To wire a standard 120V 15A plug (NEMA 5-15P), connect the black wire to the brass screw, the white wire to the silver screw, and the bare or green wire to the green grounding screw, using a 14 AWG flexible cord. For 240V 20A loads (NEMA 6-20P), use a 12 AWG cord, connect both the black and re-identified white wires to the two brass screws, and land the green/bare wire on the green ground screw. The exact plug body and cord gauge you need depends entirely on the appliance's voltage and amperage draw.

The Decision Tree: Pick Your Exact Plug and Cord Gauge

Before you strip a single wire, you must match the plug configuration to the load. Using a 15A plug on a 20A compressor will melt the plug face under sustained load. Use this decision table to select your exact parts.

Appliance Load Profile Voltage / Amps NEMA Config Min. Cord Gauge Concrete Part Pick
Standard tools, lamps, electronics (up to 15A) 120V / 15A 5-15P 14 AWG (SJTW) Leviton 5266-C
Heavy 120V tools, window ACs (up to 20A) 120V / 20A 5-20P 12 AWG (SJTW) Leviton 5269-C
Welders, large compressors, EV chargers (up to 20A) 240V / 20A 6-20P 12 AWG (SOOW) Hubbell 4730
💡 CORD SELECTION NOTE: For indoor or light outdoor use, choose SJTW (300V rated, 60°C/90°C). For shop floors, garages, or environments with oil and moisture, step up to SOOW heavy-duty rubber cord (600V rated, highly abrasion-resistant).

Tools and Materials List

Do not attempt this with a dull pocket knife and a kitchen butter knife. Proper wire prep requires specific tools to avoid nicking the copper conductors, which creates a localized hot spot.

  • Wire Strippers: Klein Tools 11055 (for 14-10 AWG solid/stranded) or equivalent automatic stripper.
  • Screwdrivers: #2 Phillips and 1/4-inch flathead (magnetic tip preferred to hold terminal screws).
  • Cutting Tool: Flush-cut diagonal pliers (Knipex or Channellock) for trimming stray strands.
  • Jacket Slitter: Dedicated cord ripper or a careful utility knife.
  • Testing Equipment: Digital Multimeter (e.g., Fluke 117) with continuity and resistance (Ohms) settings.
  • Consumables: 3M Super 33+ electrical tape (for re-identifying white wires on 240V circuits), heat shrink tubing.

Mains Safety and Preparation Protocol

⚠️ MAINS SAFETY WARNING: Even though you are working on the plug end of a cord, the other end may be hardwired into a junction box, plugged into a live receptacle out of sight, or connected to a tool with internal capacitors. Always assume the cord is live. De-energize the circuit at the breaker, lock out or tag out the panel if applicable, and verify the conductors are dead with a tested non-contact voltage tester (NCVT) and a multimeter before cutting the old plug off. Per OSHA 1910.305 standards for flexible cords, never work on energized cord ends.

Step-by-Step: How to Wire a 120V NEMA 5-15P Plug

This procedure applies to standard 15A, 120V replacement plugs using a 3-conductor (Black, White, Green/Bare) 14 AWG cord.

  1. Strip the Outer Jacket: Score the outer cord jacket about 1.5 inches from the end using a cord ripper. Pull the jacket back and trim it flush. Do not nick the inner colored insulation.
  2. Strip the Conductors: Strip exactly 5/8 inch of insulation from the black, white, and green/bare wires. If you strip too much, bare copper will sit exposed outside the terminal. If you strip too little, the screw will clamp the insulation.
  3. Twist and Form the Hook: Twist the stranded copper tightly clockwise. Bend the tip into a tight 'J' hook using the hole in your wire strippers.
  4. Land the Ground (Green/Bare): Hook the green or bare wire around the green grounding screw. The hook must wrap clockwise so tightening the screw pulls the loop closed, not pushes it out. Torque until snug, plus a quarter turn.
  5. Land the Neutral (White): Hook the white wire around the silver terminal screw. Tighten securely.
  6. Land the Hot (Black): Hook the black wire around the brass terminal screw. Tighten securely.
  7. Secure the Cord Grip: This is the most skipped step. The internal clamp (cord grip) at the base of the plug must bite down firmly on the outer jacket of the cord, not the individual colored wires. If it clamps the wires, a tug on the cord will rip the hot wire out of the terminal and cause a dead short.
  8. Close the Housing: Ensure no loose strands are bridging the gap between the brass and silver terminals. Snap or screw the plug face shut.

