UL-758:2012 pdf free download – Underwriters Laboratories Inc. Standard for Safety Appliance Wiring Material.
5.6.3 Special alloy copper conductors having a conductivity of not less than 80 percent of stranded copper are not prohibited from being used when the finished wire or cable is suitable for the use, and when the cross section of the conductor is sufficient to give it a conductivity of not less than 80 percent of the standard conductor and the tag is marked to indicate its nominal AWG size.
5.7 Stranding
5.7.1 Stranded conductors shall use a method of stranding that complies with the requirements in 5.7.2 — 5.7.7.
5.7.2 Conductors of intermediate nominal area shall comply with the requirement for the length of lay of the next smaller conductor as shown in Maximum lay of single-bunch, bunch stranded copper conductors, Table 5.3. The American Wire Gauge (AWG) sizes shown in Table 5.3 represent conductors of intermediate nominal area.
5.7.3 The individual wires used in making up a stranded conductor are usually drawn to the same diameter, which is not required to be the diameter of any AWG or other standard gauge number. The individual wires of a concentric-lay-stranded conductor are not required to be all of the same diameter.
5.7.4 A 19-wire combination round-wire unilay-stranded conductor of soft-annealed copper or an aluminum alloy indicated in this section shall be round and shall consist of:
a) A straight central wire,
b) An inner layer of six wires of the same diameter as the central wire with the six wires having identical lengths of lay, and
C) An outer layer consisting of six wires of the same diameter as the central wire alternated
with six smaller wires having a diameter of 0.732 times the diameter of the central wire and with all twelve wires of the outer layer having the same length of lay and direction of lay as the six wires of the inner layer (see Nominal strand and conductor dimensions for 19-wire combination round-wire unilay-stranded copper conductors, Table 20.6 of UL 1581).
No particular assembly of the individual wires of any other stranded conductor is required. However, simple bunching (untwisted strands) shall not be used for the entire conductor or any part thereof. The length of lay of the strands in a single-bunch bunch-stranded conductor shall not be greater than indicated in Table 5.3. The direction of lay of the strands in a single-bunch bunch-stranded conductor shall be left-hand. Any type of stranding indicated in Nominal strand and conductor dimensions for 19-wire combination round-wire unilay-stranded copper conductors, concentric-lay Classes B, C, and D (round strands), compressed-stranded 19-wire combination round-wire unilay-stranded copper or aluminum, rope-lay (constructed of concentric-lay members composed of round strands) Classes G and H, rope-lay (constructed of bunch-stranded members composed of round strands) Classes I, K, and M shall comply with 5.7.6 or 5.7.7 as applicable. The direction of lay of the outer layer shall be left-hand in all cases.
5.7.5 A compact-stranded conductor shall be a round conductor consisting of a central core wire (strand) surrounded by one or more layers of helically laid wires (strands). A compact-stranded copper conductor shall consist of uncoated strands. A compact-stranded aluminum conductor shall have all layers with the same direction of lay (left-hand unidirectional). A compact-stranded copper conductor shall be either left-hand unidirectional or have the direction of lay reversed in adjacent layers (concentric-lay-stranded with the outer layer left-handed) and with each layer rolled, drawn, or otherwise compressively formed to distort the originally round or partially preshaped strands to various close-fitting shapes that achieve almost complete filling of the spaces originally present between the strands. Each layer shall be compacted before the next layer is applied, and each compacted layer — including the outermost layer — shall have an essentially smooth, round outer surface. The length of lay of the strands in the outer layer of a 1 AWG — 1000 kcmil conductor shall be 8 — 16 times the overall diameter of that layer. The length of lay of the strands in the outer layer of a size 50—2 AWG conductor shall be 8.0 — 17.5 times the overall diameter of that layer.