For a standard 30-amp circuit, use 10 AWG copper wire paired with a 30-amp breaker. This applies to typical household loads like RV receptacles, dryers, or heavy power tools, assuming you are using THHN in conduit or NM-B cable, with an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors.
- Material: Solid or stranded copper (aluminum requires 8 AWG).
- Temperature Rating: 60°C for NM-B (Romex), 75°C for THHN/THWN-2 in conduit.
- Ambient Temperature: 30°C (86°F) or lower.
- Conduit Fill: No more than 3 current-carrying conductors in the raceway.
- Load Type: Non-continuous (operating for less than 3 hours continuously).
The Baseline Sizing Rules and NEC Ampacity
Wire sizing is not a guessing game; it is strictly governed by the National Electrical Code (NEC), specifically NFPA 70 (NEC) Article 240 for overcurrent protection and Article 310 for conductor ampacity. The breaker's sole job is to protect the wire from melting, not to protect the appliance. Therefore, the wire's ampacity must meet or exceed the breaker's rating.
According to standard manufacturer ampacity charts aligned with NEC Table 310.16, 10 AWG copper wire is rated for 30 amps in the 60°C column (used for NM-B/Romex) and 35 amps in the 75°C column (used for THHN in conduit). Because 30 amps is a standard breaker size, 10 AWG is the exact mathematical and legal fit.
| Wire Gauge (AWG) | 60°C Column (NM-B) | 75°C Column (THHN) | 90°C Column (Derating Only) | Max Standard Breaker |
|---|---|---|---|---|
| 12 AWG Copper | 20A | 25A | 30A | 20A (25A rare) |
| 10 AWG Copper | 30A | 35A | 40A | 30A |
| 8 AWG Copper | 40A | 50A | 55A | 40A / 50A |
Why 10 AWG and Not One Size Smaller?
A common DIY mistake is attempting to use 12 AWG wire on a 30-amp breaker because the wire "seems thick enough." Looking at the table above, 12 AWG is rated for 20A (60°C) or 25A (75°C). The NEC requires that the overcurrent device be rated no higher than the conductor's ampacity (with specific exceptions for next-standard-size-up rules). If you push 28 amps through a 12 AWG wire, the wire will overheat, degrade its insulation, and potentially start a fire inside your walls. A 30-amp breaker will not trip at 28 amps, meaning it fails to protect the 12 AWG wire. You must step up to 10 AWG.
When 10 AWG Isn't Enough: Derating and Voltage Drop
The baseline answer of 10 AWG assumes perfect conditions. In the real world, physics and installation environments frequently force you to upsize to 8 AWG copper. Here are the two primary culprits:
1. Voltage Drop Over Distance
The NEC recommends keeping voltage drop under 3% for branch circuits to ensure equipment operates efficiently. Copper has resistance, and over long distances, that resistance robs your appliance of voltage.
Let's run the math for a 30-amp load on 10 AWG copper (which has a resistance of roughly 1.21 ohms per 1,000 feet at 75°C):
- For a 120V Circuit (e.g., TT-30R RV Outlet): A 3% drop is 3.6 volts. At 30 amps, 10 AWG wire will hit this 3.6V drop at just 50 feet. If your panel is more than 50 feet from the RV pad, you must upsize to 8 AWG.
- For a 240V Circuit (e.g., Dryer or Welder): A 3% drop is 7.2 volts. Because the voltage is doubled, the allowable distance doubles. 10 AWG will hit the 3% drop threshold at roughly 100 feet. Beyond 100 feet, switch to 8 AWG.
2. Conduit Bundling and Ambient Heat
When you bundle multiple current-carrying conductors in a single conduit, they heat each other up. NEC Table 310.15(C)(1) mandates ampacity derating. If you pull four to six current-carrying conductors through a conduit, you must derate the wire's 90°C ampacity to 80%.
For 10 AWG, the 90°C rating is 40A. Derated to 80%, it becomes 32A. This still safely covers a 30A breaker. However, if you have seven to nine conductors in the pipe, the derating factor drops to 70%. 40A × 0.70 = 28A. Your wire is now only legally allowed to carry 28 amps, meaning it is undersized for a 30-amp breaker. In this scenario, you must pull 8 AWG wire.
Decision Matrix: Adjusting for Real-World Conditions
Use this decision tree to finalize your wire and breaker sizing based on your specific jobsite conditions.
| Installation Scenario | Condition Trigger | Required Wire Size | Breaker Size |
|---|---|---|---|
| Standard residential run under 50ft (120V) or 100ft (240V) | Normal ambient, <4 conductors, non-continuous load | 10 AWG Copper | 30A |
| Long distance run exceeding voltage drop limits | >50ft (120V) or >100ft (240V) | 8 AWG Copper | 30A |
| Using Aluminum wire instead of Copper | Aluminum has lower ampacity per AWG | 8 AWG Aluminum | 30A |
| Continuous Load (e.g., EV charger running 4+ hours) | NEC requires 125% sizing for continuous loads (30A × 1.25 = 37.5A) | 8 AWG Copper | 40A |
| High ambient temperature (e.g., 110°F attic) | Temperature correction factors reduce ampacity | 8 AWG Copper | 30A |
Frequently Asked Questions
What size of wire for a 30 amp 240V dryer or 120V RV outlet?
This is a critical distinction that causes many dangerous wiring errors. A standard 30-amp RV receptacle (NEMA TT-30R) is actually 120 volts, requiring a single-pole 30A breaker and 10/2 NM-B cable (two current-carrying wires plus a ground). Conversely, a 30-amp dryer receptacle (NEMA 14-30R) is 240 volts, requiring a double-pole 30A breaker and 10/3 NM-B cable (three current-carrying wires plus a ground). Never wire a TT-30R with 240V, or you will instantly destroy the RV's electronics and create a severe shock hazard.
Can I use 8 AWG wire on a 30 amp breaker?
Yes. In electrical work, it is always safe to use a larger wire (which has a lower AWG number) than the minimum required, provided the wire physically fits into the breaker's lug terminals. Using 8 AWG on a 30-amp breaker is highly recommended for long runs to mitigate voltage drop, or if you simply have leftover 8 AWG stock from a previous 40-amp project. The 30A breaker will still trip exactly at 30 amps, perfectly protecting the oversized wire.
What size aluminum wire do I need for 30 amps?
You must use 8 AWG aluminum for a 30-amp circuit. Never use 10 AWG aluminum. According to the 75°C column of NEC Table 310.16, 10 AWG aluminum is only rated for 25 amps. If you place it on a 30-amp breaker, you are violating code and creating a fire hazard. Furthermore, ensure your breaker terminals are explicitly rated for aluminum (marked AL/CU) and apply an approved antioxidant paste (like Noalox) to the stripped aluminum strands before torquing the lug to the manufacturer's spec to prevent galvanic corrosion and high-resistance heating.
When must I have an engineer or AHJ confirm my wire sizing?
While the guidelines above cover 95% of residential and light commercial applications, you must consult a licensed Professional Engineer (PE) or your local Authority Having Jurisdiction (AHJ) if your installation involves high ambient temperatures exceeding 50°C (122°F), complex conduit derating involving more than nine current-carrying conductors, or specialized industrial continuous loads. Furthermore, if your local municipality has amended the NEC to require AFCI/GFCI protection on 30-amp receptacles in specific locations, the AHJ inspector will have the final say on compliance. Always pull a permit for new 30-amp circuits.
