WOOD CROSSARMS USED IN TRANSMISSION LINES BASIC AND TUTORIALS

Cross arms are the most common means of supporting distribution conductors on poles. Although they are being used less frequently, their use will persist for some time.

Standard Arms
Standard cross-section dimensions for wood cross arms (width by height) are:
3-1/4 in by 4-1/4 in
3-1/2 in by 4-1/2 in
3-3/4 in by 4-3/4 in
4 in by 5 in

Of these, the first two are most commonly used for distribution purposes, and usually only one of these will be stocked by an individual utility. The larger size finds greater use in the harsher northern and western climates, while the smaller finds use in the south and southwest.

The rectangular cross section is slightly rounded or “roofed” on the top surface to shed rain and snow.

The length of the cross arm depends on the number of conductors it is to support and the spacing between them. Standard cross arms include two-, four-, six-, and eight-pin arms, although the four- and six-pin arms, 8 ft in length, are the more widely used.

Spacing between pins for the six-pin 8-ft arm is standardized at 141 in, except for the space between the two center pins, the “climbing space” for the lineman’s safety; for primary voltages up to 15,000 V, this climbing space is 30 in, and for voltages above that value it is 36 in (with spacing between the six pins reduced to 13 in).

Spacing between pins for the four-pin 8-ft cross arm is 26 in, with a space between the two center pins of 36 in, the climbing space. See Figure 10-3. Vertical spacing between cross arms is standardized at 2 ft.

Both Douglas fir and southern yellow pine are used for cross arms because of their comparatively high bending strengths and their durability. Both are treated with preservatives after holes for pins and bolts have been bored in them.

Their insulating properties are similar to those described for wood poles.