Factors Affecting Current Ratings for Underground and Air Cables

The aim of this paper is to present a parametric study to determine the major factors that influence the calculations of current rating for both air and underground cables. The current carrying capability of the power cables rely largely on the installation conditions and material properties. In this work, the influences on ampacity of conductor size, soil thermal resistivity and ambient soil temperature for underground installations are shown. The influences on the current-carrying capacity of solar heating (time of day effects and intensity of solar radiation), ambient air temperature and cable size for cables air are also presented. IEC and IEEE standards are taken as reference.

• S. H. Alwan
• J. Jasni
• M. Z. A. Ab Kadir
• N. Aziz

• Cable ampacity
• underground cable
• IEC standard
• air cables.

[1] Electric Cables – Calculation of the current rating – Part 1: Current rating equations (100% load factor) and calculation of losses – Section 1: General. IEC Standard 287-1-1 (1994-12).
[2] Electric Cables – Calculation of the current rating – Part 1: Current rating equations (100% load factor) and calculation of losses – Section 2: Sheath eddy current loss factors for two circuits in flat formation. IEC Standard 287-1-2 (1993-11).
[3] Electric Cables – Calculation of the current rating – Part 2: Thermal resistance – Section 1: Calculation of the thermal resistance. IEC Standard 287-2-1 (1994-12).
[4] Electric Cables – Calculation of the current rating – Part 2: Thermal resistance – Section 2A: A method for calculating reduction factors for groups of cables in free air, protected from solar radiation. IEC Standard 287-2-2 (1995-05).
[5] Electric Cables – Calculation of the current rating – Part 3: Sections on operating conditions – Section 1: Reference operating conditions and selection of cable type. IEC Standard 287-3-1 (1995-07).
[6] Calculation of the cyclic and emergency current rating of cables – Part 1: Cyclic rating factor for cables up to and including 18/30 (36) kV. IEC Publication 853-1 (1985).
[7] Calculation of the cyclic and emergency current rating of cables – Part 2: Cyclic rating of cables greater than 18/30 (36) kV and emergency ratings for cables of all voltages. IEC Publication 853-2 (1989-07).
[8] IEEE Standard Power Cable Ampacity Tables, IEEE Std 835-1994.
[9] J.H. Neher and M.H. McGrath, “The Calculation of the Temperature Rise and Load Capability of Cable Systems”, AIEE Transactions Part III - Power Apparatus and Systems, Vol. 76, October 1957, pp. 752-772.
[10] Olex High Voltage Cables Catalogue