IS 7098 Part 1 : 1988Cross-linked polyethylene insulated PVC sheathed cables for working voltages from 1.1 kV up to and including 3.3 kV

IS 7098 Part 1:1988 specifies cross-linked polyethylene (XLPE) insulated, PVC sheathed power cables for working voltages up to and including 1.1 kV (Part 1 of the series; Part 2 covers 3.3–33 kV, Part 3 the EHV range). It is the default LT power-cable specification for building services, substations, pumping stations and external site distribution.

Use it together with:

• IS 732 — code of practice for electrical wiring installations (how the cable is laid, derated and protected)
• IS 3961 Part 2 — recommended current ratings for cables
• IS 8130 — conductors of insulated cables (conductor class/resistance)
• NBC 2016 Part 8 Section 2 — electrical and allied installations

For flexible/domestic wiring ≤1100 V use IS 694 instead; for PVC-insulated power cables, IS 1554 Part 1 is the sibling code.

An IS 7098 Part 1 cable is built up as: conductor (aluminium or copper, stranded, class 2 per IS 8130) → XLPE insulation → inner sheath/bedding → optional armour (galvanised steel round wire or strip) → PVC outer sheath (type ST2).

The properties that drive design:

• Continuous conductor temperature 90 °C (vs 70 °C for PVC) — this is why XLPE carries more current than an equivalent PVC cable
• Short-circuit temperature 250 °C for ~1 s — sets the minimum conductor size for fault withstand
• Voltage grade 650/1100 V (U₀/U)
• Core identification and number of cores (1, 2, 3, 3½, 4) per the standard

The nameplate rating is only valid at the reference condition — actual ampacity is the tabulated value × derating factors.

Problem: feed a 100 kW, 415 V, 0.85 pf motor-control centre, cable buried in ground, ground temp 35 °C, two circuits in the same trench.

Step 1 — design current: I = P / (√3 · V · pf) = 100000 / (1.732 × 415 × 0.85) ≈ 164 A

Step 2 — base selection: from IS 3961 Part 2, a 3½ × 95 mm² Al XLPE buried cable has a base rating of ≈ 235 A.

Step 3 — derating: ground temperature factor (35 °C) ≈ 0.93; grouping (2 circuits) ≈ 0.85. Derated capacity = 235 × 0.93 × 0.85 ≈ 186 A > 164 A ✓

Step 4 — voltage drop: for ~120 m run, mV/A/m for 95 mm² ≈ 0.45 → drop = 0.45 × 164 × 120 / 1000 ≈ 8.9 V ≈ 2.1% < 3% (IS 732 limit for power) ✓

Provide 3½C × 95 mm² Al XLPE armoured cable. Always size from the derated value, never the table value.

1. Using PVC (IS 1554) current ratings for an XLPE cable. XLPE runs at 90 °C and carries materially more current — but also requires the 90 °C-rated terminations and glands.

2. Skipping derating. Ground temperature, soil thermal resistivity, depth of laying and grouping can knock 30–40% off the table ampacity; ignoring them overheats the cable.

3. Treating armour as the earth conductor without checking. Steel armour has limited fault-current capacity; verify it against the prospective earth-fault, or run a separate earth.

4. Bending radius below ~12 × overall diameter — cracks the XLPE insulation invisibly and fails months later.

5. No proper glanding/sealing at terminations — moisture ingress into XLPE causes water-treeing and premature breakdown.

• IS 1554 Part 1 — PVC-insulated power cables ≤1.1 kV (the PVC sibling)
• IS 694 — PVC-insulated cables for working voltages ≤1100 V (wiring/flexibles)
• IS 8130 — conductors of insulated cables
• IS 3961 Part 2 — recommended current ratings (paper/PVC and XLPE)
• IS 732 — electrical wiring installations code of practice
• IS 10810 (parts) — methods of test for cables
• IS 3043 — earthing
• IS 7098 Part 2/Part 3 — HT (3.3–33 kV) and EHV XLPE cables

IS 7098 Part 1 has been reaffirmed and amended rather than superseded — it remains the working LT XLPE cable spec on Indian projects, and cable makers' datasheets quote it directly. The market has moved towards FRLS / LSZH outer sheaths for life-safety in buildings (low smoke, halogen-free) — these are still IS 7098-compliant cables with an enhanced sheath, and you should specify the sheath type explicitly, not just 'IS 7098'.

For any HT distribution (11 kV/33 kV substation feeders) you are on Part 2, not Part 1 — a frequent specification slip. And remember the standard defines the *cable*; the *installation* (laying, segregation from signal cables, fire-stopping, derating) is governed by IS 732 and is where most field failures actually originate.