When you strip back the outer jacket of a standard North American 3-prong extension cord, you will find three inner conductors: Black (Hot), White (Neutral), and Green (Ground). If you are working with European or global IEC-standard cords, the colors shift to Brown (Hot), Blue (Neutral), and Green/Yellow (Ground). However, understanding the complete color code for extension cord wiring requires looking beyond just the inner copper; the printed letter codes on the outer jacket (like SJTW or SOOW) dictate exactly where and how the cord can be safely deployed.

SAFETY WARNING: Extension cords are classified as 'flexible cords' under NEC Article 400. They are intended for temporary use only. Never use an extension cord as a substitute for permanent branch circuit wiring (NM-B/ROMEX), and never bury a standard extension cord in a wall or underground. Always de-energize and unplug the cord before cutting, stripping, or testing.

Extension Cord Inner Conductor Color Codes

The table below provides the definitive reference for inner wire colors across the three major regional standards. Always verify the standard printed on the plug or jacket before making assumptions based on color alone.

Table 1: Inner Conductor Color Codes by Region
Function US / Canada (NEC / CEC) Europe / Global (IEC 60446) Old UK (Pre-2004)
Hot / Line Black Brown Red
Neutral White or Gray Blue Black
Ground / Earth Green, Green/Yellow, or Bare Green/Yellow Green/Yellow

What this means in practice: In a standard US 120V NEMA 5-15P plug, the narrow blade connects to the Black (hot) wire, the wide blade connects to the White (neutral) wire, and the U-shaped round pin connects to the Green (ground) wire. If you are repairing a plug head, reversing the hot and neutral creates a shock hazard at the appliance socket, even if the tool appears to function normally.

Decoding the Jacket: Extension Cord Letter Codes

The OSHA Standard 1926.405 strictly regulates flexible cords. The outer jacket of an extension cord is stamped with a letter code that defines its insulation material, voltage rating, and environmental resistance. A common heavy-duty outdoor cord is marked 12/3 SJTW.

Table 2: Extension Cord Jacket Letter Designations
Letter Meaning Practical Application
S General Service (600V) Standard heavy-duty shop and industrial cords.
J Junior Service (300V) Thinner insulation; standard for household 120V cords.
T Thermoplastic Jacket Standard PVC insulation (becomes stiff in extreme cold).
W Weather / Water Resistant Contains UV inhibitors; safe for temporary outdoor use.
O Oil Resistant Outer jacket won't degrade when exposed to garage oils.
E Elastomer (Thermoset) Rubber-like jacket; stays highly flexible in sub-zero temps.

For example, an SOOW cord is 600V rated, oil-resistant, and fully weather-sealed, making it ideal for a jobsite. An SJT cord is 300V, thermoplastic, and strictly for indoor, dry, climate-controlled use. Using an indoor SJT cord on a wet outdoor site violates NFPA electrical safety guidelines and risks immediate dielectric breakdown.

Rows People Get Wrong (And Dangerous Assumptions)

Even experienced DIYers make critical errors when interpreting extension cord color codes. Here are the most common pitfalls:

  • Green vs. Green/Yellow Ground: In North America, a bare copper wire or a solid green wire is the ground. In IEC regions, ground is strictly Green with a Yellow stripe. Never assume a solid green wire in an imported European appliance cord is ground without verifying the schematic; some older 3-phase equipment used solid green for a phase conductor.
  • White vs. Gray Neutral: Both white and gray are recognized as neutral under NEC Article 200. However, in older or cheaper extension cords, UV exposure can fade a white wire to look exactly like a gray wire, or a black wire to look dark brown. Never rely on visual inspection alone if the cord has been left in the sun.
  • The 'White Tape' Trick on Flexible Cords: In permanent home wiring (NM-B), electricians often wrap white wire with black tape to re-identify it as a 'hot' conductor (like in a 3-way switch loop). Under NEC Article 400, re-identifying flexible cord conductors is highly restricted. If your extension cord's hot wire is damaged, do not swap in a white wire and tape it black. Cut the cord or replace it.
  • 2-Prong vs 3-Prong Assumptions: A 2-prong (NEMA 1-15) extension cord only has two inner wires (Black and White, or Brown and Blue). It has no ground wire. Never attempt to ground a 2-prong cord by wrapping a spare wire around the outside jacket.

Frequently Asked Questions

What color is the ground wire in a 2-prong vs 3-prong extension cord?

A standard 3-prong extension cord (NEMA 5-15) will have a Green, Green/Yellow, or Bare Copper ground wire. A 2-prong extension cord (NEMA 1-15) is ungrounded and only contains two wires: Black (Hot) and White (Neutral) in North America, or Brown (Hot) and Blue (Neutral) in IEC regions. If you are powering a tool with a metal chassis or a 3-prong plug, you must use a 3-prong cord with an intact ground wire.

Can I use a white wire as the hot conductor when repairing an extension cord?

No. While the NEC allows re-identifying white wires as hot in specific permanent wiring scenarios (using permanent black tape or paint), this practice is generally prohibited and highly unsafe for flexible extension cords. Extension cords are subjected to pulling, bending, and abrasion, which will quickly wear off any tape used to re-identify the wire, creating a lethal shock hazard for the next person who plugs it in. Always use properly color-coded replacement cord stock (e.g., SOOW) or buy a new factory-molded cord.

How do I safely identify wires if the extension cord colors are faded or missing?

If UV damage or chemical exposure has faded the inner wire colors, you must use a digital multimeter (DMM) to perform a continuity test. Step 1: Ensure the cord is completely unplugged from the wall. Step 2: Set your multimeter to the continuity setting (the diode/soundwave symbol). Step 3: Touch one probe to the narrow blade of the plug (Hot) and the other probe to the stripped end of the suspected black wire. If the meter beeps or reads near 0.0 ohms, that is your Hot wire. Step 4: Repeat for the wide blade (Neutral) and the U-shaped ground pin. If the cord fails continuity on any pin, the internal copper is fractured and the cord must be destroyed and replaced.

What does the 'W' mean on my outdoor extension cord jacket?

The 'W' stands for Weather and Water resistant. This indicates the jacket contains UV inhibitors to prevent sun degradation and is formulated to remain flexible in cold temperatures. However, 'W' does not mean the cord is submersible or rated for direct burial. According to standard wiring references, outdoor flexible cords must still be kept out of standing water and protected from physical damage (like being run over by a vehicle or pinched in a door). For direct burial, you must use UF-B cable or THWN in conduit, never a flexible extension cord.