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Copper Conductor Stranding Classes

Copper conductors come solid or stranded in classes that range from a few coarse strands to hundreds of fine ones. This sheet explains the common classes, where each is used and why fine-strand wire needs terminals identified for it.

Ref EL-0016ASTM B8ASTM B174UL 486A-486B

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

Stranding class tells how many strands make up a conductor of a given size. More, finer strands make a more flexible conductor with the same copper area. ASTM B8 covers concentric-lay stranded conductors (Classes AA, A, B, C and D). ASTM B174 covers bunch-stranded conductors with very fine wires, used in flexible cords and welding cable. NEC Chapter 9, Table 10 lists the strand counts for Class B and Class C building wire.

How to identify

Building wire such as THHN and XHHW is normally Class B. Flexible cords, welding cable, DLO and some portable power cable use fine stranding, often described as Class K or Class M. Cable print may show the class; otherwise check the maker's spec sheet. Fine-strand wire feels very limp and the strands look like hair at a cut end.

Common classes

Class	Construction	Typical use
Solid	One wire	Small branch-circuit wire, NM-B, control wiring
B	Concentric; 7 strands in 2 AWG and smaller, 19 in 1 to 4/0, 37 in 250 to 500 kcmil	Standard building wire
C	Concentric; more strands than Class B	Building wire where more flexibility is wanted
D	Concentric; more strands than Class C	Special flexible conductors
K	Bunch-stranded with 30 AWG strands	Flexible cord, portable cable
M	Bunch-stranded with 34 AWG strands	Welding cable, DLO, very flexible cable

Key points

NEC 110.14 requires connectors and terminals for conductors more finely stranded than Class B and Class C to be identified for that conductor class.
Many standard mechanical lugs are listed only for Class B and C. Fine strands can splay out, cut, or creep from under a screw, causing heat.
For fine-strand conductors, use lugs and crimp connectors marked for the class, or approved ferrules where the device allows.
Class does not change the conductor size. A 4/0 Class M has about the same copper area as a 4/0 Class B, but a larger outside diameter.
Fine-strand cable insulation may differ from building wire insulation. Check that the cable type is allowed for the use.

Common mistakes

Landing welding cable or DLO in a standard set-screw lug.
Tinning fine-strand ends with solder before clamping under a screw. Solder creeps and loosens.
Choosing a conduit or lug size by gauge alone and finding the fine-strand cable does not fit.

Governing standards: ASTM B8 (concentric-lay stranded); ASTM B174 (bunch-stranded); NEC 110.14 and Chapter 9, Table 10; UL 486A-486B (wire connectors).

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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