The internal wire color code for a standard North American (NEC) extension cord is Black (Hot), White (Neutral), and Green (Ground). In Europe and regions using IEC harmonized standards, the colors are Brown (Hot), Blue (Neutral), and Green/Yellow (Ground). However, the outer jacket color (orange, yellow, black) is purely for visibility; the true electrical rating is stamped into the jacket using a specific letter code (like SJTW or SOOW).
Extension Cord Internal Wire Color Codes (NEC, IEC, and UK)
When you strip back the outer PVC or rubber jacket of a flexible cord to replace a plug head or wire a temporary receptacle box, you must identify the conductors. The colors change drastically depending on where the cord was manufactured and the standard it was built to meet. Always verify the plug face and the stamped jacket text to determine which standard applies to your specific cord.
| Region / Standard | Hot (Line) | Neutral | Ground (Earth) | Typical Voltage |
|---|---|---|---|---|
| US / Canada (NEC) | Black | White (or Gray) | Green (or Bare) | 120V / 240V |
| Europe / Global (IEC) | Brown | Blue | Green/Yellow Stripe | 230V / 400V |
| UK (Pre-2004 Legacy) | Red | Black | Green | 240V |
| Australia / NZ (AS/NZS) | Brown (or Red) | Blue (or Black) | Green/Yellow (or Green) | 230V / 400V |
Rows People Get Wrong & Faded Wire Identification
Misidentifying a conductor can result in an energized appliance chassis, a tripped GFCI that won't reset, or a dead short. Here are the most common traps and how to handle degraded cords.
If you are repairing an older UK-manufactured cord or working with imported European equipment, pay close attention to the Black wire. Under the old UK standard (pre-2004), Black was the Neutral. Under the modern IEC harmonized standard, Black is the Hot (Line) in some specific multi-core configurations, though Brown is standard for single-phase. Never assume Black is neutral on an imported cord without testing. For more on UK wiring transitions, refer to the Health and Safety Executive (HSE) wiring standards.
Safe Interpretation When Markings are Faded or Missing
Extension cords used on jobsites often suffer from UV degradation, oil staining, or paint overspray. If the white neutral wire looks gray, or the green ground is stained brown, do not guess. Use a digital multimeter in continuity mode to map the cut end to the plug blades.
| Plug Blade / Pin | Physical Characteristics | Multimeter Reading (Cut End) | Terminal Screw Color |
|---|---|---|---|
| Hot (Line) | Shorter, narrower flat blade (US) | Beeps on one specific wire | Brass (Gold) |
| Neutral | Longer, wider flat blade (US) | Beeps on one specific wire | Silver |
| Ground | Round pin (US) / Metal clips (EU) | Beeps on one specific wire | Green |
Bench Tip: If your cord has a molded, un-openable plug and the wires at the cut end are completely indistinguishable, clip the molded plug off and wire a new, high-quality replacement plug head (like a Leviton or Hubbell industrial grade) using your multimeter mapping. Never back-probe a live molded plug with multimeter leads; it is a shock hazard and can damage your meter.
Jacket Letter Codes: The Real 'Color Code' for Ampacity
While internal wire colors dictate polarity, the outer jacket letter code dictates the cord's physical durability, voltage rating, and safe ampacity. People often mistakenly believe that a yellow cord is for indoors and an orange cord is for outdoors. This is false. The NEC and OSHA flexible cord regulations rely on the stamped lettering to determine if a cord is safe for a specific environment.
| Letter | Meaning | Practical Impact on Jobsite |
|---|---|---|
| S | Severe Service | Rated for 600V. Thick, heavy-duty jacket. |
| J | Junior Service | Rated for 300V. Thinner jacket, standard for household 120V cords. |
| T | Thermoplastic | Jacket is made of PVC/Vinyl. Stiffens in freezing weather. |
| W | Weather/Water | Resists UV sunlight and moisture. Mandatory for outdoor use. |
| O | Oil Resistant | Outer jacket (first O) or inner insulation (second O) won't degrade in machine oil. |
Common Jacket Combinations in the Wild
- SJTW (Severe, Junior, Thermoplastic, Weather): The standard orange or yellow outdoor extension cord you buy at a hardware store. Rated for 300V, UV resistant, but the thermoplastic jacket will crack if left in sub-zero temperatures or run over by heavy machinery.
- SOOW (Severe, Oil, Oil, Weather): The heavy black rubber cords used in industrial settings and on film sets. Rated for 600V, completely oil and water-resistant, and the rubber remains flexible in freezing temperatures. A 12 AWG SOOW cord will outlast an SJTW cord by a decade on a commercial jobsite.
- SPT (Stranded Parallel Thermoplastic): The flat, lightweight 'lamp cord'. Strictly for indoor, light-duty, dry locations. Never use SPT outdoors or for power tools.
Regional Standards and Safe Splicing Practices
Determining which standard applies to your reader's region comes down to the physical plug attached to the cord and the voltage of the local grid. If you are in North America, you will encounter NEMA 1-15 or 5-15 plugs and NEC color codes. If you are in the EU or UK, you will encounter Schuko, BS 1363, or IEC 60309 plugs and harmonized Brown/Blue/Green-Yellow codes.
Under NEC Article 400.9 (and equivalent IEC regulations), flexible cords and cables must be used in continuous lengths without splice or tap. Splicing an extension cord with wire nuts and electrical tape is a severe fire hazard and an OSHA violation on commercial sites. If a cord is damaged, the only code-compliant repair is to cut out the damaged section and install a proper vulcanized splice kit, or more commonly, cut the cord and install new molded or screw-terminal plug/receptacle ends. Always defer to your local Authority Having Jurisdiction (AHJ) for site-specific enforcement.
When wiring a replacement plug to any of these color codes, remember the physical alignment rules: the Hot wire always terminates on the brass screw, the Neutral on the silver screw, and the Ground on the green screw. For IEC cords, ensure the Green/Yellow earth wire is cut slightly longer than the Brown and Blue conductors; this ensures that if the cord is yanked from the terminal block, the ground connection is the last one to break.
