Every substation project has the same conversation about six months too late. A relay gets added, an indication lamp gets moved, and there’s no spare core in the cable that was pulled through 80 metres of buried duct last winter. Re-pulling costs more than the cable did.
Core count is the one control cable decision you cannot revisit cheaply, so it’s worth ten minutes at the design stage.
Core counts are not a continuous range
Control cables are made in standard core counts, not in every number between 2 and 61. APAR’s LV XLPE/PVC control cables are offered at 2, 3, 4, 5, 6, 7, 10, 12, 19, 27, 37, 44, 52 and 61 cores. Notice the gaps. If your schedule says you need 8 cores, you’re buying a 10-core anyway. If it says 15, you’re buying 19. In both cases the spares arrive free, and the diameter barely moves.
That structure is the argument for generosity. Going from 7 to 10 cores takes the outside diameter from 14.5 mm to 18.0 mm on the 1.5 mm² construction. Going from 12 to 19 takes it from 18.5 mm to 21.0 mm. Cable trays and glands absorb that; a re-pull doesn’t.
Current rating falls as cores are added
Cores in the middle of a 61-core cable have nowhere to send their heat. So the per-core current rating drops steadily with core count, even though the conductor size never changes. These are the published figures for 1.5 mm² copper, XLPE insulated, to IS 7098 Part 1:
| Cores × 1.5 mm² | O.D. unarmoured (mm) | In ground, 30°C (A) | In air, 40°C (A) |
|---|---|---|---|
| 2 | 11.5 | 33 | 29 |
| 3 | 12.0 | 25 | 22 |
| 4 | 13.0 | 25 | 22 |
| 5 | 13.5 | 24 | 21 |
| 6 | 14.5 | 22 | 19 |
| 7 | 14.5 | 21 | 18 |
| 10 | 18.0 | 18 | 16 |
| 12 | 18.5 | 17 | 15 |
| 19 | 21.0 | 15 | 13 |
| 27 | 25.0 | 13 | 11 |
| 37 | 28.0 | 11 | 10 |
| 44 | 31.0 | 11 | 9 |
| 52 | 32.0 | 10 | 9 |
| 61 | 34.5 | 9 | 8 |
DC resistance stays at 12.1 Ω/km throughout, and the one-second short-circuit rating stays at 0.21 kA, because both are properties of the conductor rather than of the cable. It’s the heat that changes.
For most control duty this derating is academic, since indication and trip circuits draw very little. It stops being academic when someone runs a solenoid, a small motor space heater or a panel lighting circuit down a spare core in a 37-core cable. Check the number before you borrow a core for power.
What the rest of the spec looks like
These cables are rated 600/1000 V to 1100 V and made in 1.5 and 2.5 mm², with or without steel wire armour, sheathed in PVC, FRLS or LSOH. The 4 kV test voltage is what verifies the insulation. They’re built to IS 7098 Part 1, IEC 60502-1 and BS 5467, and also to AS/NZS 4026 and AS/NZS 5000.1 where a project calls for it. Aluminium isn’t offered in this construction, and shouldn’t be: at 1.5 mm² the termination is the weak point, not the conductor.
Practical rules
- Count the cores you need, then buy the next standard size up. The gap is usually the spare allowance you’d have specified anyway.
- Fix the spare-core percentage in the specification, not on site. Let the contractor decide and you’ll get zero.
- Terminate spare cores properly and land them on the terminal block. An unterminated spare coiled in a gland box is a spare nobody finds in five years.
- Record which cores are spare, at both ends, on the drawing that will actually survive.
- Watch the derating table before running anything with real current down a spare.
Where the same panel carries measurement signals, keep them in a separate screened instrumentation cable rather than borrowing control cores. On trackside work, the applicable family is railway signalling cable to IRS S 63/2007, which is a different specification altogether.