Conductor sizing factors and armoured cable

The conductor size of a cable is chosen so that it carries the load current without overheating, keeps the voltage drop within limits and survives the short-circuit current until the protection clears the fault.

Factors for deciding conductor size

1. Load (design) current: I = P / (√3 · V_L · cosφ · η) for 3-phase loads; the cable rating must be ≥ the design current and ≥ the rating of the protective device.
2. Derating factors: ambient temperature, grouping of cables, depth of laying, soil thermal resistivity and method of installation (air, duct, ground) reduce the rating given in the tables.
3. Voltage drop: the drop from supply to load should be within the permitted value (about 2.5–5 % of rated voltage). Long cables are often sized by voltage drop, not current.
4. Short-circuit withstand: A ≥ I_sc · √t / k, where k depends on conductor and insulation (about 115 for Cu-PVC, 143 for Cu-XLPE).
5. Type of load: motor starting current, harmonics (neutral current), duty cycle.
6. Future expansion: a margin of about 20–25 %.
7. Economy and material: copper vs aluminium, cost of energy losses over the cable life.
8. Mechanical strength: minimum sizes (e.g. 1.5 mm² Cu for lighting, 2.5 mm² Cu for power sockets).

Construction of 3½-core armoured cable

A 3½-core cable has three full-size phase conductors and one reduced-size neutral (about half the phase area), used for 3-phase 4-wire LT supply where neutral current is smaller than phase current.

        outer PVC sheath
      /  steel wire / strip armour
     /  /  bedding (inner sheath)
    /  /  /   filler
   (  (  (  ___   ___  )  )  )
          ( R )  ( Y )
             \    /
          ( B )  (n)  <- reduced neutral
   each core: conductor + PVC/XLPE insulation

Layers from inside to outside:

1. Conductors: stranded copper or aluminium, often sector shaped to reduce the overall diameter; three phase cores plus one half-size neutral.
2. Insulation: PVC or XLPE over each conductor, colour coded (red, yellow, blue, black).
3. Fillers: non-hygroscopic PVC or polypropylene fillers that make the laid-up cores round.
4. Inner sheath (bedding): extruded PVC layer that protects the cores and gives a bed for the armour.
5. Armour: galvanised steel wires (round) or steel strips laid helically; gives mechanical protection and acts as an earth-continuity path. Single-core cables use non-magnetic (aluminium) armour.
6. Outer sheath (serving): tough PVC (or FRLS) sheath against moisture, chemicals and corrosion, with voltage grade and size printed on it.

Such cables (e.g. 1.1 kV, 3½ × 95 mm² Al XLPE armoured) are laid directly in the ground or on trays between the transformer, MDB and SDBs.