For a standard 30-amp circuit, you need 10 AWG copper wire paired with a 30-amp breaker. This applies to typical 120V or 240V residential branch circuits like RV receptacles, dryers, or water heaters. Always verify local codes, but 10 AWG is the universal baseline for this amperage.
- Material: Copper (Aluminum requires a different size)
- Temperature Rating: 75°C column (NEC Table 310.16)
- Ambient Temperature: 30°C (86°F) or less
- Installation: Standard conduit or NM-B cable, not more than 3 current-carrying conductors bundled together
The Baseline Answer and Core NEC Rules
When determining what size wire for a 30 amp breaker, the immediate answer is 10 AWG copper. However, the path through the National Electrical Code (NEC) to get to this answer reveals a critical rule that trips up many DIYers: the "Small Conductor Rule" found in NEC Article 240.4(D).
If you look at the 90°C column of NEC Table 310.16 for 10 AWG copper THHN wire, the ampacity is listed as 40 amps. In the 75°C column, it is 35 amps. Logically, you might assume you could protect 10 AWG wire with a 35A or 40A breaker. You cannot. NEC 240.4(D) explicitly hard-caps the overcurrent protective device (OCPD) for 10 AWG copper at exactly 30 amps, regardless of the insulation's higher thermal rating. This rule exists because smaller wires are more susceptible to damage from high-fault currents and mechanical stress at termination points.
| Insulation Type | Temp Column | Base Ampacity | Max Breaker Size (NEC 240.4(D)) |
|---|---|---|---|
| THHN / THWN-2 | 90°C | 40A | 30A |
| THW / THWN | 75°C | 35A | 30A |
| NM-B (Romex) | 60°C (per NEC 334.80) | 30A | 30A |
Notice the NM-B (Romex) row. Even if the individual wires inside the yellow sheath are rated for 90°C, NEC 334.80 mandates that NM-B cable ampacity must be calculated using the 60°C column. Fortunately, 10 AWG at 60°C is exactly 30 amps, making it perfectly compliant for a 30-amp breaker.
Why 10 AWG Copper? (And Why Not 12 AWG)
A common question on the workbench is whether 12 AWG wire can handle a 30-amp load for a short run or a lightweight appliance. The answer is an absolute no. 12 AWG copper is strictly limited to a 20-amp breaker under the same 240.4(D) small conductor rule.
Circuit breakers use an inverse-time trip curve. A 30-amp breaker will not trip instantly at 31 amps; it uses a thermal bimetallic strip that bends as it heats up. If you push 28 amps through a 12 AWG wire (which is rated for 20A), the wire's insulation will begin to degrade, soften, and eventually melt or catch fire long before the 30-amp breaker recognizes the thermal overload and trips. The breaker is there to protect the wire, not the appliance. Sizing the breaker larger than the wire's ampacity defeats the entire purpose of the overcurrent protection system.
Decision Tree: Adjusting for Distance, Bundling, and Material
While 10 AWG copper is the default, real-world jobsite conditions frequently force an upsize. Use this decision path to determine your final wire pick.
| Condition / Variable | Impact on 10 AWG Copper | Required Action |
|---|---|---|
| Standard Run (< 50 ft) | No derating required. Voltage drop is negligible. | Use 10 AWG Copper. |
| Long Run (> 75 ft at 120V) | Voltage drop exceeds 3% recommendation. | Upsize to 8 AWG Copper. |
| Conduit Bundling (4-6 conductors) | Ampacity derates to 80% (35A x 0.8 = 28A). | Upsize to 8 AWG Copper. |
| High Ambient Temp (113°F+) | Ampacity derates based on NEC Table 310.15(B)(1). | Upsize to 8 AWG Copper. |
| Using Aluminum Wire (SER/USE-2) | Aluminum has higher resistance and different thermal limits. | Use 8 AWG Aluminum. |
If you are pulling individual THHN conductors through a conduit alongside other circuits, count your current-carrying conductors. If you have four or more, NEC 310.15(C)(1) requires you to apply a derating factor. For 4-6 conductors, you multiply the base ampacity by 0.80. If you start with 10 AWG THHN (90°C column = 40A for derating purposes), 40 x 0.80 = 32A. While 32A is technically above 30A, the 240.4(D) cap still forces you to use a 30A breaker, leaving you with almost no thermal headroom. In bundled conduit scenarios, pulling 8 AWG is the professional standard to ensure the wire runs cool and passes inspection without debate.
Voltage Drop Check: The 50-Foot and 100-Foot Tests
Ampacity tells you if the wire will melt; voltage drop tells you if your appliance will actually function correctly. The NEC recommends (via Informational Notes in Article 210.19) that branch circuit voltage drop should not exceed 3%, and the total feeder-plus-branch drop should not exceed 5%.
Let us run the math for a 30-amp load on a 120V circuit using 10 AWG copper, utilizing the standard voltage drop formula: Vd = (2 × K × I × D) / CM.
- K (Copper resistivity) = 12.9
- I (Current) = 30 Amps
- CM (Circular Mils for 10 AWG) = 10,380
Scenario A: 50-Foot Run (One-Way Distance)
Vd = (2 × 12.9 × 30 × 50) / 10,380 = 3.73 Volts.
Percentage: (3.73 / 120) × 100 = 3.1%.
Verdict: This is right on the edge of the 3% recommendation. For a 240V circuit (like a dryer), this same 50-foot run yields only a 1.55% drop, which is excellent. You can confidently use 10 AWG here. You can verify these calculations using tools like the Southwire Voltage Drop Calculator.
Scenario B: 100-Foot Run (One-Way Distance)
Vd = (2 × 12.9 × 30 × 100) / 10,380 = 7.46 Volts.
Percentage: (7.46 / 120) × 100 = 6.2%.
Verdict: This fails the 3% branch circuit recommendation and approaches the 5% total system limit. At 100 feet on a 120V RV pedestal or workshop outlet, a 30A load will experience significant voltage sag, which can damage compressor motors in RV air conditioners. You must upsize to 8 AWG copper for a 100-foot 120V run.
When to Upgrade to 8 AWG or Call an Inspector
There is a massive distinction between a "30-amp breaker" and a "30-amp continuous load." Under NEC Article 100, a continuous load is one where the maximum current is expected to continue for 3 hours or more. Examples include commercial kilns, heavy-duty space heating, or certain EV charging setups.
NEC 210.20(A) requires that the overcurrent device be rated at no less than 125% of the continuous load. If your equipment draws a continuous 30 amps, you cannot use a 30-amp breaker. You must calculate 30A × 1.25 = 37.5A. This forces you to step up to a 40-amp breaker. Consequently, a 40-amp breaker requires 8 AWG copper wire (which has a base ampacity of 50A at 75°C, safely accommodating the 40A OCPD).
When to involve the AHJ (Authority Having Jurisdiction) or an Engineer:
You must pull a permit and have the local electrical inspector verify your work if you are installing a new 30A circuit in a commercial space, running conduit through high-ambient-temperature environments (like an uninsulated attic in a southern climate where temps exceed 113°F), or if you are paralleling conductors. Local amendments to the NEC can supersede standard tables, particularly regarding aluminum wire allowances and specific derating requirements for solar or battery storage interconnections. When in doubt, default to 8 AWG copper; it is universally accepted on a 30A breaker, provides superior voltage drop performance, and eliminates inspector pushback regarding thermal headroom.
