30 Amp Wiring Size Guide: What Wire Gauge for a 30A Breaker?

The correct 30 amp wiring size is 10 AWG copper wire, which safely carries up to 30 amps of current without exceeding its thermal limits under standard National Electrical Code (NEC) conditions. Selecting the right gauge dictates the physical diameter of the conductor, which directly controls resistive heating, voltage drop over distance, and whether the wire will physically fit into the breaker's terminal lugs. DIYers frequently confuse the breaker rating with the wire's actual ampacity, mistakenly assuming 12 AWG wire (rated for 20A) is "close enough" for a short 30A run—a severe fire hazard. Another common mix-up is applying copper ampacity tables to aluminum wire, which requires a thicker gauge for the same current.

The Core Rule: Sizing Wire for a 30 Amp Breaker

When sizing conductors, the NEC requires you to look at the temperature rating of the weakest link in the circuit. Most residential breakers and receptacles are rated for 75°C, but NEC 110.14(C)(1)(a) generally mandates using the 60°C ampacity column for circuits 100A or less, unless the equipment is specifically listed and marked for 75°C terminations. Fortunately, 10 AWG copper is rated 30A at 60°C, 35A at 75°C, and 40A at 90°C. This makes 10 AWG copper the universal, code-compliant baseline for a 30-amp breaker regardless of the terminal rating.

If you are using aluminum wire (common in older feeder runs or specific utility applications), 10 AWG aluminum is only rated for 25A at 60°C. You must step up to 8 AWG aluminum to safely carry 30 amps.

Where You Meet 30 Amp Wiring Size in Practice

You will rarely see a 30-amp circuit powering standard household receptacles. This specific amperage is reserved for dedicated, high-draw appliances and specialized outlets. Here is where 10 AWG wire shows up on the jobsite:

• RV Receptacles (TT-30R): The standard 120V, 30-amp travel trailer outlet uses 10/2 NM-B with ground. The wiring consists of one hot (black), one neutral (white), and one ground (bare). It is critical not to confuse this with a 240V dryer outlet; wiring a TT-30R with 240V will instantly destroy the RV's power converter.
• Solar Charge Controllers: High-current MPPT charge controllers (like the Victron SmartSolar 100/30 or 150/35) often require 10 AWG stranded THHN or flexible welding cable for the battery bank connection to handle the continuous 30A output without voltage sag.
• Compact Electric Dryers & Point-of-Use Water Heaters: While standard full-size dryers require 30A or 40A (often 10/3 or 8/3), some 240V compact ventless dryers and small tankless water heaters use a dedicated 240V 30A circuit. This requires 10/2 with ground (two hots and a ground, no neutral).
• HVAC Disconnects: The outdoor service disconnect for a 2-ton or 2.5-ton central air conditioning condenser unit is frequently protected by a 30A breaker or fused disconnect, fed by 10 AWG THHN pulled through liquid-tight metallic conduit.

Bench Tip: 10 AWG solid copper (like you find in yellow NM-B Romex) is notoriously stiff. When wiring a tight RV receptacle box or a small HVAC disconnect, the physical tension of the wire can pull the terminals loose over time if not seated perfectly. Where code permits, using 10 AWG stranded THHN wire makes terminations significantly more reliable and easier to torque.

Worked Example: Voltage Drop and Distance Limits

Ampacity tells you the wire won't melt, but it doesn't guarantee your appliance will actually work. If a wire is too long, resistance causes voltage drop. The NEC recommends a maximum 3% voltage drop on branch circuits for reasonable efficiency. Let's calculate the exact limits for a 30 amp wiring size over distance.

The Scenario: You are wiring a 240V baseboard heater drawing a full 30A. The one-way distance from the panel to the heater is 80 feet. You plan to use 10 AWG copper.

The Math:
VD = (2 × 12.9 × 30 × 80) / 10,380
VD = 61,920 / 10,380 = 5.96 Volts

The Result: 5.96V divided by 240V equals a 2.48% voltage drop. This is well under the 3% limit, meaning 10 AWG copper is perfectly adequate for this 80-foot 240V run.

The 120V Trap: What if you were wiring a 120V, 30A RV outlet at that same 80-foot distance? The voltage drop in absolute terms remains 5.96V. However, 5.96V divided by 120V equals a 4.97% voltage drop. This exceeds the 3% recommendation and will cause noticeable dimming and poor appliance performance. For a 120V, 30A circuit at 80 feet, you must upsize to 8 AWG copper to keep the drop under 3%.

For complex runs, you can verify these manual calculations using the Southwire Voltage Drop Calculator, which accounts for AC impedance and power factor in addition to basic DC resistance.

Common Mistakes and Code Caveats

Even when you buy the right 10 AWG wire, installation errors can compromise the circuit. Avoid these frequent jobsite and workbench mistakes:

1. The "Short Run" Fallacy: Never use 12 AWG wire on a 30A breaker just because the run is only 5 feet long. The breaker protects the wire. If a fault draws 35A, a 30A breaker will not trip immediately, but 12 AWG wire will rapidly overheat and melt its insulation. The wire gauge must always match or exceed the breaker size.
2. Misunderstanding the "Next Size Up" Rule: NEC 240.4(B) allows you to use the next standard breaker size up if the wire ampacity doesn't match a standard breaker. However, this only applies if the calculated load is lower than the wire's ampacity. You cannot put 10 AWG wire (30A at 60°C) on a 35A breaker just because you ran out of 30A breakers. The 30A breaker is a standard size, so the next-size-up rule does not apply here.
3. Ignoring Torque Specifications: 10 AWG wire requires significant mechanical pressure to maintain a low-resistance connection. A loose lug will arc and overheat under a 30A load. Most modern 30A breakers (like the Square D QO or Homeline series) require roughly 25 inch-pounds of torque. Use a calibrated torque screwdriver; guessing by hand is insufficient.
4. Continuous Load Derating: If the 30A load will run for 3 hours or more (like an EV charger or a continuous space heater), NEC 210.20(A) requires the breaker and wire to be sized at 125% of the continuous load. A true 30A continuous load requires a 40A breaker and 8 AWG wire. A 30A breaker and 10 AWG wire are only legal for a maximum continuous load of 24A.