If you are wiring a heavy-duty appliance or an RV pedestal, the direct answer to what size Romex for 30 amp circuits you need is 10 AWG copper. Romex (technically known as NM-B cable) sized for a 30-amp circuit requires 10 AWG copper conductors, which have a baseline ampacity of 30 amps under the 60°C temperature column of the National Electrical Code (NEC). Using the correct wire size ensures the cable can safely carry the continuous thermal load without the insulation degrading, melting, or creating a hidden fire hazard inside your walls.
The Core NEC Rule: Why 10 AWG is the 30-Amp Standard
When determining wire size, electricians rely on the National Electrical Code (NEC) published by the NFPA. Two specific articles dictate why 10 AWG is the absolute minimum for a 30-amp breaker when using Romex (NM-B).
While the individual THHN conductors inside a modern Romex jacket are technically rated for 90°C, the outer PVC jacket cannot handle that heat. Therefore, you must use the 60°C column in NEC Table 310.16. In that column, 10 AWG copper is rated for exactly 30 amps.
Furthermore, NEC Article 240.4(D)—often called the 'small conductor rule'—explicitly caps the overcurrent protection for 10 AWG copper at 30 amps. Think of wire gauge like a hallway; a 30-amp crowd generates heat, and a 10 AWG hallway is the minimum width to let that heat escape without melting the walls. If you attempt to protect 12 AWG wire (rated for 20A) with a 30A breaker, the wire will overheat and catch fire long before the breaker ever trips.
Worked Numeric Example: Voltage Drop on a 50-Foot Run
Ampacity isn't the only factor; voltage drop changes how a real circuit performs over distance. Let's calculate the voltage drop for a 120V, 30-amp RV outlet (NEMA TT-30R) located 50 feet from the main panel using 10/2 Romex.
The formula for single-phase voltage drop is:
Vd = (2 × K × I × L) / CM
- K (Copper resistivity) = 12.9 ohms
- I (Current) = 30 amps
- L (One-way length) = 50 feet
- CM (Circular mils for 10 AWG) = 10,380
Calculation:
Vd = (2 × 12.9 × 30 × 50) / 10,380
Vd = 38,700 / 10,380
Vd = 3.72 Volts
To find the percentage: (3.72V / 120V) × 100 = 3.1%.
The NEC recommends a maximum voltage drop of 3% for branch circuits. At exactly 50 feet, 10 AWG Romex is right on the edge of acceptable performance. If your run is 75 feet, the drop jumps to 4.65%, which can cause RV air conditioners to stall or overheat. In that scenario, you must upsize to 8 AWG wire for the run, even though the breaker remains 30 amps.
Where You Meet This in Practice
You will typically encounter the 30-amp Romex requirement in specific high-draw residential and hobbyist applications. Knowing what changes in a real installation helps you choose between 10/2 and 10/3 cable.
| Application | Receptacle Type | Romex Configuration | Why It Matters |
|---|---|---|---|
| RV Pedestal (120V) | NEMA TT-30R | 10/2 NM-B | Requires one hot, one neutral, one ground. 30A at 120V. |
| Heavy Air Compressor | NEMA L6-30R (Twist-lock) | 10/2 NM-B | Straight 240V. Requires two hots and a ground; no neutral needed. |
| Older Electric Dryer | NEMA 10-30R | 10/3 NM-B | 120/240V. Legacy 3-prong setup (two hots, one neutral acting as ground). |
| Modern Electric Dryer | NEMA 14-30R | 10/3 NM-B | 120/240V. Requires two hots, one neutral, and a separate ground. |
Common Confusion: People frequently confuse the physical wire gauge (10 AWG) with the number of conductors (the '/2' or '/3'). 'What size Romex' refers strictly to the thickness of the copper (10 AWG), while the application dictates whether you need two or three insulated conductors inside the jacket.
The Temperature Column Trap: Romex vs. THHN
The most common mistake DIYers make is looking at a spool of individual THHN wire in conduit, seeing that 10 AWG THHN is rated for 40 amps in the 90°C column, and assuming they can put a 40-amp breaker on 10 AWG Romex. This is a severe code violation and a fire hazard.
Individual THHN wires in conduit can dissipate heat differently, and their 90°C insulation can handle higher temperatures. However, NEC 240.4(D) overrides the 90°C column for small conductors. Regardless of whether you are using 90°C THHN in conduit or 90°C THHN inside a Romex jacket, the overcurrent protection for 10 AWG copper is hard-capped at 30 amps. You can use the 90°C column for derating purposes (like adjusting for high ambient attic temperatures), but the final breaker size can never exceed 30A for 10 AWG.
Frequently Asked Questions
Can I use 8 AWG Romex for a 30 amp breaker?
Yes, the NEC allows you to upsize your wire to combat voltage drop over long distances. Using 8 AWG Romex (rated for 40A in the 60°C column) on a 30-amp breaker is perfectly safe and legal. The breaker will still protect the circuit at 30 amps. The only drawback is the physical difficulty of bending 8 AWG NM-B inside a standard single-gang or double-gang receptacle box and fitting it into the terminal lugs.
What size Romex for a 30 amp 220V circuit?
You still need 10 AWG copper. The voltage (120V vs 240V) does not change the ampacity rating of the wire; a 10 AWG wire carrying 30 amps generates the exact same amount of heat regardless of the system voltage. However, for a 240V circuit, you will typically use 10/2 Romex (black, white, bare) if the appliance doesn't require 120V for controls, or 10/3 Romex (black, red, white, bare) if it's a 120/240V appliance like an electric dryer.
How far can I run 10 gauge Romex on a 30 amp breaker?
For a standard 120V, 30-amp circuit, you should keep 10 AWG Romex runs under 50 feet to maintain a voltage drop below the recommended 3%. For a 240V, 30-amp circuit, the allowable distance doubles to roughly 100 feet before voltage drop becomes a functional issue, because the higher baseline voltage makes the absolute voltage drop a smaller percentage of the total. If your run exceeds these distances, upsize to 8 AWG.
Will a 30 amp breaker protect 12 AWG wire?
No, and this is a highly dangerous configuration. 12 AWG Romex is rated for a maximum of 20 amps. If a fault or overload draws 28 amps, the 12 AWG wire will overheat, the insulation will melt, and a fire can start inside the wall cavity. The 30-amp breaker will not trip because it is designed to tolerate up to 30 amps. Always match the breaker to the smallest wire in the circuit: 15A for 14 AWG, 20A for 12 AWG, and 30A for 10 AWG. For more on breaker sizing, refer to OSHA's electrical safety guidelines and standard NEC breaker sizing charts.
