Design Geometry of Multi-Conductor Cables
To those outside of the industry, the geometric design principles used in cable-making may not be apparent.
To assist the customer in comprehensively discussing his needs with our factory engineering staff, we offer
a brief guide to the major design options and tradeoffs available. A designers check list for specifying cable
is offered elsewhere in this section. Not all are aware that for a given number of wires, several different
geometry's may be used to form the wires into a helical cable bundle. Any of them may be justified, depending
on cost, intended use and performance, or limitations of the manufacturers' equipment. National Wire & Cable
Corp. has perhaps the widest and most versatile selection of modern cable-making equipment in the custom cable
industry. We welcome your knotty design problems. Our 40 years of experience in wire & cable field are at your disposal.
Conductor Layup Geometry Options
Layer Upon Layer
When all conductors have equal diameters they can be cabled in simple layers around a suitable core or central
wire. Theoretically, every layer will contain 6 more wires than the preceding inner layer. This can be shown
with a few round disks or coins. In practice this is not always true. Tolerances of insulation diameters enter
the picture. Further, the conductors spiral in a helical path when formed into cable, and thus occupy an
elliptically-shaped area around the circumference of the core. Typically, the eccentricity of the ellipse
is about 5%. Thus about 19 conductors may fit, where 20 should if they were assumed to occupy a circular cross-section.
Subcabling
For some constructions small groups of wires are helically cabled into subcables, which are then helically
cabled to form the finished bundle. They may take these forms due to the end use requirements, (i.e.: where
the design calls for twisted pairs, trios, etc.) or may do so mainly for the convenience of the lay-up of
the required number of wires in an available site within the cable cross-section. Advantages of this method
are improved flexibility of the cable, possible convenience in the intended end-use for the cable and the
wide selection of geometry's it offers the designer. Disadvantages compared to the layer-on layer method
are usually increased diameter and cost. Where there are hundreds of conductors in a cable, this method is often used to permit cabling on relatively small cabling machines.
Filling Interstices
When a cable contains a wide mixture of wire diameters, the larger wire sizes or subcables may be cable-formed
so as to deliberately leave interstices large enough for the smaller wires to lie within tangent to two adjacent large wires, but not in contact with the inner layer or core.
Use of Fillers
Where a layer requires more wires than are available to properly fill all positions in that layer, round fillers
are used to occupy the otherwise vacant sites. Their use ensures a round shape and provides adequate support for
the outer layers of the construction. These may be spare wires, or of plastic rod, jute, or twisted yarns or
tapes. They are often used as a core member, and may be deliberately designed into a layer to permit circumferential compression in a layer during flexing of the cable.
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Cable Flexibility
When a helically cabled bundle is flexed, each of the wires in a layer tend to slide along their helical path
slightly with respect to the wires adjacent. If the wires are in firm contact around the entire layer, the friction
between them inhibits the desired sliding action, and additional stiffness is imparted to the cable. Interlayer
friction can also contribute to cable stiffness. In general, any stored radial forces which contribute to the
friction should be avoided by proper design and manufacturing techniques. These forces may be from unduly snug jackets, braids, wraps, or serves.
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Design Assistance
Due to the many variable factors which influence cable designs, we strongly recommend that the customer consult
with our technical staff to ensure proper choice of dimensions and tolerances when he generates his own specification.
For design reference, a few of the common geometry's for subcabled and intersticed constructions are shown. The
diameter of the large members is taken to be one unit of diameter. The size of the other members are shown as some
decimal which relates their size to that of the large member. For layer-on-layer constructions, the table shows the
factor by which the wire diameter can be multiplied to obtain the cabled bundle diameter.
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