If you are wiring a NEMA 10-30 plug, you are dealing with a legacy 125/250V, 30-amp, 3-wire configuration. The direct answer for your wire mapping is: Black to X (Hot 1), Red to Y (Hot 2), and White to W (Neutral/Ground). The bare copper wire in a 10/3 NM-B cable is not used and must be capped.

Before you strip a single wire, you must understand the critical safety deficit of this design: The NEMA 10-30 has no dedicated equipment grounding conductor. The neutral (W) carries unbalanced return current and bonds the appliance chassis to ground. If that neutral connection fails, the metal chassis of your dryer or welder becomes energized at up to 240V. Because of this lethal edge case, the National Electrical Code (NEC) banned NEMA 10-30 for new dryer and range installations in 1996, mandating the 4-wire NEMA 14-30 instead. You should only wire a 10-30 plug when replacing a damaged cord on an existing, grandfathered 10-30 receptacle.

NEMA 10-30 Terminal Pinout and Wire Mapping Table

When looking at the face of a NEMA 10-30R receptacle (or the back of a 10-30P plug where the wires terminate), you will see three terminals. There is no green grounding screw. Use this spec-sheet table to map your physical wires to the correct terminals.

Terminal Marking Physical Position (Receptacle Face) Function 10/3 NM-B Wire Color THHN in Conduit Voltage to W
X Right (Straight Slot) Hot Line 1 (120V) Black Black 120V
Y Left (Straight Slot) Hot Line 2 (120V) Red Red 120V
W Bottom (L-Shaped Slot) Neutral / Equipment Ground White White 0V (Reference)
G (N/A) None Dedicated Ground (Does not exist) Bare (Cap off) N/A N/A
Callout Tip: Torque Matters. When terminating 10 AWG wire on a 30A plug or receptacle, do not just tighten until it 'feels tight.' Use a torque screwdriver set to the manufacturer's specification (typically 12 to 14 in-lbs for standard 10-32 terminal screws). A loose neutral (W) connection on a 10-30 doesn't just cause a voltage drop; it electrifies the appliance chassis.

Node-by-Node Wiring Trace: Panel to Plug

To understand why the diagram is drawn the way it is, we need to trace the current path from the source to the load. In standard electrical schematics, you will see parallel lines representing the 2-pole breaker, straight lines for the conductors, and a circle with 'X', 'Y', and 'W' representing the receptacle terminals.

1. The Panel (Source)

The circuit originates at a 2-pole, 30A common-trip breaker.

  • Hot 1 (Black): Terminates on one of the breaker's line lugs.
  • Hot 2 (Red): Terminates on the second breaker lug.
  • Neutral (White): Terminates strictly on the Neutral Bus Bar.
Critical Code Note: In a main panel, the neutral and ground bars are bonded. However, in a subpanel, they are isolated. The white wire for a 10-30 circuit must land on the neutral bar, never the isolated ground bar, because it carries unbalanced 120V return current from the appliance's control boards and motors.

2. The Cable Run

The conductors travel via 10/3 NM-B (Romex) or three individual #10 AWG THHN wires in a raceway. If using 10/3 NM-B, you will have a bare copper wire. Do not connect the bare wire at either end. Cap it with a wire nut at the panel and cut it flush at the receptacle box. Connecting it to the ground bar at the panel but leaving it floating at the receptacle creates a dangerous parallel path if the neutral fails.

3. The Receptacle and Plug (Load)

At the wall, the Black, Red, and White wires land on the X, Y, and W screws of the NEMA 10-30R.

At the appliance, the NEMA 10-30P plug mirrors this. Inside the appliance's terminal block, you will find a bonding strap or wire. This strap physically connects the W (White/Neutral) terminal to the metal chassis of the dryer or range. This strap is what provides your equipment ground. If you ever upgrade this appliance to a 4-wire NEMA 14-30 cord in the future, you must remove this bonding strap.

Meter Verification and Polarity Checks

Never assume the previous electrician wired the wall receptacle correctly. Legacy 10-30 receptacles are notorious for swapped hots or, worse, a neutral landed on a ground bar. Follow this OSHA-compliant lockout/tagout and verification sequence with your digital multimeter (DMM).

Phase 1: De-Energized Continuity Test (At the Plug)

Before plugging the appliance in, verify the cord itself.

  1. Set your DMM to Continuity (the diode/beep symbol).
  2. Probe the X prong on the plug to the X terminal inside the cord cap. It should beep (< 1 ohm).
  3. Repeat for Y to Y, and W to W.
  4. Probe W to the metal chassis of the appliance. It must beep, confirming the internal bonding strap is intact.
  5. Probe X and Y to the chassis. It must read 'OL' (Open Loop). If it beeps, you have a dead short.

Phase 2: Live Voltage Test (At the Receptacle)

Energize the breaker and set your DMM to AC Voltage (V~).

  1. X to Y: Should read 240V (±5%, so 228V to 252V is acceptable). This confirms both poles of the breaker are live.
  2. X to W: Should read 120V.
  3. Y to W: Should read 120V.
  4. X/Y to True Earth (Grounding screw on the cover plate or a known ground): Should read ~120V. If X to the cover plate reads 0V, your receptacle has no reference to earth, indicating a broken neutral path back to the panel.

Safety Warning: If your X-to-W and Y-to-W readings are erratic, or if you measure 240V from X to the metal box but 0V from X to W, stop immediately and de-energize the circuit. This indicates a floating or broken neutral. In a 10-30 system, a broken neutral means the appliance chassis is currently sitting at 120V or 240V relative to the floor you are standing on.

NEMA 10-30 vs. 14-30: The Grounding Deficit

If you are wiring a new shop, adding a new EV charger, or running a new circuit for a kiln, you should not be looking at a NEMA 10-30 diagram at all. You need a NEMA 14-30. Here is how the legacy 3-wire system compares to the modern 4-wire standard.

Feature NEMA 10-30 (Legacy) NEMA 14-30 (Modern Standard)
Wire Count 3 (Hot, Hot, Neutral) 4 (Hot, Hot, Neutral, Ground)
Equipment Grounding Relies on Neutral wire + Bonding Strap Dedicated Green/Bare Ground wire (No strap)
Chassis Safety if Neutral Fails Chassis becomes energized (Lethal) Chassis remains safely grounded; breaker may trip or appliance loses 120V logic
NEC Status for New Installs Banned for new Dryer/Range circuits (NEC 250.140) Required for all new 30A 125/250V installations
Cable Required 10/3 NM-B (Bare wire unused) or 3x #10 THHN 10/3 NM-B (Bare wire used as ground) or 4x #10 THHN

The fundamental difference lies in the separation of current-carrying and non-current-carrying paths. In a 14-30 setup, the white neutral wire only handles the 120V unbalanced load (like a dryer's timer or a range's control board). The bare/green ground wire sits idle unless a fault occurs, providing a safe, zero-resistance path back to the panel to trip the breaker. As detailed in comprehensive NEMA configuration guides, the 10-series (10-30, 10-50) are strictly legacy holdovers.

If your home has a 10-30 receptacle and you are installing a modern appliance or an EV charger adapter, the safest path forward is to have a licensed electrician pull a new 4-wire feed and install a 14-30R. If you must use the existing 10-30, ensure your terminal torques are exact, your neutral connections are pristine, and you understand the physical path the current takes from the panel bus bar, through the white wire, and into the appliance chassis.