Although overhead distribution system is losing favor in residential areas, it is still widely used in many parts of the country, particularly in towns and rural areas. Nevertheless, overhead distribution still has many economic and technical advantage
- Old or faulty transformers on existing poles can easily be replaced and new transformers can be added on existing poles with minimum service interruption.
- New poles can be added to extend existing service, and poles can be moved to accommodate the widening of old roads or the addition of new ones.
- Maintenance of overhead transformers and cables is easier and less costly than it is for pad-mounted transformers and underground cables.
- If local power requirements are to be upgraded, changes in cables and insulators are more easily accomplished. (By contrast, the entire underground cable system might have to be replaced to be upgraded.)
OVERHEAD DISTRIBUTION CABLE AND CLEARANCE
Service drops are cables from pole-mounted overhead distribution system transformers to the customer’s premises, and service laterals are the underground cables from pad-mounted distribution transformers to the customer’s premises. These terminate the local power company’s distribution network. The power company is, however, obligated to pull the cables to the designated service points or points of attachment on homes or buildings and make the splices either to the overhead or underground service entrance conductors.
The builder or electrical contractor must establish these entry points according to specifications issued by the power utility. These are available to the electrical contractor as written documents that conform to the NEC. However, the utility specifications and local building codes can be more restrictive than the NEC requirements.
The installation of an overhead service drop is more complex than the installation of a service lateral because the connection point must be high enough above the grade to allow the minimum clearance distances specified by the power utility.
The location of the service point must be calculated so that it will be high enough to provide the minimum clearance distances between the estimated cable droop and the various ground or structural surfaces. The service point can be an anchor bolt driven into the outside wall of most two-story buildings, but single-story structures typically require the installation of a rigid steel pipe mast to meet the clearance requirements.
The power company will string the service drop from the pole to the service point and fasten it to the anchor bolt and insulator installed by the electrical contractor.
The following statements supplement the information given graphically in below Figure .
- The service point shall not be less than 12 ft above grade or greater than 21 ft, unless approved in writing.
- All service connections must be reachable from a ladder placed on the ground, so their placement above roofs of garages or other building extensions is not allowed.
- There shall be 2-ft minimum clearances between power cables and lateral telephone lines and TV cables.
- A 3-ft clearance is required from the service drop connections to all windows, doors, and mountings on the building wall if a pipe mast is not used.
- Add 6 in. to all clearances for any open wire construction.
below Figure illustrates the specialized service drop clearances over swimming pools and fountains or spas (hot tubs). It also shows the minimum slant distances from the edge of the swimming pool to the service drop connection on the pole, and the high-voltage conductor on the top of the pole. Minimum cable clearance or droop must take into account the height of any diving boards or other pool-related structures such as viewing stands or pool heating and water-circulating machinery enclosures.