POWER CABLE AS USED IN UNDERGROUND DISTRIBUTION SYSTEM
How are conductor and materials for power cables being specified?
Conductor material and insulation type will be specified. Restricting extensions of existing systems to a specific conductor material and insulation type in order to match an existing cable type is permitted only when a need has been established. Neutral cables, where required, will be installed with 600V insulation unless concentric neutral cable is used. In duct lines, neutrals will be installed in the same conduit with associated phase cables.
Conductor material. Since underground conductors are continuously supported, soft-drawn copper or aluminum alloy 5005 provides adequate strength. However, the selection of copper or aluminum will be justified based upon an analysis using life, environmental, and cost factors. The need for mechanical flexibility requires that conducts be stranded, and the NEC makes this mandatory for cables larger than No. 8 AWG installed in raceways. The installation of conductors larger than 500 kcmil is not economical, and such large cables should be used only under exceptional circumstances. Large ampacities can be served by parallel or multiple circuits. Three 15 kV, single conductor, nonmetallic-jacketed cables larger than No. 4/0 AWG will require use of ducts larger than the standard four-inch size (i.e. three singleconductor cables making up a three-phase circuit and each having individual overall diameters greater than 1.25 inches will need to be installed in a duct larger than four inches). One three conductor cable is more costly than three single conductor cables, and use of multiple-conductor cable will be restricted to special conditions. Metallic-armored cable is such a special condition.
Insulation and jacket material. The type of insulation used will be dependent upon the voltage level and type of service required. Factors affecting selection will be the effects of the surrounding environment, the importance of the load in regard to operation of the installation, and whether peak loading is continuous or intermittent.
1. Medium-voltage cable. Cable will be specitied as 133 percent insulation level (ungrounded) which allows greater margin for voltage surges, insulation deterioration, and fault clearing time than does the use of the 100 percent insulation level (grounded). When marking guide specifications, refer to NFPA 70, which currently limits the minimum size to No. 1 AWG at 133 percent insulation for 15 kV to 28 kV systems and No. 2 AWG at 133 percent insulation for 8 kV to 15 kV systems. Medium-voltage cable above 3 kV will be shielded.
a. Nonmetallic-jacketed cable. Nonmetallic jacketed cable will be used, except where circumstances warrant other coverings. Insulation will be either crosslinked-polyethylene (XLP) for short life requirements, or ethylene-propylene-rubber (EPR) for long life requirements, in accordance with NEMA WC-7 and WC-8. Comparisons of various cable insulations, as shown in table 7-1, indicate the advantages of these two insulations over other types. Coverings (jackets) will be any of the rubber or plastic options covered by NEMA specifications. This option allows the use of cables which are available as stock items in small quantities. In some environments, however, selection of other jacket materials may be necessary because properties of some jacket materials may not provide adequate cable protection. Special shielding or coverings will not be specified, unless the designer has checked that the footage installed for each different cable diameter is large enough for manufacturers to make the special runs required.
b. Metallic-armored cable. Armored cable is justified only when cable is installed under water (submarine cables) and sometimes when installed in cable trays or trenches. Armored cable will have XLP or EPR insulation covered with a thermoplastic core covering and then provided with an interlocked-metal tape armor. A non-metallic jacket is required for underground installations, where corrosion and moisture protection is required, for installations in outdoor cable trays, or for submarine cables. Submarine cable may also require a lead covering. Cable having a steel armor will be three-conductor type to avoid the high hysteresis and eddy current losses which can result when single-conductor cable is used.
2. Low-voltage cables. Cables suitable for below grade installations are listed in the NEC. Insulation will be either XLP (NEMA WC 7) or EPR (NEMA WC 8) and jackets or other protection will be in accordance with the applicable Underwriter’s Laboratories (UL) specification covering that NEC type. Use of metal-clad (MC) cable will be limited as previously discussed for metallicarmored cable. The use of the less expensive Moisture-and-Heat-Resistant Thermoplastic (THWN) or Moisture-and-Heat-Resistant Cross- Linked Synthetic Polymer (XHHW) is not recommended for underground work as their thinner insulation has been designed for interior usage. Moisture-and-Heat Resistant Thermoplastic (THW) wiring does have the same thickness of insulation as Heat-Resistant Rubber (RHH)/Moisture-and- Heat Resistant Rubber (RHW)/Underground Service-Entrance (USE) wire, but polyvinylchloride insulation is considered to have only fair electrical and mechanical insulation properties as compared to the excellent properties exhibited by XLP and EPR insulation. UF cable may have a greater insulation thickness, but some sizes have a lower ampacity rating than does USE cable.
Cable ampacity. The current carrying capacities of cable will be in accordance with ampacities given in the NEC and IEEE/ICEA publications. There are many factors taken into account in determining these allowable ampacities such as operating temperatures, soil effects, shielding losses, and conductor configurations, but the variables which cause the most concern are circuit loading and location in a duct bank. Because of load diversity, peak demands for cables in a duct bank will not occur concurrently in most cases. This diversity factor will be taken into account when computating expected heat build-up in a duct bank. Heat dissipation from a cable is also influenced by the position occupied by the cable in a duck bank. Cables in duck bank corners dissipate heat more effectively than cables in interior ducts, because of the greater soil dissipating area and the smaller heat contribution from neighbouring cables. Calculations of the position effect indicate that, to equalize operating temperatures, full load ratings of cables appropriate for isolated (one-way) ducts should be decreased for multiple duct banks. For example, in an eight-way-duct bank the recommended full-load percentage decrease for each corner duct is 95 percent and for each interior duct is 83 percent giving an average load percentage decrease of 89 percent. This derating still allows provision for loads in excess of the normal feeder capacity usually found on military installations, as the summation of feeder capacities is generally from three to eight times the overall capacity of a main electric supply station.
Power cable joints. A splice which connects cables rated 2.5 kV and above is known as a power cable joint. Cable joints are composed of connectors to join two or more cables for the purpose of providing a continuous electric path plus necessary components for maintaining symmetrical stress distribution, minimizing voltage gradients, and maximizing environmental protection.
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