IEEE Std 525-2007 IEEE Guide for the Design and Installation of Cable Systems in Substations

Annex C (normative) Cable selection

This annex provides guidance for cable selection for various types of installations. The proper design of cable systems requires the consideration of many factors. These factors include ambient temperature, conductor temperature, earth thermal resistivity, load factor, current loading, system fault level, voltage drop, system nominal voltage and grounding, method of installation, and number of conductors being installed.

C.1 Conductor

The cable conductor is selected based upon cost-efficient material, industry sizes, to meet ampacity requirements, voltage drop, and short-circuit criteria. The selection of power cables may include consideration of the cost of losses.

C.1.1 Material

One of the most important properties of a conductor material is its conductivity. In 1913, the International Electrotechnical Commission adopted the International Annealed Copper Standard (IACS) that set the conductivity of copper to be 100. Conductors are typically specified based on this standard.

Copper conductor may be uncoated or coated with tin, lead alloy, or nickel. Normally uncoated conductor is used, but coated conductor may be used to ease stripping of the insulation from the conductor and to make soldering easier. Note that soldering is not a typical termination method for utilities.

Aluminum conductor is usually electrical conductor grade, which has a volume conductivity of approximately 61% that of copper. For the same diameter, aluminum conductors have a lower conductivity than copper. Aluminum’s advantage is a 20% lower mass for equivalent conductivity.

Control and instrumentation cable conductor is almost always copper. Aluminum conductor should be considered for larger power cables. Factors that influence the selection of either copper or aluminum for conductors include:

a) Aluminum metal has historically been less expensive than copper.

b) Aluminum conductor terminations require special treatment, copper terminations do not.

c) For equivalent ampacity, aluminum conductor has a lower mass that makes it easier to handle for larger cable sizes.

d) For equivalent ampacity, copper conductor is smaller and can be installed in smaller raceways.

C.1.2 Size

Conductor size is measured by its cross-sectional area expressed in circular mils (cmil) or mm2. One circular mil is defined as the area of a circle 1 mil (0.001 in) in diameter. In North America, conductors below 250 kcmil are assigned American Wire Gauge (AWG) numbers for easy reference. The AWG number increases as the cross-sectional area decreases.

1 cmil = 5.067 × 10−4 mm2 (0.7854 × 10−6 in2)

Conductor size is selected to meet ampacity, voltage drop, and short-circuit criteria. The selection of power cables may include consideration of the cost of losses.

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