- Material: Copper conductors only (aluminum requires different sizing).
- Temperature Rating: 75°C terminations (standard for modern breakers/receptacles), with NM-B cable limited to the 60°C column per NEC 334.80.
- Ambient Temperature: 30°C (86°F) or lower.
- Conduit/Raceway: EMT, PVC, or standard NM-B romex with no more than 3 current-carrying conductors (CCCs) bundled together.
- Receptacle Type: NEMA TT-30 (120V, 30A, 3-prong), standard for travel trailers and smaller motorhomes.
The Direct Answer and Why 12 AWG Fails
The National Electrical Code (NEC) mandates that the overcurrent protection device (breaker) must protect the weakest link in the circuit. For a 30-amp circuit, 10 AWG copper is the minimum allowable size.
A common DIY mistake is attempting to use 12 AWG wire because it is physically easier to bend and terminate in tight weatherproof boxes. However, 12 AWG copper is only rated for 20 amps in the 60°C column and 25 amps in the 75°C column. If you pull a sustained 28-amp load (like an RV air conditioner plus a microwave) through 12 AWG wire on a 30-amp breaker, the breaker will not trip, but the wire will overheat. This degrades the insulation over time and creates a severe fire hazard inside the walls or conduit. NEC 240.4 strictly prohibits protecting 12 AWG wire with a 30-amp breaker.
NEC Ampacity Rules: NM-B vs THHN
When sizing wire, you must reference NEC Table 310.16 (often published by wire manufacturers like Cerrowire or Southwire). The allowable ampacity depends heavily on the insulation type you are pulling.
| Wire Size (AWG) | 60°C Column (NM-B / Romex) | 75°C Column (THHN in Conduit) | 90°C Column (THHN Derating Base) |
|---|---|---|---|
| 12 AWG | 20A | 25A | 30A |
| 10 AWG | 30A | 35A | 40A |
| 8 AWG | 40A | 50A | 55A |
The NM-B Catch: If you are running 10/2 NM-B (Romex) cable from your subpanel to the RV pedestal, NEC 334.80 dictates that you must use the 60°C column for ampacity, regardless of the fact that the THHN wires inside the jacket are technically rated for 90°C. Fortunately, 10 AWG in the 60°C column is exactly 30 amps, making it perfectly compliant for a 30-amp breaker.
The THHN Advantage: If you pull individual 10 AWG THHN/THWN-2 wires through PVC or EMT conduit, you use the 75°C column (assuming your breaker and TT-30 receptacle terminals are rated for 75°C). Here, 10 AWG is rated for 35 amps. You still use a 30-amp breaker to match the receptacle rating, but the wire runs cooler and has a slight thermal buffer.
Voltage Drop: When 10 AWG Isn't Enough
Ampacity tells you what size wire will prevent a fire. Voltage drop tells you what size wire will actually let your RV appliances run properly. The NEC recommends a maximum 3% voltage drop on branch circuits. For a 120V RV plug, 3% equals a 3.6V drop, meaning your RV should see no less than 116.4V under full load.
RV air conditioners draw massive startup currents (Locked Rotor Amps). If your voltage sags below 115V due to a long wire run, the compressor will struggle to start, overheat, and eventually trip its internal thermal overload or destroy the capacitor.
| One-Way Run Distance | Recommended Wire Size | Estimated Voltage Drop (at 30A) | Drop Percentage | Verdict |
|---|---|---|---|---|
| 25 feet | 10 AWG Copper | 1.7V | 1.4% | Pass |
| 50 feet | 10 AWG Copper | 3.4V | 2.8% | Pass (Limit) |
| 75 feet | 10 AWG Copper | 5.1V | 4.2% | Fail (Upsize) |
| 100 feet | 8 AWG Copper | 4.1V | 3.4% | Acceptable* |
| 150 feet | 6 AWG Copper | 3.7V | 3.1% | Pass |
*Note: While 8 AWG at 100 feet yields a 3.4% drop (slightly over the 3% ideal), it is generally acceptable for RV pedestals where the load is intermittent. For continuous 30A loads, step up to 6 AWG at 100 feet.
