Cable Derating Factors - Electrical Engineering 1 of 5 https://www.electrical-engineering.net/cable-derating-factors/ Home » Engineering » Design Consideration » Cable Derating Factors Cable Derating Factors March 19, 2021 A conductor section size will carry a rated current without damage to cable insulation. Cable manufacturer will announce that rated current in their product catalog. Rated current should be read in conjunction with ambient conditions. However, site actual conditions are always different from the manufacturer statements. In addition, there is a stage during a project cable data is not available. IEC standards also provide cable ampacity, of course at their standards ambient and installation conditions. This article will explain cable derating factors and provide these factors for several installations. Contents [ hide ] • What Affect Cable Ampacity ? • Cable Derating Factor Due to Ambient Air Temperature • Cable Derating Factor Due to Ground Temperature • Cable Derating Factor Due to Ground Thermal Resistivity • Cable Derating Factor Due to Cable Arrangement • Most Adverse Installation • What Affect Cable Ampacity ? Cable conductors will need to carry current to maintain a surface temperature according to their insulation material. For example, XLPE insulation cable will allow a normal operating temperature of 90°C. Adding more layers beyond thermal insulation i.e. screens, sheathing, armor will reduce rated current at the same ambient temperature. See also: Cable Construction In short, cable construction and surrounding arrangement will affect thermal dissipation and therefore impact cable carrying current. There are four (4) factors composed of overall cable derating factor namely ambient or soil temperature, buried depth, soil thermal resistivity, and cable arrangement. 13/02/2023, 15:27 Cable Derating Factors - Electrical Engineering 2 of 5 https://www.electrical-engineering.net/cable-derating-factors/ • Cable Derating Factor Due to Ambient Air Temperature The following table will give the conversion factor between different ambient air temperatures. Note that all IEC cable ratings are based on air temperature of 30 oC Table 1 – Ambient Air Temperature Derating Other than 30 degree C See also: Electrical Cable Sizing • Aboveground Cable Derating Calculation • Underground Cable Derating Calculation • Cable Installation 13/02/2023, 15:27 Cable Derating Factors - Electrical Engineering 3 of 5 https://www.electrical-engineering.net/cable-derating-factors/ • Cable Derating Factor Due to Ground Temperature The following table will give conversion factor between different ground temperatures. Table 2 – Ambient Ground Temperature Derating Other than 20 degree C • Cable Derating Factor Due to Ground Thermal Resistivity Cables that are laid in the ground need to dissipate heat into their surroundings. The thermal conductivity of soil varies considerably due to soil characteristics i.e. near a water source, coastal areas, dry soil, desert sand. The higher the value of thermal resistivity the more difficult it becomes to remove the heat from the cable. 13/02/2023, 15:27 The following table will give conversion factor between different ground thermal resistivities. Table 3 – Soil Thermal Resistivity Derating Other than 2,5 K*m/W • Cable Derating Factor Due to Cable Arrangement The cable manufacturer will provide derating factors for several possibilities i.e., number of installed cables in a layer, number of layers, cable spacing horizontally and vertically. The actual condition is a combination of cable installation and cable manufacturer factors are not sufficient. International standards (IEC, ERA, etc) has developed to cover this concern. Likewise, standards introduced deduction factor for a group of cables in cable ladder or inside duct bank, see detail in IEC 60364-5-52. • • Most Adverse Installation It is commonplace that electrical cable experiences various different arrangements and ambient conditions along its route. The environment that creates the most derating factor of the rated current should be taken and used for the whole cable route. However, this requirement can normally neglected if the length of the cable route is less than 0,35 m (IEC 60364-5-52) or 6 ft (IEEE Std 141). Design Consideration Cable MV Variable Speed Drive Standards Voltage Drop Calculation Cable Derating Factors - Electrical Engineering 5 of 5 https://www.electrical-engineering.net/cable-derating-factors/ Shock Risk Assessment Arc Flash Calculator Conduit Fill Chart Cable Short Circuit Rating Aboveground Cable Derating Calculation Underground Cable Derating Calculation Voltage Drop Calculator Conduit Fill Calculation Cable Construction Usage Terms Privacy Contact Us 2023 © Electrical-Engineering.net 13/02/2023, 15:27
