What Size Wire for a 20 Amp 240V Circuit? (NEC Sizing Guide)

The Short Answer: Sizing Wire for a 20-Amp 240V Circuit

The correct wire size for a 20 amp 240v circuit is 12 AWG copper, which is the minimum gauge required to safely carry 20 amps of current without overheating. According to NEC Article 310.16, 12 AWG copper wire with standard 60°C or 75°C insulation (like NM-B or THHN) is rated for exactly 20 amps to 25 amps, making it the perfect match for a 20-amp double-pole breaker. While 12 AWG is the baseline for thermal safety, long wire runs will force you to upsize to 10 AWG to prevent voltage drop, a factor that trips up many DIYers and even some apprentice electricians.

Why Voltage Doesn't Change Wire Thickness (The Common Confusion)

A massive point of confusion on the jobsite is the assumption that higher voltage requires thicker wire. It doesn't. Wire gauge is dictated strictly by current (amps), which generates heat, not voltage. A 20-amp load on a 120V circuit and a 20-amp load on a 240V circuit both generate the exact same amount of resistive heat in a 12 AWG wire.

So, what does 240V change in a real installation? Two things: insulation requirements and voltage drop characteristics. First, the wire insulation must be rated for the system voltage. Standard NM-B (Romex) and THHN are rated for 600V, so they easily handle 240V. Second, because you are starting with a higher baseline voltage (240V instead of 120V), you can push the same amount of wattage over a much longer physical distance before hitting the NEC's recommended 3% voltage drop limit. This is why 240V is used for heavy loads—it's inherently more efficient for power transmission.

Where You Meet This in Practice: Common 240V/20A Loads

You will typically pull a 20-amp 240V circuit for specific mid-size appliances that need more power than a standard 120V/15A outlet can provide, but don't require the heavy 30A or 50A feeds used for electric ranges or EV chargers. These circuits usually terminate at a NEMA 6-20R receptacle or hardwire directly into a junction box.

The Math: When to Upsize to 10 AWG for Voltage Drop

While 12 AWG handles the thermal load of 20 amps perfectly, the NEC recommends keeping voltage drop under 3% for branch circuits to ensure equipment runs efficiently (NEC Informational Note 210.19(A)). For a 240V circuit, a 3% drop means you can lose a maximum of 7.2 volts before the equipment suffers.

Let's run a worked numeric example using the standard voltage drop formula: $VD = \frac{2 \times K \times I \times L}{CM}$.

Because 7.9V is greater than our 7.2V maximum allowable drop, 12 AWG fails the voltage drop test at 100 feet. The motor in your workshop will run hot and inefficiently. To fix this, you must upsize to 10 AWG wire, which has 10,380 Circular Mils. Running the math again: 51,600 / 10,380 = 4.97 Volts. This is well under the 7.2V limit, making 10 AWG the correct choice for this specific 100-foot run.

War Story: The Workshop AC Unit That Kept Tripping

Theory is great, but real-world installations have edge cases. Last summer, I was called out to troubleshoot a newly installed 240V, 18,000 BTU window AC unit in a detached garage. The setup looked correct on paper: the unit drew 16 amps running, and the homeowner had wired it with 12 AWG NM-B cable on a 20-amp double-pole breaker. The run was about 85 feet, with the cable routed through a hot, unventilated attic space.

The Outcome: The AC compressor would struggle to start, hum loudly, and then trip the 20-amp breaker after about three seconds. Furthermore, after resetting and running for just five minutes, the NM-B cable in the attic was warm to the touch.

What Went Wrong: There were two compounding failures here. First, the attic ambient temperature regularly hit 110°F (43°C). According to NEC Table 310.15(B)(1)(1), 12 AWG NM-B (rated at 60°C for ampacity derating purposes) must be derated by 0.58 at that temperature. That effectively reduced the wire's safe ampacity to just 11.6 amps, causing the wire to overheat under the 16-amp continuous load.

Second, the 85-foot run of undersized wire caused a severe voltage sag during startup. Induction motors require high starting torque. When the voltage at the compressor terminals sagged from 240V down to 212V due to wire resistance, the starting torque dropped by the square of the voltage. The motor couldn't spin up fast enough, staying in the high-current 'locked rotor' phase (drawing 45+ amps) for too long, which tripped the thermal element inside the breaker.

The Fix: We pulled out the NM-B and ran 10 AWG THHN individual conductors through PVC conduit. The conduit protected the wire from the attic heat (THHN is rated for 90°C), and the thicker 10 AWG copper eliminated the voltage drop, allowing the compressor to spin up in under a second.

Step-by-Step: Terminating a 20A 240V Circuit

If you are wiring a pure 240V circuit (no 120V accessories on the load), you do not need a neutral wire. You will use two hot wires and an equipment ground. If you are using NM-B cable, it contains a black, white, and bare wire.

1. De-energize and Verify: Turn off the main breaker or the specific panel feed. Use a non-contact voltage tester and a multimeter to verify the bus bars are dead. Mains voltage is lethal; if you are unsure, hire a licensed electrician.
2. Strip the Jacket: Strip the NM-B outer jacket back at least 8 inches inside the panel, leaving the individual wire insulation intact until you reach the terminal.
3. Re-identify the White Wire: Per NEC 200.7(C)(2), you must permanently re-identify the white wire as a hot conductor. Wrap it with black electrical tape or use a black sharpie at both the panel and the receptacle ends.
4. Terminate the Hots: Land the black wire and the re-identified white wire on the two brass terminals of the 20-amp double-pole breaker. Ensure no bare copper is exposed outside the terminal lug.
5. Land the Ground: Terminate the bare copper wire on the panel's equipment grounding bus bar. Never land a ground wire on the neutral bus bar in a subpanel.
6. Torque to Spec: Use an inch-pound torque screwdriver to tighten the breaker terminals to the manufacturer's specified torque (usually printed on the breaker label, often around 25-30 in-lbs). Loose connections cause arcing and fires.

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