Flexible Cords
Flexible cords are designed to move. Their conductors use many small strands and their jackets are selected for bending, abrasion, moisture, oil, temperature, and service conditions. They are used on portable tools, appliances, equipment, extension cords, pendants, and machinery where movement is part of normal use.
Common cord letters
Cord designations such as S, SJ, SOOW, SJT, and related markings describe service class, jacket and insulation characteristics, oil resistance, weather/water suitability, and thermoplastic or thermoset construction. The complete marking matters more than memorizing one nickname.
| Example | General idea |
|---|---|
| S | Hard-service flexible cord family. |
| SJ | Junior hard-service cord family with a lower voltage class than full S types. |
| SO / SOO | Oil-resistant designations; repeated O indicates oil resistance of both insulation and jacket in common cord nomenclature. |
| W | Weather/water-resistant service marking used on appropriate cord types. |
| T | Thermoplastic construction designation in common flexible-cord nomenclature. |
Flexible does not mean permanent substitute
The NEC and OSHA restrict where flexible cords and cables may be used. A cord is not a general substitute for permanent building wiring simply because it is convenient to route.
Strain relief
Pull on the outer cord should not be transmitted directly to conductor terminations. A proper cord grip, clamp, or other listed strain-relief method keeps mechanical stress away from terminal screws and splices.
Damage
Extension cords are exposed to doors, sharp edges, vehicle traffic, abrasion, water, tools, and repeated flexing. Damaged jackets, crushed sections, missing grounding pins, overheated connectors, and loose strain relief are reasons to remove a cord from service.
Grounding
OSHA construction rules require extension cords used with portable electric tools and appliances to be three-wire types so an equipment grounding path is available. The grounding conductor and grounding pin must not be defeated or used for another purpose.
Wet conditions
Wet cord connectors can create shock hazards. Outdoor or wet-location use requires cords, plugs, connectors, receptacles, and protection suitable for the environment. A cord marked for indoor use does not become weather-resistant because it is plugged into a GFCI.
Voltage drop in extension cords
Long cords and small conductors can cause enough voltage drop to make motors, compressors, welders, heaters, and power tools perform poorly or overheat. Higher-current equipment often needs a shorter or larger-conductor cord.