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, heavy-duty window AC units, or small subpanels. But this baseline assumes specific installation conditions that we must verify before you pull any wire.
The Baseline: 10 AWG Copper and the 75°C Column
Before we look at edge cases, we need to establish the baseline assumptions for this sizing. Wire ampacity isn't a single universal number; it changes based on insulation, ambient heat, and how the wire is installed.
- Material: Solid or stranded Copper
- Temperature Rating: 75°C column (standard for modern residential breakers and terminals)
- Ambient Temperature: 30°C (86°F) or lower
- Installation Method: Installed in a raceway (EMT conduit) or as part of a standard cable assembly (like NM-B/Romex)
- Current-Carrying Conductors: 3 or fewer in the same conduit/raceway
Under these conditions, we reference NEC Table 310.16 to find our allowable ampacity. Here is the exact spec-sheet row for 10 AWG copper:
| Wire Size (AWG) | 60°C Column (Amps) | 75°C Column (Amps) | 90°C Column (Amps) |
|---|---|---|---|
| 10 AWG Copper | 30A | 35A | 40A |
Why 10 AWG and Not One Size Smaller?
You might look at the 60°C column and think 10 AWG is rated for exactly 30A, so why not use 12 AWG? Under NEC Table 310.16, 12 AWG copper is rated for 25A at 75°C. However, NEC 240.4(D) places strict overcurrent protection limits on small conductors. It explicitly forbids protecting 12 AWG copper with anything larger than a 20-amp breaker. If you put 12 AWG wire on a 30-amp breaker, a 28-amp fault will melt the wire's insulation and start a fire inside your walls before the breaker ever trips. 10 AWG is the absolute minimum legal size for a 30A breaker.
When 10 AWG Isn't Enough: Voltage Drop and Derating
The NEC baseline assumes a relatively short run and standard temperatures. In the real world, physics and installation realities often force us to upsize.
The Voltage Drop Check (100-Foot Run)
Ampacity tells you what the wire can handle without melting; voltage drop tells you what the appliance will actually receive. Let's check a 240V, 30A circuit running to a detached garage or RV pedestal 100 feet away. We use the standard single-phase voltage drop formula: VD = (2 × K × I × L) / CM.
- K (Copper resistivity): 12.9
- I (Current): 30A
- L (One-way length): 100 ft
- CM (Circular mils for 10 AWG): 10,380
Plugging in the numbers: (2 × 12.9 × 30 × 100) / 10,380 = 7.45 Volts dropped.
Divide 7.45V by 240V, and you get a 3.1% voltage drop. The NEC recommends keeping branch circuit voltage drop under 3%. At 3.1%, your RV's air conditioner compressor might struggle to start on a hot day, drawing locked-rotor amps and overheating. Fix: Bump up to 8 AWG copper for any 30A run exceeding 90 feet at full load. You can verify your specific runs using the Southwire Voltage Drop Calculator.
Conduit Bundling and Derating
What if you are pulling wire through EMT conduit and sharing the pipe with other circuits? Heat builds up when wires are bundled. NEC Table 310.15(C)(1) requires you to 'derate' the ampacity.
If you pull four to six current-carrying conductors in one conduit, you must apply an 80% derating factor. Here is where the 90°C column saves you: if you use THHN wire (rated 90°C), you start with 40A. 40A × 0.80 = 32A. Because 32A is still greater than your 30A breaker, 10 AWG THHN is still legal. However, if you pull seven to nine conductors (70% derating: 40A × 0.70 = 28A), you fall below 30A. You must step up to 8 AWG copper.
Aluminum vs. Copper: The 8 AWG Shift
Never treat aluminum and copper interchangeably. Aluminum has higher electrical resistance, expands and contracts more under heat (leading to loose connections), and requires specific termination techniques.
If you are running a feeder to a subpanel or an underground URD cable to an RV pedestal using aluminum, you must step up to 8 AWG aluminum. While 10 AWG aluminum technically holds 30A at 75°C on paper, the mechanical fragility of 10 AWG aluminum and the high risk of terminal creep make it a poor choice for 30A lugs. 8 AWG aluminum is rated for 40A at 75°C, giving you a safe thermal buffer and accommodating the higher resistance of the material.
- Only terminate aluminum wire on breakers and lugs explicitly marked CU/AL or ALR.
- Always coat stripped aluminum conductors with an anti-oxidant paste like Noalox or Penetrox to prevent aluminum oxide buildup, which is highly resistive and causes terminal fires.
- Torque aluminum lugs to the exact inch-pound specification printed on the breaker label; aluminum cold-flows and will loosen if under-torqued.
Decision Matrix: Do You Need an Engineer or AHJ Review?
Most 30-amp circuits (dryers, RV plugs, window ACs) are straightforward. But certain conditions push the project out of standard DIY territory and require a review by a licensed electrical engineer or your local Authority Having Jurisdiction (AHJ / electrical inspector).
| Installation Condition | Action Required | Who Confirms? |
|---|---|---|
| Standard run < 90 ft, copper, < 30°C ambient | Use 10 AWG Copper, 30A Breaker | DIY / Standard Inspection |
| Continuous load (runs 3+ hours, e.g., kiln, heater) | Upsize to 8 AWG Copper, 40A Breaker (NEC 210.20(A) 125% rule) | DIY / Standard Inspection |
| Ambient temp > 113°F (45°C) (e.g., hot attic) | Apply Table 310.15(B)(1) temp correction factors; likely upsize to 8 AWG | Electrician / AHJ |
| Mixed wire types or transitioning from overhead to underground | Verify splice ratings, grounding electrode requirements, and physical protection | Engineer / AHJ |
For deeper code interpretations, especially regarding continuous loads and derating, resources like EC&M's National Electrical Code hub provide excellent real-world scenarios and inspector perspectives.
Frequently Asked Questions
Can I use 12 gauge wire for a 30 amp breaker if the run is short?
No. NEC 240.4(D) strictly prohibits protecting 12 AWG copper wire with a breaker larger than 20 amps, regardless of how short the run is. The breaker's job is to protect the wire, not the appliance. If a fault pulls 28 amps, a 30-amp breaker won't trip, but the 12 AWG wire will overheat and melt. Always use a minimum of 10 AWG for a 30-amp breaker.
What gauge wire for a 30 amp 240V circuit compared to 120V?
The wire gauge remains exactly the same: 10 AWG copper. Ampacity is determined by the current (amps) flowing through the wire and the heat it generates, not the voltage. Whether you are pushing 30 amps at 120V (3,600 watts) or 30 amps at 240V (7,200 watts), the physical wire experiences the exact same thermal stress. The only difference is that the 240V circuit will suffer half the percentage of voltage drop over the same distance.
What size wire for a 30 amp RV plug at the pedestal?
For a standard NEMA TT-30R (120V, 30A) receptacle, use 10 AWG copper wire. However, RV pedestals are often located far from the main service panel. If the one-way wire run from your breaker panel to the pedestal exceeds 90 feet, you must upsize to 8 AWG copper to keep the voltage drop under the recommended 3% threshold, ensuring your RV's air conditioning and microwave can operate simultaneously without tripping the RV's internal main breaker.
Can I use 10 AWG wire on a 20 amp breaker?
Yes, it is perfectly legal and safe to use a larger wire on a smaller breaker. 10 AWG wire on a 20-amp breaker will run incredibly cool and have virtually zero voltage drop. The only downsides are cost (10 AWG is more expensive) and physical termination difficulty; 10 AWG is stiffer and might be hard to fold into a standard single-gang junction box or fit under the smaller terminal screws of a 20-amp receptacle.
