For a three-layer stranded wire, the number of strands in the first, second, and third layer respectively are:

• A. 1, 5 and 10
• B. 1, 4 and 8
• C. 1, 7 and 14
• D. 1, 6 and 12

Answer: (D)

The total number of the conductor in stranded cable is given by
N = 3x2 − 3x + 1
Where
x = no. of layer
For layer 1
N1 = 3(1)2 − 3(1) + 1
N1 = 1
For layer 2
N2 = 3(2)2 − 3(2) + 1
N2 = 7
In layer 2 no. of conductor = N2 − N1 = 7 − 1 = 6
For layer 3
N3 = 3(3)2 − 3(3) + 1
N3 = 19
In layer 3 no. of conductor = N3 − N2 = 19 − 7 = 12