Sags & Tensions of Span Attachments

Service wires to residential and commercial buildings deserve special mention. Most wires or cables used for servicing customers can be run either pole to pole or pole to customer. When run pole to pole they are always tensioned to higher values than pole to customer, as the clearance and separation requirements are harder to achieve over these longer distances. Many organizations have different installation tensions defined for service connections than for mainline attachments. That is part of the answer as those tensions were established to meet the maximum service span length anticipated; which is usually 38 meters.

There is a requirement in the Electrical Codes of Canada (CEC Part 1) and USA (NESC), and perhaps other countries as well, for the power service connection point to withstand up to 600 pounds of combined load, under worst case conditions, from all attachments on the smallest of services from an amperage perspective. This covers residential and many commercial buildings. Larger commercial buildings would have higher tension load capacities with corresponding larger conduit and mast pipe sizes. In many cases, communication drops will attach to this same service mast as power. In other cases, especially commercial buildings, communication attachments have their own support at the building. This is important context as Power Utilities do not want to field complaints from customers if their power mast is pulled off their building. This has driven behavior in installation tensions for spans shorter than the maximum which should be considered.

Like mainline wire and cable attachments, it is convenient and desirable to assume with confidence that specific tensions were utilized at installation time. In the case of power service wires, often only tensions for the maximum span length are provided in Utility standards. This leaves designers wondering what values to use as the maximum values are obviously inappropriate for many typical installations. While power service mast strength is one consideration, we also don't want to design the attached pole line assuming unreasonable support is available from customer connections. Also, it is impractical to perform survey measurements on all services when assessing a pole line. There are just too many of them, creating too much of a burden on the pole line owner or attacher to do a detailed and accurate assessment. A simpler, but reasonable way is needed to estimate the tensions of power service wires.

One company in Canada surveyed and measured tensions on many different power service wire installations of various lengths in two different cities. From the results, they analyzed the data collected along with feedback they received from several Canadian Power Utilities on their installation practices. The outcome, which was suspected but not proven before, is that Power Utility technicians do not pull their service wires to full maximum tension unless it is needed for clearance purposes. In almost all cases, vertical sag below the attachment point at the customer's location of about 0.5m was being achieved at installation time. This is consistent with the desire of the power utility to stay well below the 600 pound limitation of most service masts; especially those of older buildings built before this load requirement was well understood. If the designer has no other information to use, the results of this study may prove useful to them. The ability to use this study's results is incorporated into Quick Pole.