If you are running a dedicated circuit for a heavy-duty shop tool, a compact welder, or a high-capacity window AC unit, you need a 240 volt outlet 20 amp. For a pure 240V load drawing up to 20 amps, the direct answer is a NEMA 6-20R receptacle, fed by a 20A double-pole breaker using 12 AWG copper wire. There is no neutral wire in this configuration—just two hot legs and a ground.

Getting this right means understanding exactly which wire lands on which terminal, how to handle the white conductor in a standard 12/2 cable, and how to verify the circuit before plugging in your equipment. Here is the exact blueprint to get it done safely and to code.

The 240 Volt Outlet 20 Amp Decision Matrix

Before you buy parts, you must confirm your load's actual requirements. Plugging a 120/240V appliance into a pure 240V receptacle will fry its 120V control boards. Use this decision table to lock in your exact hardware.

If your load requires... Then you need... Concrete Pick (Default)
Pure 240V, up to 20A (e.g., 240V heaters, pure 240V ACs, some welders) 2-wire + ground (No neutral) NEMA 6-20R (Stop here, proceed below)
120V and 240V combined, up to 20A (e.g., RV adapters, some smart appliances) 3-wire + ground (Requires neutral) NEMA 14-20R (Requires 12/3 wire; see 14-series guide)
Pure 240V, up to 15A (e.g., older small tools) 2-wire + ground NEMA 6-15R (Undersized for modern 20A tools)
Decision Lock: If your tool's nameplate reads "240V, 60Hz, 20A" with no mention of 120V, you are wiring a NEMA 6-20R. We will proceed exclusively with this configuration.

Tools, Materials, and Mains Safety Protocol

Working inside a breaker panel is unforgiving. The bus bars remain lethal even when the main breaker is off, unless you are pulling the meter. Treat every surface as live until your meter proves otherwise.

CRITICAL MAINS SAFETY WARNING: Before opening your panel, turn off the main breaker. Use a non-contact voltage tester (NCVT) on the branch breakers, then use a CAT III or CAT IV digital multimeter to verify zero voltage across the main lugs and the bus bars. NEC-style guidance requires de-energizing and verifying dead before any terminal work. If you are uncomfortable working near the main service lugs, hire a licensed electrician. For more on safe panel work, refer to OSHA's electrical safety guidelines.

Materials List

  • Receptacle: 20A, 125/250V, NEMA 6-20R (e.g., Leviton 5028-W or Bryant 620R).
  • Breaker: 20A Double-Pole, matching your panel's brand and series (e.g., Square D Homeline HOM220, Eaton BR220, or Siemens Q220). Never mix brands unless the breaker is explicitly UL-classified for that panel.
  • Wire: 12/2 NM-B (Romex) with ground for indoor dry locations, OR two 12 AWG THHN conductors (Black, White) plus a 12 AWG THHN Green ground in conduit.
  • Re-identification: Black or red electrical tape (mandatory for the white wire).

Tools List

  • Digital Multimeter (CAT III minimum)
  • Non-Contact Voltage Tester (NCVT)
  • Wire strippers (calibrated for 12 AWG)
  • Torque screwdriver (to meet NEC 110.14(D) torque requirements)
  • Lineman's pliers and flathead/Phillips screwdrivers

Step-by-Step Wiring: Panel to Receptacle

For this installation, we are using standard 12/2 NM-B cable. Because 12/2 contains a black, white, and bare wire, the National Electrical Code (NEC) requires you to permanently re-identify the white wire as a hot conductor at both ends. See the NEMA WD-6 standard configurations for visual reference on plug prongs.

  1. Prep the Cable and Re-identify: Strip back 3/4 inch of insulation from the ends of the black and white wires. Wrap the last 2 inches of the white wire tightly with black (or red) electrical tape at both the panel end and the receptacle end. This legally designates it as a 240V hot leg.
  2. Land the Ground at the Panel: Route the 12/2 cable into the panel. Terminate the bare copper ground wire to an open slot on the equipment grounding bus bar. Torque to the panel label's specification (usually 30-35 in-lbs).
  3. Land the Hots at the Breaker: Insert the black wire into Pole 1 of the 20A double-pole breaker. Insert the taped white wire into Pole 2 of the same breaker. Torque both terminal screws to the breaker's printed spec (typically 35 in-lbs for standard 20A breakers). Snap the breaker into the panel's bus stab.
  4. Prep the Receptacle Box: At the outlet location, strip the cable jacket back to where it enters the box, leaving at least 6 inches of working conductor length. Strip 5/8 inch of insulation from the black and taped white wires.
  5. Terminate the Ground at the Receptacle: Form a hook in the bare copper wire and loop it clockwise around the Green terminal screw on the NEMA 6-20R. Tighten firmly.
  6. Terminate Hot 1: Hook the black wire clockwise around one of the Brass terminal screws (usually labeled 'X' on the back of the yoke). Tighten securely.
  7. Terminate Hot 2: Hook the taped white wire clockwise around the remaining Brass terminal screw (labeled 'Y'). Tighten securely. Ensure no bare copper is exposed outside the terminal clamp.
  8. Secure the Device: Carefully fold the wires into the box, keeping the ground wire pushed to the back so it doesn't pinch against the hot terminals. Mount the receptacle to the box and install the cover plate.

Verify and Test: Expected Meter Readings

Do not plug in your tool yet. Put the cover on the panel, turn the main breaker back on, and flip the new 20A double-pole breaker to the ON position. Set your multimeter to AC Voltage (V~) and take the following measurements at the receptacle slots.

Probe Placement Expected Reading Acceptable Range
Left Slot (X) to Right Slot (Y) ~240V 228V - 252V
Left Slot (X) to Ground U-Slot ~120V 114V - 126V
Right Slot (Y) to Ground U-Slot ~120V 114V - 126V

If you read 240V across the two hot slots, and exactly 120V from each hot slot to the ground slot, your circuit is wired correctly and balanced. If you read 0V from hot to ground but 240V across the hots, you have an open ground—turn off the breaker and check your ground bar termination immediately.

The Most Common Botch (and How to Avoid It)

The single most frequent mistake DIYers make when wiring a 240 volt outlet 20 amp with 12/2 NM-B cable is failing to re-identify the white wire and landing it on the neutral bus bar instead of the breaker.

The Symptom: The moment you flip the breaker on, it trips instantly with a loud, violent bang. In worse cases, it can trip the main service breaker or cause arcing damage to the bus bar.

The Cause: In a standard 120V circuit, the white wire returns current to the neutral bar. But in a 6-20R circuit, the white wire is being used as a second 240V hot leg. If you land it on the neutral/ground bar in the panel, you are creating a direct, dead-short between the 120V breaker pole and the grounded neutral system. The breaker is simply doing its job by tripping to prevent a fire.

The Fix: Always wrap the white wire in black or red tape at both ends. This isn't just a suggestion; it is a strict NEC requirement (Article 200.7(C)(1)). The tape signals to you, and to the next electrician who opens that panel, that this white wire is carrying lethal line voltage, not returning neutral current.

By following this exact decision path, terminating the correct colors to the correct brass and green screws, and verifying your 240V/120V split with a meter, your NEMA 6-20R will provide years of safe, high-capacity power to your shop.