The correct 30 amp 240 volt wire size is the minimum American Wire Gauge (AWG) cross-section required to safely carry 30 amps of current at 240 volts without exceeding the thermal limits of the wire's insulation or the connected terminals. For standard residential and commercial installations, the definitive answer is 10 AWG copper or 8 AWG aluminum. This assumes standard termination temperatures and runs under 50 feet; if you are pulling wire through conduit or using specific cable assemblies, the exact insulation type and distance dictate your final pick.
The Direct Answer: Sizing for 30 Amps at 240 Volts
When sizing wire, amperage dictates the physical thickness of the conductor, while voltage dictates the required insulation rating. A 30-amp circuit requires the same wire gauge whether it is carrying 120V, 240V, or 480V. The 240V aspect simply means your wire insulation must be rated for at least 300V (standard THHN/THWN-2 is rated for 600V, so this is never an issue in standard building wire).
For a standard 30A 240V double-pole breaker, you need a conductor rated for at least 30 amps. According to the NFPA 70 (National Electrical Code) Table 310.16, 10 AWG copper is rated for 30A at 60°C and 35A at 75°C. Therefore, 10 AWG copper is the exact, code-compliant minimum. If you are using aluminum to save on material costs for longer runs, you must step up to 8 AWG, as 8 AWG aluminum is rated for 30A at 60°C and 40A at 75°C.
Where You Meet This in Practice
You will encounter the 30A 240V requirement in several specific residential and light-commercial scenarios. Understanding what the wire size changes in a real installation helps you avoid physical fitment and code-compliance issues.
- Electric Water Heaters: A standard 4500-watt, 240V water heater draws 18.75 amps. Because the NEC defines water heaters as continuous loads, the circuit must be sized at 125% of the load (18.75 x 1.25 = 23.4A). A 30A breaker with 10 AWG wire is the industry standard here.
- Electric Dryers (NEMA 14-30): Modern 4-prong dryer receptacles are rated for 30 amps at 125/250V. They require two hot legs (10 AWG), a neutral (10 AWG), and a ground (10 AWG).
- Pure 240V Equipment (NEMA 6-30): Small MIG/TIG welders, heavy-duty air compressors, and commercial power tools often use a NEMA 6-30 receptacle. This requires two hots and a ground, all 10 AWG copper.
- RV Receptacles: While many RVs use a 30A 120V outlet (NEMA TT-30), larger motorhomes sometimes utilize 240V 30A or 50A setups. Always verify the receptacle pinout before pulling wire.
What it changes in the real world: Choosing the correct 10 AWG wire ensures the conductor physically fits into the lugs of standard 30A breakers and NEMA receptacles. If you mistakenly oversize to 8 AWG copper 'for safety,' you will likely find the wire too thick to bend cleanly inside a standard single-gang receptacle box, and it may not fit into the terminal lugs of a standard 30A breaker without aggressive filing or pigtailing, which violates code if not done with specific rated lugs.
The Common Confusion: Temperature Columns and the '240V Myth'
The most frequent mistake DIYers and junior electricians make when consulting NEC Table 310.16 is looking at the wrong temperature column.
Under NEC Article 110.14(C), most modern residential breakers and receptacles are rated for 75°C terminations. Therefore, you must use the 75°C column. In the 75°C column, 10 AWG copper is rated for 35 amps. Because 35A is greater than your 30A load and 30A breaker, 10 AWG is perfectly legal and safe.
If you are using NM-B cable (commonly known as Romex), the NEC mandates that you must use the 60°C column for ampacity, regardless of the fact that the wire's internal THHN conductors are technically rated for 90°C. Fortunately, 10 AWG copper in the 60°C column is rated for exactly 30 amps, which perfectly matches a 30-amp breaker.
The second common confusion is the '240V Myth'—the assumption that because 240V is 'more dangerous' or 'more powerful' than 120V, it requires thicker insulation or thicker wire. Again, wire thickness (AWG) is purely a function of current (Amps) and heat dissipation. Voltage only matters for the insulation jacket rating, and standard 600V building wire covers both 120V and 240V systems with massive headroom.
