Cable Sizing for Indian Buildings — IS 3961 + IS 7098 vs IEC 60364-5-52 vs NEC 2023
By MEPVAULT Editorial Team · MEP Consultant · Electrical · 11 May 2026
Reading time ~ 9 min · Originally published: 10 May 2026 · Last revised: 11 May 2026
A 1C × 25 mm² Cu XLPE cable in tray at 35 °C ambient: 132 A per IS, 127 A per IEC, 138 A per NEC. The 11 A spread is the difference between 95 % utilisation and 86 % — and the difference between needing a 240 mm² cable and getting away with a 185 mm². When IS governs, when IEC harmonises, when NEC takes over (FM Global, data centres), worked end-to-end for a 1,200 kVA transformer feeder.
Three standards, three ampacity tables, one cable
A 1C × 25 mm² copper XLPE cable installed in cable tray at 35 °C ambient carries 132 A per IS 3961 + IS 7098. Same cable carries 127 A per IEC 60364-5-52. NEC 2023 Table 310.16 gives 138 A.
The 11 A spread (8 % of the IEC value) is the difference between picking a cable that runs at 95 % design utilisation and one that runs at 86 %. Over a 25-year cable life, this matters for insulation ageing, fault levels, and protection coordination.
Why each standard returns a different number
IS 3961 + IS 7098 use the European tradition. Reference ambient is 30 °C for air installations, 20 °C ground for buried. Derating curves for grouping (number of circuits laid together), depth, soil thermal resistivity (default 1.5 K·m/W). Ampacity decreases monotonically as you add cables in a tray or trench.
IEC 60364-5-52 harmonises with IS at ~95 % agreement — the small gap is rounding-up rules and method-of-installation classification. IEC’s Method E is what IS Tables 3-12 call “in air on perforated tray with > 30 % free area”. Same physics, slightly different conservatism.
NEC 2023 uses 30 °C reference for buried, 40 °C for air, but the tables come out higher than IS/IEC because NEC permits XLPE to operate at 90 °C conductor temperature in many use-cases. IS limits XLPE to 90 °C for current rating but caps short-circuit temperature at 250 °C — design clears for IS submissions on the conservative limit.
A worked 1,200 kVA transformer downstream cable selection
A 1,200 kVA 11 kV/0.433 kV transformer feeds a main MCCB rated 1,600 A at the LT side. Cable from transformer LT to MCCB — buried in a concrete duct bank, 0.8 m deep, 30 °C ground temperature.
| Standard | Cable size selected | Ampacity at design conditions | Utilisation factor at 1,600 A | Voltage drop at 30 m (full load) |
|---|---|---|---|---|
| IS 3961+7098 | 2 × (3.5C × 240 mm² Al XLPE) parallel | 2 × 410 = 820 A → not OK | — | — |
| IS 3961+7098 | 3 × (3.5C × 300 mm² Al XLPE) parallel | 3 × 470 = 1,410 A → OK | 113 % | 2.8 V (0.64 %) |
| IS 3961+7098 (final) | 3 × (3.5C × 400 mm² Al XLPE) parallel | 3 × 540 = 1,620 A → OK | 99 % | 2.2 V (0.51 %) |
| IEC 60364-5-52 | 3 × (3C × 400 mm² Al XLPE) + 1C × 240 mm² N | Same as IS, marginal differences | 98 % | 2.1 V (0.49 %) |
| NEC 2023 (informative) | 3 × (3C × 500 kcmil + 4/0 ground) Al | Higher ampacity → 1,800 A | 89 % | — |
Indian submissions go with IS sizing. Two practical things on the BoQ that matter more than the standard chosen:
- Ground temperature assumption — for ducts under direct sunlight in summer Indian cities, the actual ground temperature in the top 0.8 m can exceed 35 °C. Always use 30 °C as IS but verify thermal resistivity of fill ≤ 2.0 K·m/W per IS 11892.
- Grouping factor — three 3.5C × 400 mm² in a single duct triggers IS 3961 Table 11 derating factor of 0.74 for grouped cables. Apply this, not 1.0.
- Short-circuit withstand — for a 1,200 kVA transformer the prospective short circuit at LT terminals is ~25 kA. Verify that 400 mm² Al XLPE has 1-second I²t ≥ 25² × 1 = 625 MA²·s. IS 7098 Annex B gives 200 mm² Cu and 400 mm² Al as the minimum for this protection coordination.
When to deviate from IS
FM Global insured plants — NEC 2023 sizing is mandatory; the carrier audits to NEC tables, not IS. The capex impact is generally ±5 % on cable cost; over-sizing for NEC is the lesser problem than failing the carrier audit.
Solar PV farms — NEC 2023 Article 690 has specific PV cable derating + DC voltage rules that IS does not address. Use NEC for PV-side, IS for AC-side.
Multinational data centres — IEC 60364-5-52 is the global default; some hyperscaler standards (Microsoft DC, Meta DC) reference NFPA 70 (NEC) for fire-stop and conduit rules but IEC for ampacity. Always confirm the brand standard at design-basis stage, not at construction stage.
References
- IS 3961 (Part 1): 1967 (reaffirmed) — Recommended Current Ratings for Cables — PVC Insulated and PVC Sheathed Heavy Duty Cables, BIS.
- IS 7098 (Part 1): 1988 (reaffirmed) — Cross-Linked Polyethylene Insulated PVC Sheathed Cables for Working Voltages up to and Including 1100 V, BIS.
- IS 1554 (Part 1): 1988 — PVC Insulated (Heavy Duty) Electric Cables, BIS.
- IEC 60364-5-52: 2009+A1:2019 — Electrical Installations of Buildings — Selection and Erection of Electrical Equipment — Wiring Systems, IEC Geneva.
- NEC 2023 (NFPA 70) — National Electrical Code, NFPA Quincy MA, Article 310 + Tables 310.16, 310.17, 310.18.
- IS 11892: 1986 — Thermal Resistivity of Soil — Method of Measurement, Bureau of Indian Standards.
- BS 7671: 2018+A2:2022 — Requirements for Electrical Installations (UK Wiring Regulations).
- FM Global Property Loss Prevention Data Sheet 5-31 — Cables and Bus Ducts, FM Global 2023 ed.
// About the Author
MEPVAULT Editorial Team — A team of practising MEP consultants based in India. ISHRAE Mumbai chapter member; FSAI affiliate.