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Flexible Cords (SJOOW, SOOW, SJTW)

SOOW, SJOOW and SJTW are the most common portable cord families, and the letters tell you the service grade, jacket material and outdoor and oil ratings. This sheet compares the three and explains where flexible cord is and is not allowed.

Ref EL-0011NEC 400UL 62

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

Flexible cord is covered by NEC Article 400 and listed to UL 62. It is built for equipment that moves, vibrates or must be disconnected, such as portable tools, extension cords and pendant drops. The letters on the jacket name the cord type in NEC Table 400.4. Each letter has a meaning, so SOOW and SJOOW differ mainly in service grade and voltage. The full letter list is on the flexible cord letter code decoder sheet.

How to identify

Read the jacket print: conductor count and size (for example 12/3), type letters, voltage, temperature rating and any added marks such as water resistant or sunlight resistant. Rubber and thermoset jackets feel softer and stay flexible in cold. Thermoplastic jackets (the T in SJTW) are stiffer and smoother. Junior hard service cord has a thinner jacket than extra hard usage cord of the same gauge.

Comparing common cords

LettersService gradeVoltage ratingJacketOil resistanceOutdoor
SOOWExtra hard usage600 VThermoset (rubber type)Jacket and insulationYes (W)
SJOOWJunior hard service300 VThermoset (rubber type)Jacket and insulationYes (W)
SJTWJunior hard service300 VThermoplasticNot ratedYes (W)
SJTOWJunior hard service300 VThermoplasticJacket onlyYes (W)
SEOOWExtra hard usage600 VThermoplastic elastomerJacket and insulationYes (W)

Letter meanings used above

LetterMeaning
SExtra hard usage when alone; with J, part of "junior" service
JJunior hard service, 300 V
EThermoplastic elastomer
TThermoplastic
OOil-resistant jacket
OOOil-resistant jacket and insulation
WWet locations and outdoor, sunlight resistant

Key points

  • NEC 400.10 lists permitted uses. NEC 400.12 lists uses not permitted, including use as a substitute for fixed wiring and running through walls, ceilings or floors.
  • Ampacity of cords comes from NEC Table 400.5(A)(1) and (A)(2). Do not use building wire ampacity for cord.
  • Choose extra hard usage cord for construction, industrial and heavy-traffic use. Junior cord is for lighter duty.
  • Cord used with strain relief fittings must match the fitting's diameter range.
  • Cold-flexibility ratings vary by maker. Check the spec sheet for outdoor winter use.

Common mistakes

  • Selling SJTW for oily shop floors.
  • Using cord as permanent wiring above a ceiling.
  • Choosing a 300 V junior cord for a 480 V machine.
  • Ordering strain relief by cord gauge instead of the cord's outside diameter.

Governing standards: NEC Article 400 (flexible cords and flexible cables); UL 62 (flexible cords and cables). See the flexible cord letter code decoder.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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