XLPE Cable — Insulation, Rating and Derating

What it is

Power cable with cross-linked polyethylene insulation. The cross-linking turns a thermoplastic into a thermoset, so the insulation does not soften at the temperatures that limit PVC.

Specification at a glance

Property	Typical specification
Standard	IS 7098, part by voltage grade
Conductor	Copper or aluminium, class stated
Continuous rating	90°C conductor temperature
Short circuit	250°C for a short duration
Armour	Steel wire, or aluminium strip on single core
Sheath	PVC standard; FRLS where required

Derating is the design, not the catalogue

A cable's published rating assumes reference conditions. Real installations rarely match them, and every departure reduces capacity:

Factor	Effect
High ambient temperature	Reduces rating
Grouping in a tray or conduit	Reduces rating, significantly when bunched
Installation method	Enclosed runs derate more than free air
Soil thermal resistivity	Governs buried cable capacity

Applying these under IS 3961 and IS 732 is what separates a cable that runs warm from one that runs hot for twenty years and fails early.

Where projects go wrong

• Sizing from the catalogue rating with no derating applied.
• Bunched cables in a hot shaft, where the combined grouping and ambient factors are severe.
• Bending radius ignored during pulling, damaging the insulation invisibly.
• Single-core cables with steel wire armour, inducing circulating currents.
• Gland and armour termination done badly, so the armour earth path is not continuous — an electrical safety defect, not a finish issue.

Standards referenced

Cable construction under IS 7098. Installation practice under IS 732. Current ratings and derating factors under IS 3961.