The correct 30 amp wire size for a 240V circuit is the minimum American Wire Gauge (AWG) conductor capable of safely carrying 30 amps of continuous or non-continuous current without exceeding its thermal insulation rating or violating National Electrical Code (NEC) voltage drop limits. Selecting the precise gauge changes the thermal safety margin of your installation, prevents breaker nuisance tripping under sustained loads, and eliminates severe voltage sag that starves 240V appliances—like EV chargers and electric dryers—of the electrical pressure they need to operate efficiently.
The Core Sizing Table: 30 Amp Wire Size 240V at a Glance
Wire sizing is governed by the NEC Table 310.16 ampacity charts, but the 'correct' wire depends heavily on your insulation type and installation method. Below is the data-dense reference table for 30A 240V circuits, factoring in base ampacity and maximum recommended run lengths to maintain a voltage drop under 3% (the NEC recommended limit for branch circuits).
| Wire Gauge (AWG) | Material | Insulation / Cable Type | Temp Column Used | Base Ampacity | Max Run Length (240V @ 3% Drop) |
|---|---|---|---|---|---|
| 10 AWG | Copper | NM-B (Romex) | 60°C | 30A | 45 feet |
| 10 AWG | Copper | THHN (in conduit) | 75°C | 35A* | 55 feet |
| 8 AWG | Copper | THHN (in conduit) | 75°C | 50A | 110 feet |
| 8 AWG | Aluminum | XHHW (in conduit) | 75°C | 40A | 70 feet |
| 6 AWG | Aluminum | XHHW (in conduit) | 75°C | 50A | 140 feet |
*Note: While 10 AWG THHN has a 90°C base ampacity of 40A, NEC 110.14(C) termination rules and standard 30A breaker sizing cap the practical overcurrent protection at 30A. Source: Cerro Wire Ampacity Charts.
Worked Example: Voltage Drop and Derating on a 100-Foot Run
Let’s look at a real-world scenario: You are wiring a hardwired 240V Level 2 EV charger (like an Emporia Vue or ChargePoint Home Flex) configured to draw 24 amps continuously. The charger is located in a detached garage, exactly 100 feet from your main electrical panel.
Because an EV charger is a continuous load (operating for 3 hours or more), NEC Article 210.19(A)(1) requires the conductors to be sized at 125% of the continuous load.
At first glance, 10 AWG copper wire (rated 30A in the 60°C column) seems perfectly legal. But let's run the voltage drop calculation using the standard single-phase formula: VD = (2 × L × I × R) / 1000.
- Length (L): 100 feet
- Current (I): 24 amps (actual continuous draw)
- Resistance (R) for 10 AWG Copper: ~1.24 ohms per 1,000 ft (NEC Chapter 9, Table 8)
10 AWG Voltage Drop: (2 × 100 × 24 × 1.24) / 1000 = 5.95 Volts.
Percentage: (5.95 / 240) × 100 = 2.48%.
While 2.48% is technically under the 3% NEC recommendation, it leaves almost zero headroom. If you route that wire through a hot attic (ambient temperature of 110°F), NEC Table 310.15(B)(1) requires a temperature correction factor of 0.87 for 90°C THHN. Your 10 AWG wire's ampacity derates to 34.8A, which is fine for the 30A breaker, but the heat will compound the resistance, pushing your voltage drop closer to the 3% threshold and causing the wire to run warm to the touch.
The Professional Fix: Bump up to 8 AWG Copper THHN (Resistance = ~0.778 ohms/kft).
8 AWG Voltage Drop: (2 × 100 × 24 × 0.778) / 1000 = 3.73 Volts (1.55%). This guarantees a cool-running circuit, eliminates voltage sag, and provides ample headroom for future derating factors.
Where You Meet 30A 240V Circuits in Practice
You will typically encounter the 30 amp 240V requirement in specific residential and light-commercial applications. Recognizing these helps you anticipate the correct receptacle types and wire configurations.
- Electric Dryers: Most modern full-size electric dryers require a 30A circuit. Older homes use a 3-prong NEMA 10-30R (which lacks a dedicated equipment ground), while modern NEC-compliant installations require a 4-prong NEMA 14-30R, necessitating 10/3 NM-B cable with a ground.
- Level 2 EV Chargers: Many plug-in EV chargers use a NEMA 6-30R or 14-30R plug. As noted in our worked example, continuous load rules make 8 AWG wire the superior choice for runs over 50 feet.
- Small Subpanels: Feeding a small shed or detached garage subpanel for basic lighting and a single receptacle circuit. (Note: For modern workshops, a 60A feeder is highly recommended over 30A to allow for future tool expansion).
- Instant / Tankless Water Heaters: Small point-of-use electric tankless water heaters often require a dedicated 30A 240V double-pole breaker.
Common Confusions and NEC Code Traps
When sizing wire for 240V circuits, DIYers and even junior electricians frequently fall into a few specific traps that can lead to failed inspections or unsafe installations.
Confusion 1: '240V Needs Thicker Wire Than 120V'
This is false. Wire gauge is dictated strictly by current (amps), not voltage. A 30A load requires the same physical copper mass whether it is pushed by 120V or 240V. The voltage rating of the wire (e.g., 600V printed on THHN) dictates the insulation thickness, not the copper diameter. However, because 240V systems operate at a higher electrical pressure, the percentage of voltage drop over a long distance is mathematically more forgiving than a 120V circuit carrying the same wattage.
Confusion 2: The 60°C NM-B 'Romex' Trap
Many builders buy 10 AWG THHN wire, look at the 90°C column on the ampacity chart, see '40 Amps', and assume they have massive headroom. But if you are using standard NM-B (Romex) cable inside residential walls, NEC Article 334.80 legally forces you to use the 60°C column for ampacity, regardless of the fact that the internal conductors are technically rated for 90°C. In the 60°C column, 10 AWG copper is capped at exactly 30A. You cannot use 10 AWG NM-B for a continuous load that requires a 35A or 40A breaker.
Confusion 3: Ignoring the Equipment Grounding Conductor (EGC)
When pulling wire through conduit for a 30A 240V circuit, you must include a separate equipment grounding conductor. For a 30A circuit, NEC Table 250.122 mandates a minimum 10 AWG copper ground wire. You cannot rely on the metal conduit alone as the ground path in residential branch circuits unless specifically permitted and properly bonded with approved fittings, which is a frequent point of failure in DIY garage builds.
For further reading on branch circuit sizing and continuous load calculations, refer to the NFPA 70 National Electrical Code (specifically Articles 210, 250, and 310). Always consult your local Authority Having Jurisdiction (AHJ), as local amendments can supersede baseline NEC guidance.
