The letters printed on an extension cord’s jacket describe how it’s built. In SJTW, S stands for service cord, J for junior (a 300 volt cord), T for a thermoplastic (vinyl) jacket and W for weather resistant, the outdoor rating.
Type the letters and numbers printed on the cord’s jacket, such as 16/3 SJTW.
What it means
12/3 SJOOW
Junior (300 V) service cord with an oil-resistant jacket and oil-resistant insulation, rated for outdoor use. 12 gauge wire, 3 conductors.
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The numbers next to the letters, such as 16/3, give the wire gauge and the number of conductors.
Where to find the letters
The type letters are printed along the jacket, about every foot under OSHA’s rules for workplace cords. A Carol cord reads like this: 16/3 SJTW OUTDOOR E-XXXXX (UL). That’s the gauge and conductors, the jacket type, where it can be used, the UL identification number and the UL listing.
Where the makers word it differently
OSHA groups the J types (SJ, SJO, SJT, SJTO) as junior hard service, and S, ST, SO and STO as hard service.
Carol’s catalog calls SJTW extra-hard-usage cord. An OSHA letter classes SJTW as junior hard service cord, approved for hard usage.
Extension cord letters chart
Extension cord letters
| Letter | Carol | Southwire |
|---|---|---|
| S | Service (600 V) | Extra hard service, 600 V (if not followed by J, V or P) |
| J | Junior (300 V) | Hard service, 300 V |
| T | Thermoplastic/vinyl | Thermoplastic |
| E | Thermoplastic elastomer (TPE) | Thermoplastic elastomer (UL/NEC designation only) |
| O | Oil-resistant | Oil resistant |
| W | Weather approved (water-, moisture-, damp-, sunlight-resistant) | Outdoor: sunlight-resistant jacket and wet-location rated conductors (formerly W-A) |
| P | Parallel | Parallel cord, also known as zip cord: always light duty |
| V | Not in its list of letters (type SV: vacuum cleaner cord) | Vacuum cleaner cord, also light duty cable |
| H | Not listed | Heater cable |
One O (as in SJEOW) means only the outer jacket is oil resistant. Two (as in SEOOW) means the insulation on the conductors and the outer jacket both are.
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Indoor or outdoor
Use an extension cord outdoors only when it’s marked for outdoor use. The W in the letters is the weather rating. Carol’s example legend prints OUTDOOR after the letters, and Worx’s pressure washer manual says extension cords intended for outdoor use are identified by the marking “Acceptable for use with outdoor appliances; store indoors while not in use.”
For lights outside, see indoor vs outdoor Christmas lights.
Keep plugs and connections off the ground and dry.