Step-by-Step: How to Wire a 240V NEMA 6-20P Plug

Wiring a 240V plug for a welder or compressor requires a critical code-compliant adjustment regarding the white wire.

  1. Re-identify the White Wire: Standard 3-conductor cords use a white wire for neutral. In a 240V circuit, there is no neutral; both current-carrying conductors are hot. Per NFPA 70 (NEC) Article 200.7(C), you must permanently re-identify the white wire as a hot conductor. Wrap the last 2 inches of the white wire's insulation tightly in black electrical tape or slide black heat shrink over it.
  2. Strip and Prep: Strip 1.5 inches of jacket and 5/8 inch of conductor insulation, just as with the 120V plug, using your 12 AWG SOOW cord.
  3. Land the Ground (Green/Bare): Connect to the green grounding screw.
  4. Land Hot 1 (Black): Connect the black wire to the first brass terminal screw (often marked 'X').
  5. Land Hot 2 (White with Black Tape): Connect the re-identified white wire to the second brass terminal screw (often marked 'Y'). Never land a re-identified hot wire on a silver screw.
  6. Secure Cord Grip and Close: Clamp the outer jacket firmly and assemble the housing.

Verification and Testing Protocol

Never plug a newly wired cord into a live receptacle without testing it first. A dead short will result in an explosive arc flash at the panel or receptacle.

  1. Set Multimeter to Continuity/Ohms (Ω): Ensure the meter reads 'OL' (Open Loop) or infinite resistance when the probes are separated.
  2. Test for Shorts (Prong to Prong): Place one probe on the brass (hot) prong and the other on the silver (neutral) prong (or the second brass prong on a 240V plug). Expected Reading: OL (Infinite). If it reads near 0 Ω, you have a stray strand short. Open the plug and fix it.
  3. Test Ground Isolation: Place one probe on the ground prong (the U-shape or round pin) and the other on the hot prong. Expected Reading: OL. Repeat for neutral to ground. Both must be OL.
  4. Test Cord Continuity (If applicable): If the other end of the cord is a female receptacle or exposed wires, test from the male plug prong to the corresponding female slot/wire. Expected Reading: Less than 1.0 Ω (indicating a solid, unbroken path).

The Most Common Botch: Stray Strands and Insulation Clamping

The most frequent failure in DIY plug wiring is the "Stray Strand Short" or the "Insulation Clamp." Both stem from rushing the wire prep phase.

The Symptom: You plug the tool in, and the breaker trips instantly with a loud pop (Stray Strand Short). Alternatively, the tool runs, but after 10 minutes, the plug face becomes too hot to touch and eventually melts, emitting a burning plastic smell (Insulation Clamp).

The Cause:
1. Stray Strand: When twisting the stranded wire, a single microscopic copper whisker escapes the bundle and bridges the 3mm gap between the brass (hot) and silver (neutral) terminal blocks.
2. Insulation Clamp: The wire wasn't stripped far enough (less than 1/2 inch). The terminal screw bites down on the plastic PVC insulation instead of the copper. Plastic is an insulator; it creates a high-resistance connection. Under a 12A load, this high resistance generates massive heat (I²R losses), melting the plug.

The Fix: Always strip exactly 5/8 inch. After hooking the wire under the screw, visually inspect the terminal from the side. You should see copper completely filling the terminal gap, with no bare copper extending past the metal washer, and no plastic insulation caught under the screw head. Trim any stray whiskers with flush-cut diagonal pliers before tightening.