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Article 338 of the National Electrical Code (NEC) covers the use, installation and construction specifications of service-entrance (SE) cable. Electricians commonly use these cables for service conductors and for feeders and branch circuits in residential and small commercial installations. There were some very subtle changes in Article 338 in the 2008 NEC that are not readily obvious, so care must be taken to analyze these changes.
The definition of service-entrance cable in 338.2 is “a single conductor or a multiconductor assembly provided with or without an overall covering, primarily used for services.”
According to 338.2, there are two types of service-entrance cables: “Type SE or service entrance cable having a flame-retardant, moisture-resistant covering or Type USE which is service entrance cable, identified for underground use, having a moisture-resistant covering, but not required to have a flame-retardant covering.” All service-entrance cable is rated at 600V and is listed in sizes 14 AWG and larger for copper and 12 AWG and larger for aluminum or copper-clad aluminum.
The term “SE cable” indicates the cable jacket and the internal conductors are designed for above-ground installations and are listed for installations where the cable jacket and internal conductors are exposed to the sun in wet locations. Type SE cable contains Type RHW, RHW-2, XHHW, XHHW-2, THWN or THWN-2 insulated conductors, and the outer jacket of the cable is flame-retardant. Type SE cable, containing two or more conductors, is permitted to have one conductor that is uninsulated.
Types USE and USE-2 cables are designed and listed for underground installations, including direct burial in the earth and are available in both single-conductor and multiconductor cables. Multiconductor Type USE cable contains conductors with insulation equivalent to RHW or XHHW with the conductors rated at 90°C in a damp or dry installation and 75°C for a wet application. Type USE-2 contains conductors with insulation equivalent to RHW-2 or XHHW-2 and is rated at 90°C in a wet, damp or dry installation. Type USE cable, containing two or more conductors, is permitted to have one conductor that is uninsulated.
Section 338.12 has been added to the 2008 NEC and covers uses not permitted for both Type SE and Type USE cables. It states that Type SE cable cannot be used where subject to physical abuse, unless adequately protected, and can never be installed in an underground installation, with or without a raceway. SE cable also cannot be used for exterior branch circuit or feeder wiring, unless the installation complies with the provisions of Part I of Article 225 and is properly supported. USE shall not be used for interior wiring at all since it does not have a flame-retardant covering. In fact, USE cannot be used above ground except where it emerges from ground and terminates in an enclosure at an outdoor location. USE cable cannot be used as an aerial cable, unless it is a multiconductor cable, identified for use above ground and is installed as a messenger-supported wiring.
However, the most sweeping change in Article 338 in the 2008 NEC is not readily identifiable since text was deleted, not added. The text in 338.10(B)(4)(a) in the 2005 NEC reads as follows: “Interior Installations. In addition to the provisions of this article, Type SE service-entrance cable used for interior wiring shall comply with the installation requirements of Parts I and II of Article 334, excluding 334.80.”
Since Part I of Article 334 for NM cable had absolutely nothing to do with SE cable, Part I was deleted in the first paragraph of 338.10(B)(4)(a) for the 2008 NEC. In the 2005 NEC, SE cable was required to comply with Part II of Article 334 with the exception of 334.80. In the 2005 NEC, this section required NM cable, but not SE cable, to comply with the following: “the ampacity shall be in accordance with the 60°C (140°F) conductor temperature rating. The 90°C (194°F) rating shall be permitted to be used for ampacity derating purposes, provided the final derated ampacity does not exceed that for a 60°C (140°F) rated conductor.” By deleting “excluding 334.80” in the 2008 NEC, SE cable, where used as branch circuits or feeders, now must comply with the same ampacity rating as NM cable. This change was made for the 2008 NEC since SE cable often contains similar conductor insulation as NM cable, and where installed inside walls and in insulation, SE cable has heat dissipation similar to NM cable.
SE cable installed as branch circuit or feeder wiring in the interior of a building or structure is required to have an ampacity rating based on an insulation rating of a 60°C conductor and will reduce the allowable ampacity for the cable requiring a larger conductor based on the 2008 NEC.
ODE is a staff engineering associate at Underwriters Laboratories Inc., in Research Triangle Park, N.C. He can be reached at 919.549.1726 or at [email protected].
About The Author
ODE is a retired lead engineering instructor at Underwriters Laboratories and is owner of Southwest Electrical Training and Consulting. Contact him at 919.949.2576 and [email protected].