Rubber Cable are also rated as heavy duty, medium duty and light duty. These designations are given based on the jacket used on the cord. All four types (SJ cable, SJO cable, SJOW cable; and SJOOW cable) are considered heavy duty, due to their thermoset rubber jackets. Portable cord with thermoplastic elastomer jackets are considered medium duty and those with thermoplastic jackets are designated as light duty.Also can tell what type of jacket the cord has by reading the letters as well. A thermoplastic elastomer jacket will have the letter "E", usually after the "S" or "SJ" depending on the service type. A thermoplastic jacket will have a"T" instead of an "E" in the same spot. The absence of an "E" or a "T" means it has a thermoset rubber jacket.