Color Codes for Extension Cords: Internal Wires & Jacket Letters

When you strip back the outer jacket of a standard 120V extension cord to repair a plug, wire a temporary drop, or adapt a tool, you will expose three insulated conductors. The exact color codes for extension cords depend entirely on the regional standard (NEC/UL in North America vs. IEC in Europe/UK) and the manufacturing era. Furthermore, the letters printed on the outside of the jacket dictate the cord's environmental survival rating. Below is the immediate reference data you need before making any connections.

Internal Wire Color Codes by Region

The internal conductors dictate the electrical function of the wire. North America follows NFPA 70 (National Electrical Code) guidelines, while Europe and most of the world follow IEC 60446. Always verify the origin of the cord before splicing.

Function	US / Canada (NEC/UL)	Europe (IEC 60446)	Old UK (Pre-2004)	Australia / NZ
Line / Hot	Black (Red for 2nd hot in 240V)	Brown	Red	Active (Brown)
Neutral	White (or Gray)	Blue	Black	Neutral (Blue)
Ground	Green, Green/Yellow, or Bare	Green/Yellow Stripe	Green (or Bare)	Earth (Green/Yellow)

Rows People Get Wrong: Internal Colors

• US White vs. EU Blue (Neutral): If you are splicing a US-manufactured cord to a European-imported appliance, the US White wire connects to the EU Blue wire. Never connect US White to EU Brown, or you will create a dead short.
• Old UK Red/Black Reversal: A vintage UK cord uses Red (Live) and Black (Neutral). If you wire this to a modern EU plug, the Black wire is Neutral, not Ground. Connecting old UK Black to the modern Green/Yellow ground pin will energize the chassis of your tool.
• Ground Variations: US standards allow bare copper for the ground conductor inside some flexible cords, while IEC strictly mandates the Green/Yellow bi-color stripe. If you see a bare wire in a 3-conductor cord, it is always ground—never neutral.

Decoding Extension Cord Jacket Letter Codes

While internal colors tell you what the wire does, the jacket letters tell you what the cord survives. The OSHA 1926.405 standard for flexible cords relies on UL/NEC letter designations to determine if a cord is safe for a specific jobsite environment. You will find these letters embossed directly into the outer vinyl or rubber jacket.

Letter	Stands For	Voltage Rating & Practical Use
S	Service (Standard)	600V. Heavy-duty, thick insulation. Standard for commercial sites.
J	Junior	300V. Lighter duty, thinner jacket. Standard for household 120V use.
T	Thermoplastic	Vinyl/PVC jacket. Stiffens significantly in cold weather.
W	Weather/Water	Outdoor rated. UV stabilized and moisture resistant.
O	Oil-Resistant	Outer jacket resists oil, gas, and chemical degradation.
OO	Oil-Resistant (Both)	Both outer jacket AND inner wire insulation resist oil.
E	Elastomer	Rubber-like jacket (TPE/Neoprene). Stays highly flexible in freezing temps.

Rows People Get Wrong: Jacket Letters

• "W" vs "O" (Environment): "W" means the cord survives rain, mud, and UV exposure (use it in the yard). "O" means the jacket survives being run over by a lawnmower leaking gas or sitting in a puddle of motor oil (use it in the garage). An "SJTW" cord will dissolve if left in a puddle of gasoline.
• Missing "J" (Voltage): An "STW" cord is rated for 600V, while an "SJTW" cord is only rated for 300V. For standard 120V home use, 300V is perfectly adequate. However, if you are building a 240V extension cord for a welder or RV, you must drop the "J" and use an "STW" or "SOOW" jacket to meet the 600V insulation requirement.
• "T" in Winter (Flexibility): A standard "SJTW" (Thermoplastic) cord will turn into a stiff, brittle hockey stick at 20°F (-6°C), leading to cracked insulation and exposed copper. For winter outdoor use or cold-storage warehouses, you must buy an "SEOW" or "SJOOW" (Elastomer) cord.

Safe Interpretation for Faded, Missing, or Non-Standard Cords

UV exposure and physical abrasion frequently destroy jacket lettering and fade internal wire colors. A white neutral wire left in the sun for two summers will turn yellowish-grey, and imported appliance cords often use non-standard colors like brown, black, and grey. When markings are compromised, never guess based on physical position inside the plug. Follow this strict testing protocol.

1. De-energize and Isolate: Unplug the extension cord from the wall. Ensure both ends are completely disconnected from any power source or load.
2. Set the Multimeter: Turn your digital multimeter to the Continuity setting (the diode/beep symbol) or the lowest Ohms (Ω) range. Touch the probes together to verify the meter reads near 0.0Ω and beeps.
3. Identify the Plug Blades (US Standard): Look at the male plug. The narrow flat blade is Hot. The wide flat blade is Neutral. The U-shaped or D-shaped pin is Ground.
4. Map the Conductors: Place one probe firmly on the narrow (Hot) blade. Touch the other probe to each stripped wire end at the female connector. The wire that yields a reading of < 1 ohm (and triggers the beep) is your Hot wire. Tag it with black electrical tape.
5. Map Neutral and Ground: Repeat the process using the wide blade (Neutral) and the ground pin. Tag the corresponding wires with white and green tape, respectively.
6. Discard if Open: If the multimeter reads "OL" (Open Loop) or infinite resistance between a plug blade and any wire at the opposite end, the cord has an internal break. Do not attempt to repair it; cut the ends off and discard the cord to prevent someone else from using it.

By cross-referencing the internal conductor colors with the outer jacket letter codes, you ensure that your temporary wiring is not only electrically correct but mechanically suited for the environment. When in doubt, cut the cord, map it with a meter, and remark it yourself before putting it into service.