A recent addition to the 2008 National Electrical Code (NEC) has prompted a possible change for the 2011 NEC and a number of questions from electricians, electrical contractors, electrical inspectors and electrical engineers. Section 690.31(F) in the 2008 NEC now requires flexible, fine-stranded cables be terminated only with terminal lugs, termination devices or connectors identified and listed for use with fine-stranded conductors. Questions have been raised about the acceptability of very flexible, fine-stranded conductors in general electrical installations and the expanded use of these cables for photovoltaic systems, wind turbines, cranes, welders, elevators, battery banks, uninterruptible power supplies, computers and similar installations. A further study of these issues may help provide the answers to the questions and help clarify the use of both the conductors and the termination devices.
Basic understanding of flexible cables and the appropriate terminal cable connectors for these stranded cables begins with understanding of class and construction of cables used for specific installations and applications. The most common general-use conductor construction is the bare concentric-lay stranded conductor made from round copper wires, either uncoated or coated with tin, lead or lead alloy. These conductors are constructed with a central core surrounded by one or more layers of helically laid stranded wires. The term “helically stranded” means that each smaller conductor is twisted together to form a circle or a helix with the small conductors starting at the top of the cable, wrapping around to the bottom, and then back up to the top of the cable.
These concentric lay conductors are classified as Class AA for bare conductors usually used in overhead lines and as Class A for conductors covered with weather-resistant (weather-proof), slow-burning materials and for bare conductors where greater flexibility than that afforded by Class AA is required. Class B conductors have regular stranding and flexibility, commonly called concentric cables; may be compressed into smaller overall cables; are insulated with various materials, such as thermoplastic and thermoset insulation; and are used for conductor installations where greater flexibility is required than Class A cables will provide. Class C power cables are used where more flexibility is required than regular Class B conductors, and Class D power cables are used where even more flexibility is required than either Class B or Class C.
Generally, most standard concentric-lay stranded cables in the NEC are available in sizes No. 18 American wire gauge (AWG) to 2,000 kcmil. As can be found in Table 8 of Chapter 9 in the NEC, 18 AWG through 2 AWG conductors have 7 strands, 1 AWG through 4/0 AWG conductors have 19 strands, 250 kcmil through 500 kcmil conductors have 37 strands, 600 kcmil through 1,000 kcmil have 61 strands, 1,250 through 1,500 kcmil conductors have 91 strands, and 1,750 kcmil through 2,000 kcmil conductors have 127 strands. Any conductors having more strands than shown in Table 8 would probably not be Class B cables and would most likely be Class C or Class D cables with much smaller strands within the cables.
Connectors or terminals for standard stranded conductors often are screw-type terminals, since the larger strands in Class B cables can withstand the torquing requirements for appropriate tightening of the terminal without causing damage to the strands within the cable. Using these terminals on finer stranded conductors, such as Class C and Class D cables can cause damage to the much smaller strands, since the screw has a tendency to break these smaller strands at the point of contact. A reduction in the number of strands at the point of connection results in a possible hot point at the terminal, and the heating causes conductor creepage from the connector. In addition, the screw-type terminal compresses the smaller strands, often resulting in cold flow (the conductors continue to compress after the completion of the torquing of the connector terminal), which adds to the flow of the conductor out of the terminal.
Very fine-stranded conductors are required to have terminals listed for fine-stranded conductors by 690.31(F) in the 2008 NEC and are being proposed for inclusion in 110.14 to cover all fine-stranded conductor connections. Many of these listed fine-stranded terminations are crimped-type terminals where the crimping is provided with the appropriate crimping tool to ensure the compression of the terminal does not damage the individual fine strands. The wire size, wire range and stranding classification of the conductor must be marked on the connector, so all personnel can verify that the proper connector has been used. These requirements were in other National Electrical Code editions since 110.3(B) required listed and labeled electrical equipment to be installed and used in accordance with all the listing and labeling requirements in the installation instructions, but this addition will ensure a safe connection.
ODE is a staff engineering associate at Underwriters Laboratories Inc., in Research Triangle Park, N.C. He can be reached at 919.549.1726 and firstname.lastname@example.org.