Derating, Aluminum, and AHJ Sign-Off
Several real-world jobsite conditions will force you to abandon 10 AWG copper and upsize your wire, regardless of the run length.
Conduit Bundling (Derating): If you are pulling wires for multiple RV pedestals through a single large PVC conduit, you must apply NEC 310.15(C)(1) adjustment factors. If you have 4 to 6 current-carrying conductors in one pipe, you must derate the ampacity to 80%. The 90°C column for 10 AWG is 40A. 80% of 40A is 32A. This still technically covers a 30A breaker, but if you have 7 to 9 conductors (derated to 70%), 10 AWG drops to 28A and is no longer legal for a 30A breaker. You must upsize to 8 AWG.
High Ambient Heat: If your conduit runs across a sun-baked black roof or through an unventilated attic in the Southwest where ambient temperatures exceed 113°F (45°C), you must apply temperature correction factors. At 122°F (50°C), the 90°C THHN ampacity is multiplied by 0.82. Again, this eats into your safety margin and often requires upsizing to 8 AWG.
The Aluminum Question: Never present aluminum and copper interchangeably. 10 AWG aluminum is only rated for 25A. To carry 30A with aluminum, you need a minimum of 8 AWG (rated 40A at 75°C). However, most standard NEMA TT-30 receptacles feature copper-alloy terminals that are not UL-listed for aluminum wire, or they require specific anti-oxidant compound (like Noalox) and precise torque settings. For short pedestal runs, the cost savings of aluminum are negligible compared to the termination risks. Stick to copper.
When to Call the AHJ: You must consult your local Authority Having Jurisdiction (AHJ) or a licensed professional engineer if your run exceeds 200 feet (which may require parallel conductors or a step-up transformer), if you are installing a multi-pedestal RV park setup requiring complex load calculations per NFPA 70 (NEC) Article 551, or if your local municipality has adopted amendments that strictly mandate GFCI protection on outdoor RV receptacles (a known pain point due to nuisance tripping from RV inverters).
Frequently Asked Questions
Can I use 8 AWG wire instead of 10 AWG for my TT-30 receptacle?
Yes, upsizing your wire is always electrically safe and code-compliant, provided the wire physically fits into the termination lugs. 8 AWG copper will run cooler and reduce voltage drop. However, some cheaper NEMA TT-30 receptacles have small pressure plates designed strictly for 10 AWG. If you use 8 AWG, you may need to trim a few strands (which is bad practice) or pigtail the 8 AWG to a short 10 AWG whip inside the box using a properly sized wire nut or Wago connector. Always check the receptacle manufacturer's spec sheet for maximum wire size.
What size wire for a 30 amp RV plug if the run is over 100 feet?
For a 120V, 30-amp circuit stretching past 100 feet, 10 AWG copper will result in a voltage drop exceeding 5%, which can damage RV air conditioner compressors. You must upsize to 6 AWG copper wire to keep the voltage drop under the recommended 3% threshold at full 30-amp load. If the run is exactly 100 feet, 8 AWG is the absolute minimum, but 6 AWG is the professional standard for long trench runs.
Do I need a GFCI breaker for an outdoor 30 amp RV pedestal?
Under NEC 2020 and 2023 (specifically 210.8(F)), GFCI protection is required for all outdoor receptacles rated 150 volts to ground or less, and 50 amperes or less. This technically includes 30-amp TT-30 RV receptacles. However, many RV owners and electricians report severe nuisance tripping issues because RV onboard converters, inverters, and older appliances leak small amounts of current to ground. While the NEC mandates it, some local AHJs have issued temporary exceptions or specific interpretations for dedicated RV park pedestals. Always verify with your local inspector before purchasing a $150 30-amp GFCI breaker.
