For a standard 30-amp circuit, use 10 AWG copper wire paired with a 30-amp breaker. This assumes THHN/THWN-2 insulation in a 75°C temperature column, a maximum ambient temperature of 30°C (86°F), and no more than three current-carrying conductors in the raceway. If your run exceeds 50 feet, you must calculate voltage drop.
Baseline Assumptions: Copper conductor, THHN/THWN-2 or NM-B insulation, 30°C (86°F) ambient temperature, installed in a standard conduit or cable with a maximum of three current-carrying conductors, and standard 75°C rated terminations. Non-continuous load (under 3 hours).

The Baseline Spec Sheet: 10 AWG Copper at 30 Amps

When determining what size wire for 30 amps is required, we start with the National Electrical Code (NEC) Table 310.16. For 10 AWG copper wire, the ampacity varies by temperature column: 30A in the 60°C column, 35A in the 75°C column, and 40A in the 90°C column. However, you cannot simply pick the highest number.

NEC 110.14(C) dictates that the ampacity of a conductor must be selected based on the lowest temperature rating of any connected termination, conductor, or device. Since most standard residential breakers (like Square D Homeline or Siemens QT series) and receptacles are rated for 75°C, we are legally bound to the 75°C column, which lists 10 AWG copper at 35 Amps.

Furthermore, NEC 240.4(D) establishes specific overcurrent protection limits for small conductors. It explicitly states that the overcurrent protection for 10 AWG copper shall not exceed 30 Amperes. Therefore, even though the wire can technically handle 35A of heat dissipation at 75°C, the breaker must be capped at 30A. This pairing—10 AWG copper and a 30A breaker—is the bedrock of circuits powering heavy appliances like RV outlets (TT-30), window air conditioners, and heavy-duty workshop equipment.

Why 10 AWG and Not 12 AWG? The Physics and the Code

A common mistake among novice DIYers is looking at the 90°C column in Table 310.16, seeing that 12 AWG THHN is rated for 30 Amps, and attempting to use it on a 30A breaker. This is a severe code violation and a fire hazard for two distinct reasons.

First, the physics of heat dissipation. When current flows through a conductor, it generates heat proportional to the square of the current (I²R). A 12 AWG wire has a smaller cross-sectional area (6,530 circular mils) compared to 10 AWG (10,380 circular mils). Pushing 30 Amps through 12 AWG wire will cause it to heat up significantly faster. If that wire is buried in insulation or routed through a hot attic, the heat cannot escape, degrading the insulation and eventually causing a short circuit.

Second, the legal code restriction. NEC 240.4(D)(4) strictly limits 12 AWG copper to a maximum 20-amp breaker. There are no exceptions for standard branch circuits. If you install a 30A breaker on 12 AWG wire, the breaker will not trip until the current exceeds 30A, but the wire will begin to melt and fail at sustained loads above 20A. The breaker is there to protect the wire, not the appliance; using an oversized breaker on undersized wire defeats the entire purpose of the overcurrent protective device.

Decision Tree: When to Upsize Your Wire

The 10 AWG baseline only holds true under perfect conditions. Real-world jobsites introduce variables that force you to upsize. Use the decision matrix below to determine your exact wire and breaker pick.

Installation Condition Code / Physics Constraint Concrete Pick (Wire + Breaker)
Run under 50 ft, 1-3 conductors in conduit Standard NEC 310.16 ampacity applies. 10 AWG Cu + 30A Breaker
Run between 50 ft and 80 ft (120V circuit) Voltage drop exceeds 3% recommendation. 8 AWG Cu + 30A Breaker
4 to 6 current-carrying conductors in one conduit NEC 310.15(C)(1) requires 80% derating. 10 AWG (35A * 0.8 = 28A) is now underprotected. 8 AWG Cu + 30A Breaker
Using Aluminum wire instead of Copper Aluminum has higher resistance. 8 AWG Al is rated 40A at 75°C, safely clearing the 30A hurdle. 8 AWG Al + 30A Breaker
Load is 'Continuous' (runs for 3+ hours) NEC 210.20(A) requires 125% sizing. 30A * 1.25 = 37.5A minimum circuit ampacity. 8 AWG Cu + 40A Breaker
Aluminum Termination Warning: If you choose 8 AWG Aluminum to save on material costs, you must use an anti-oxidant compound (like Noalox) on the terminations and torque the lugs to the manufacturer's exact specification using a calibrated torque screwdriver, per NEC 110.14(D). Aluminum is prone to cold creep and oxidation, which causes loose connections and arcing fires if not torqued correctly.

Voltage Drop Check: The 50-Foot Threshold

Ampacity tells you if the wire will melt; voltage drop tells you if your appliance will actually work. The NEC recommends (in Informational Note 4 to 210.19) that branch circuit voltage drop be limited to 3% for optimal efficiency.

Let us run a concrete numeric example. Suppose you are wiring a 120V, 30-amp RV pedestal outlet (TT-30R) located 80 feet from the main panel. Using the standard voltage drop formula: VD = (2 × K × I × L) / CM.

  • K (Copper resistivity) = 12.9
  • I (Current) = 30 Amps
  • L (One-way length) = 80 feet
  • CM (Circular mils for 10 AWG) = 10,380

VD = (2 × 12.9 × 30 × 80) / 10,380 = 5.96 Volts.

On a 120V circuit, a 5.96V drop represents a 4.96% loss. This exceeds the 3% branch circuit recommendation and approaches the 5% total system limit. Your RV's air conditioner compressor may struggle to start, drawing locked-rotor amps and potentially tripping the breaker or damaging the compressor windings.

The Fix: Upsize to 8 AWG copper (CM = 16,510). Recalculating: VD = (2 × 12.9 × 30 × 80) / 16,510 = 3.75 Volts (3.1%). While still slightly above the ideal 3%, it is vastly superior and represents the practical limit before jumping to an expensive 6 AWG run. For 240V circuits (like a dryer or welder), the 5.96V drop on 10 AWG represents only 2.48%, which is perfectly acceptable, meaning 10 AWG remains valid for 240V runs up to about 100 feet.

When an Engineer or the AHJ Must Confirm

While the decision tree above covers 95% of residential and light-commercial scenarios, certain edge cases require formal review by a licensed Professional Engineer (PE) or your local Authority Having Jurisdiction (AHJ / electrical inspector).

First, if your ambient temperature exceeds 30°C (86°F). If you are routing THHN through an attic in a southern climate where temperatures routinely hit 110°F (43°C), you must apply the temperature correction factors in NEC Table 310.15(B)(1). At 41-45°C, the correction factor for 90°C insulation is 0.87. You must calculate the derated ampacity and likely upsize to 8 AWG just to maintain baseline thermal safety.

Second, if the circuit involves utility interconnection, such as a 30-amp backfeed breaker for a solar inverter or a standby generator transfer switch. The available fault current at the panel busbars may exceed the AIC (Ampere Interrupting Capacity) rating of standard breakers, requiring specific breaker selections and potentially altering wire routing requirements.

Always pull a permit for a new 30-amp circuit. Local inspectors have the final say on whether your specific installation meets the adopted version of the NFPA 70 (National Electrical Code) in your municipality.

Final Verdict: For a standard, non-continuous 30-amp load under 50 feet in a normal ambient environment, buy 10 AWG copper wire and a 30-amp breaker. If your run is longer, bundled with other wires, or powering a continuous load, step up to 8 AWG copper. Never use 12 AWG, and never defeat the breaker to accommodate an undersized wire.