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In case of electrical installations, if cable conductors are spiralling, then the resistance/unit length will ___________.
decrease
remain the same
become zero
increase
increase
When cable conductors are spiralled (or stranded), their actual length is greater than the apparent length of the cable. Since electrical resistance is directly proportional to the physical length of the conductor, the effective resistance per unit length of the finished cable increases.
IS 8130:2013
When cable conductors are spiralled (or stranded), their actual length is greater than the apparent length of the cable. Since electrical resistance is directly proportional to the physical length of the conductor, the effective resistance per unit length of the finished cable increases.
R=╧БALтАЛ тАФ Basic resistance formula where L is length, ╧Б is resistivity, and A is area.
Lay┬аFactor=1+(P╧АDтАЛ)2тАЛ тАФ Where D is the diameter and P is the pitch of the spiral.
In a stranded conductor, the individual wires follow a helical path. The 'lay factor' is defined as the ratio of the length of the strand along the helix to the axial length of the cable. Because the current must travel along the spiral path, the physical length of the current path is LactualтАЛ=LaxialтАЛ├ЧLay┬аFactor. Since R=╧БALтАЛ, an increase in the path length L directly results in an increase in resistance.
Stranding increases the total physical length of the wire compared to the cable length.
The increase in resistance is typically accounted for by a factor (e.g., 2% to 5%) depending on the lay ratio.
Increased spiralling improves the flexibility of the cable but sacrifices electrical efficiency.
DC resistance of stranded conductors is calculated using the total length of individual strands.
Improved mechanical flexibility
Reduced risk of conductor breakage under bending
Easier installation in conduits and cable trays
Increased ohmic losses due to higher total conductor length
Slightly higher material cost per unit of cable length
Flexible building wires (FR/FRLS)
Submarine power cables
Control and instrumentation cables
The resistance increase is often referred to as the 'lay effect'.
Option A is incorrect because resistance is directly proportional to length.
Option B is incorrect as it ignores the geometric increase in conductor path length.
Option C is physically impossible for any standard metallic conductor.
D is correct тАФ Spiralling increases the actual length of the conducting path, which leads to an increase in the resistance per unit length of the cable.
Remember that in electrical exams, 'effective length' is always greater than 'axial length' for spiralled conductors, making resistance and mass slightly higher than expected for a straight rod of the same material.
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