Long Cable Runs | 120V run exceeds 48 feet, or 240V run exceeds 96 feet at full 30A load. | Upsize to 8 AWG to keep voltage drop below the 3% NEC informational recommendation. |
| Continuous Loads | The load will run at maximum capacity for 3 hours or more (e.g., EV chargers, heavy heaters). | Apply the 125% rule (NEC 210.20). 30A x 1.25 = 37.5A. Upsize to 8 AWG wire and a 40A breaker. |
| Conductor Bundling | Pulling 4 to 6 current-carrying conductors through the same conduit nipple or raceway. | Apply 80% derating factor (NEC 310.15(C)(1)). Upsize to 8 AWG to maintain 30A capacity after derating. |
| High Ambient Heat | Routing wire through an attic space that exceeds 30°C (86°F) or near a boiler chase. | Apply temperature correction factors from NEC Table 310.15(B)(1). Usually requires upsizing to 8 AWG. |
| Aluminum Wire | Using SER cable or aluminum branch circuit wire to save on material costs. | Aluminum has higher resistance. You must use 8 AWG Aluminum (rated 40A at 75°C) for a 30A breaker. |
The NM-B (Romex) vs. THHN Derating Trap
Here is a mistake that catches many DIYers and even junior apprentices. If you are using NM-B cable (commonly known as Romex), NEC 334.80 dictates that while the wire insulation is rated 90°C, the final allowable ampacity is strictly capped at the 60°C column.
Fortunately, 10 AWG in the 60°C column is exactly 30 amps, so it works perfectly for a standard 30A breaker. However, if you have to bundle two NM-B cables together through a single bored hole in a top plate (creating 4 current-carrying conductors), you must apply an 80% derating factor. You must start your derating math from the 90°C column (40A for 10 AWG). 40A x 0.80 = 32A. Since 32A is above the 60°C cap of 30A, 10 AWG NM-B actually survives this specific bundling scenario. But if you add a third cable (6 conductors, 70% derating), 40A x 0.70 = 28A. Your wire is now only good for 28 amps, and you must rip it out and replace it with 8 AWG NM-B.
By contrast, if you pull individual THHN/THWN-2 wires in conduit, you use the 75°C column for your baseline (35A for 10 AWG), giving you much more mathematical headroom before derating forces an upsize.
When an Engineer or the AHJ Must Confirm
While the NEC provides the baseline framework, local Authorities Having Jurisdiction (AHJ) and professional engineers must step in under specific conditions. Do not finalize your wire and breaker sizing without professional sign-off if your project involves any of the following:
- Service Entrance Conductors: If this 30A circuit is part of a larger service upgrade or feeder tap, the calculations for let-through current and fault tolerance require an engineered load study.
- Motor Circuits: If the 30A breaker is protecting a large motor (like an air compressor or well pump), NEC Article 430 allows for specific exceptions regarding breaker sizing versus wire ampacity to handle locked-rotor currents. The inverse-time breaker size can often exceed the standard wire ampacity limits.
- Local Code Amendments: Some municipalities (particularly in regions with extreme summer heat or strict energy codes) amend the NEC to require voltage drop calculations to be mandatory rather than informational, forcing larger wire sizes for all exterior runs.
- Aluminum Terminations: If you are terminating 8 AWG aluminum wire onto a breaker or lug that is not explicitly rated 'AL/CU' and marked for 75°C, the AHJ will fail the inspection. Never assume a lug is rated for aluminum without checking the manufacturer's stamp.
Always verify your final wire size against the specific terminal temperature ratings printed on the breaker chassis and the equipment nameplate. When in doubt, stepping up one wire size (e.g., using 8 AWG copper instead of 10 AWG) is always safe, provided the physical terminal lugs on the breaker and receptacle are rated to accept the larger gauge.
