Current ratings for XLPE insulated cables for ‘ground’ and ‘duce’ installation are derived from the latest issue of ERA Report 69-30 Part 5 which is based upon IEC Publication 60287. The ratings for ‘In Air’ installation are taken from IEE Wiring Regulations.
All the ratings given are for single circuits installed thermally independent of other circuits or any other heat source and on the basis of the standard conditions of installation given in relevant Tables between 17 to 33. For other ambient or ground temperatures, depth of laying, soil thermal resistivity, the rating must be multiplied by relevant rating factors in Tables 2 to 6 and 8 to 12.
It should be noted that if XLPE insulated cables, are subjected to operating temperatures appreciably higher than the 90˚C permissible for continuous operation, the insulation will undergo premature ageing thus affecting the normal life of the cable. However, limiting maximum conductor temperature to 105˚C during overloads with duration not exceeding 4 hours on any one occasion, or a maximum of 100 hours in any 12 consecutive months, or a total of 500 hours in the cable’s lifetime, would be tenable.