Worked Numeric Example: Voltage Drop on a 60-Foot Run
Ampacity tables assume standard conditions, but they do not account for voltage drop over distance. The NEC recommends a maximum voltage drop of 3% for branch circuits to ensure equipment operates efficiently and motors do not overheat. Let us calculate the voltage drop for a 30A, 240V water heater located 60 feet from the main panel using 10 AWG copper wire.
The Formula:
Voltage Drop (VD) = (2 × K × I × L) / CM
- K = 12.9 (Ohms-cmil/ft for Copper)
- I = 30 Amps (Current)
- L = 60 Feet (One-way length)
- CM = 10,380 (Circular mils for 10 AWG wire)
The Math:
VD = (2 × 12.9 × 30 × 60) / 10,380
VD = 46,440 / 10,380
VD = 4.47 Volts
The Percentage:
(4.47V / 240V) × 100 = 1.86%
Because 1.86% is well below the 3% NEC recommendation, 10 AWG copper is the correct and optimal choice for this 60-foot run. However, if that same water heater was located 150 feet away, the voltage drop would be 11.18V (4.65%). At that distance, you would be forced to upgrade to 8 AWG copper to maintain code compliance and equipment safety.
Decision Tree: Picking Your Exact Wire Type and Gauge
Use this decision path to select the exact part and material for your specific installation environment.
| Installation Scenario | If your condition is... | Then your exact pick is... |
|---|---|---|
| Standard Indoor Run (Conduit) | Pulling individual wires through EMT/PVC conduit, under 75 feet. | 10 AWG Copper THHN/THWN-2. (Buy 3 strands if adding a neutral, plus 10 AWG bare/green ground). |
| Standard Indoor Run (Cable) | Running cable through wall cavities, studs, or attics without conduit. | 10/2 NM-B with Ground (for pure 240V) or 10/3 NM-B with Ground (if a neutral is required for 120/240V appliances). |
| Long Distance Run | The one-way distance from the panel to the receptacle exceeds 75 feet. | 8 AWG Copper THHN/THWN-2 to mitigate voltage drop below the 3% threshold. |
| Budget / Feeder Alternative | You are running a dedicated line and want to save on copper costs, and lugs are rated for aluminum. | 8 AWG Aluminum XHHW-2. (Never use aluminum for standard indoor NM-B branch circuits; use it in conduit or as SER cable). |
| High Ambient Heat | The wire runs through an attic that exceeds 86°F (30°C) regularly, or is bundled tightly with other circuits. | 8 AWG Copper THHN. The 90°C rating of THHN allows you to apply derating factors without dropping below the 30A requirement. |
Frequently Asked Questions
Can I use 12 AWG wire if my 240V appliance only draws 20 amps?
No. The wire size must be matched to the breaker protecting the circuit, not just the appliance draw. If the appliance requires a 30A breaker (often due to startup surge or manufacturer NEC compliance instructions), you must use wire rated for 30A (10 AWG). If the appliance only requires a 20A breaker, then 12 AWG wire is perfectly fine.
Does a 240V 30-amp circuit require a neutral wire?
It depends entirely on the load. Pure 240V loads like baseboard heaters, small welders, and air compressors do not require a neutral; they only need two hot legs and a ground (NEMA 6-30). However, appliances that use 120V for internal electronics, timers, or lights—like electric dryers and ranges—require a neutral to complete the 120V circuit (NEMA 14-30).
What size ground wire do I need for a 30-amp circuit?
According to NEC Table 250.122, the minimum equipment grounding conductor for a 30-amp overcurrent device is 10 AWG copper. While some older codes allowed smaller grounds, modern best practice and code require the ground to be the same gauge as the current-carrying conductors for standard branch circuits of this size.
Default Recommendation: For 95% of residential 30A 240V installations under 75 feet, purchase 10 AWG Copper THHN/THWN-2 for conduit runs, or 10/2 or 10/3 NM-B Romex for in-wall cable runs. Do not overcomplicate the build with 8 AWG copper unless your voltage drop calculations demand it.
