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.
• 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.
• 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
See also: Electrical Cable Sizing • Aboveground Cable Derating Calculation • Underground Cable Derating Calculation • Cable Installation
• Cable Derating Factor Due to Ground Temperature
The following table will give conversion factor between different ground temperatures.
• 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.
The following table will give conversion factor between different ground thermal resistivities.
• 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).