Armoured, flexible or FR: choosing cable for a plant
Pick the conductor by run length and cost, the construction by where the cable physically lives, and the size by volt drop rather than by current. On a plant of any size the cable that fails first is almost never undersized for current — it is a cable that was sized for a shade temperature it never sees, or run 200 metres across a yard without anyone checking the volt drop.
- Volt drop limit5% total to the furthest point, 3% for lighting (IS 732)
- Al vs CuAluminium needs ≈1.6× the cross-section for the same current
- XLPE vs PVC90 °C conductor temperature against 70 °C — more current, same size
- ArmourMechanical protection only. Buried, yard, vehicle or rodent risk
Aluminium or copper?
Aluminium conducts about 61 per cent as well as copper by volume, so you need roughly 1.6 times the cross-sectional area for the same current. Against that, it is far cheaper and much lighter, which matters when you are pulling 240 sq mm four-core three hundred metres across a plant.
Our rule at the counter: aluminium for power runs above about 25 sq mm, copper below that and for anything that gets disturbed. Control wiring, small motor tails, panel internals, and any termination that will be opened and remade repeatedly should be copper — aluminium terminations creep and need re-torquing, and a loose aluminium lug is a hot joint waiting to happen.
If you do use aluminium, insist on bimetallic lugs at every termination onto copper busbar or copper equipment terminals. Aluminium bolted straight to copper in a damp plant sets up galvanic corrosion, and the joint resistance climbs until something glows.
XLPE or PVC insulation?
XLPE runs at a 90 °C conductor temperature against 70 °C for PVC, which means a given size of XLPE cable carries more current than the same size in PVC. It also handles short-circuit temperature rise better and is more resistant to moisture.
For plant power distribution, specify XLPE. PVC still has its place in domestic and light commercial wiring, and FR — flame retardant — PVC house wire is the right thing for a building. For a hospital or any premises where people have to evacuate, ask for FRLS or LSZH, which produces far less smoke and no halogen gas when it burns.
| Type | Use it for | Do not use it for |
|---|---|---|
| XLPE armoured (Al) | Buried runs, yard crossings, transformer to PCC, main feeders | Short panel tails, anything needing frequent re-termination |
| XLPE unarmoured | Inside buildings, on tray in protected areas, in conduit | Direct burial, open yards, rodent-prone areas |
| Flexible copper | Motor tails, moving machinery, panel interconnection, portable tools | Fixed long runs — it costs far more per amp |
| FR / FRLS house wire | Building wiring in conduit, lighting and socket circuits | Any industrial power feeder |
| Control cable (Cu, screened) | Signal runs, sensors, drive control, anything alongside power cables | Carrying load current |
Sizing: current first, then volt drop, then derating
Three checks. The cable must pass all three, and the biggest of the three sizes wins.
1. Current carrying capacity. Start from the load current, add the protective device rating, and read the cable catalogue figure for the installation method — in air, on tray, in duct, buried direct. These are all different numbers for the same cable.
2. Volt drop. This is the one that catches people. Volt drop is proportional to current times length. Under IS 732, total drop from the origin to the furthest point should not exceed 5 per cent at full load, and lighting is generally held to 3. A 400 metre run to a crusher at the far corner of a yard will usually need a cable one or two sizes above what the current alone demands — and if you get it wrong, the motor runs at low voltage, draws extra current to make the same power, and cooks itself slowly.
3. Derating. Multiply the catalogue rating by every applicable factor:
- Ambient temperature — the catalogue assumes 40 °C air or 30 °C ground. A tray near a kiln or a dryer is much hotter.
- Grouping — cables bunched together cannot shed heat. Six cables in one duct bank can be down to 60 per cent of the single-cable rating.
- Depth and soil resistivity for buried runs. Dry, sandy or rocky soil carries heat away far worse than damp loam, and western Odisha soil in April is not damp loam.
The failure we are called about most often in the sponge iron belt is cable that has been sized correctly on paper and then run along a tray beside a kiln. At 60 °C ambient the derating factor is brutal, the cable runs continuously at its limit, and the XLPE ages years in months. When we go and look, the insulation is hard and the cable is discoloured for exactly the length that passes the hot zone.
The second is undersized cable to a distant crusher, diagnosed by the owner as "the motor is weak". Measure the voltage at the motor terminals while it is running under load, not at the panel. If you have lost more than a few per cent, the cable is the problem and no amount of motor rewinding will fix it.
The third is buried unarmoured cable, laid by whoever was cheapest, found three years later by a JCB.
Buying practicalities
- Buy to ISI marked standard. IS 7098 for XLPE, IS 1554 for PVC insulated. Ask for the test certificate on a large order; a reputable make will produce it without fuss.
- Check the conductor, not just the drum. Undersized conductor inside correctly marked sheathing is the commonest fraud in cable, and it is invisible until you strip an end. Weigh a metre if you are unsure — copper and aluminium have known densities and the sums are easy.
- Order 8 to 10 per cent extra on long runs for routing, terminations and the bend you did not plan for. Joining a run mid-yard because you were two metres short introduces the weakest point in the whole circuit.
- Get the glands right. Double compression glands for armoured cable, correctly sized to the armour. A gland that does not grip the armour leaves the armour unearthed, which defeats the point of buying it.
The short version
- Aluminium above 25 sq mm for power; copper below, and for anything re-terminated.
- XLPE for plant, FR or FRLS house wire for buildings, FRLS for hospitals.
- Armour wherever the cable is buried, crossed by vehicles, or exposed to rodents.
- Size on current, then check volt drop, then apply every derating factor.
- Ambient temperature is the derating everyone forgets and the one that kills cable.
Send us the load, the run length, and how the cable will be installed — buried, on tray, in duct, in air, and roughly how hot that place gets. We will size it and quote it.