Grouping or bundling certain wires, such as electrically unprotected power wiring and wiring to duplicate vital equipment, should be avoided. Wire bundles should generally be limited in size to a bundle of 75 wires, or 2 inches in diameter where practicable. When several wires are grouped at junction boxes, terminal blocks, panels, etc., the identity of the group within a bundle can be retained. [Figure 4-84]
When specified on the engineering drawing, parallel wires must be twisted. The most common examples are:
- Wiring in the vicinity of magnetic compass or flux valve,
- Three-phase distribution wiring, and
- Certain other wires (usually radio wiring).
Twist the wires so that they lie snugly against each other, making approximately the number of twists per foot as listed in Figure 4-85. Always check wire insulation for damage after twisting. If the insulation is torn or frayed, replace the wire.
Spliced Connections in Wire Bundles
Spliced connections in wire groups or bundles should be located so that they can be easily inspected. Splices should also be staggered so that the bundle does not become excessively enlarged. [Figure 4-86] All noninsulated splices should be covered with plastic, securely tied at both ends.
Slack in Wiring Bundles
Single wires or wire bundles should not be installed with excessive slack. Slack between supports should normally not exceed ½ inch. This is the maximum it should be possible to deflect the wire with normal hand force. However, this may be exceeded if the wire bundle is thin and the clamps are far apart. But the slack should never be so great that the wire bundle can abrade against any surface it touches. [Figure 4-87] A sufficient amount of slack should be allowed near each end of a bundle to:
- Permit easy maintenance;
- Allow replacement of terminals;
- Present mechanical strain on the wires, wire junctions, or supports;
- Permit free movement of shock and vibration-mounted equipment; and
- Permit shifting of equipment for purposes of maintenance.
Bends in wire groups or bundles should not be less than ten times the outside diameter of the wire group or bundle. However, at terminal strips, where wire is suitably supported at each end of the bend, a minimum radius of three times the outside diameter of the wire, or wire bundle, is usually acceptable. There are exceptions to these guidelines in the case of certain types of cable; for example, coaxial cable should never be bent to a smaller radius than six times the outside diameter.