The three sizing criteria
A cable must carry the load current continuously without exceeding its insulation temperature, deliver acceptable voltage at the far end, and survive the prospective fault current for as long as the protective device takes to clear. Each criterion produces a minimum conductor size, and the cable you buy has to satisfy the largest of them.
- Current-carrying capacity — the base rating, before derating is applied
- Voltage drop — usually the governing criterion on long runs, where a cable rated comfortably for current still delivers too little voltage at the load
- Fault withstand — the conductor must not be damaged by the let-through energy before the breaker or fuse operates
Derating — where published ratings stop applying
Published current ratings assume reference installation conditions. Real installations rarely match them, and the applicable derating factors multiply together, so several modest factors combine into a substantial reduction.
- Ambient temperature above the reference value reduces capacity — significant in Pakistani summer conditions
- Grouping cables together in a tray or conduit reduces each cable’s rating
- Buried cables depend on soil thermal resistivity and depth of laying
- Enclosure in conduit or trunking restricts heat dissipation compared with free air
Insulation and sheath materials
Insulation sets the conductor operating temperature and therefore the current rating; the outer sheath protects against the external environment.
| Material | Characteristic | Typical use |
|---|---|---|
| PVC | Lower operating temperature, economical | General indoor power and control wiring |
| XLPE | Higher operating temperature and current rating | Power distribution, main feeders |
| LSZH | Low smoke, zero halogen when burning | Enclosed and occupied areas where smoke toxicity matters |
| Rubber / EPR | Very flexible, tolerant of movement | Trailing cables, portable and mobile equipment |
| PTFE and high-temperature | Withstands elevated ambient temperature | Furnaces, hot process areas |
Armouring and mechanical protection
Armour protects the cable from mechanical damage and can serve as an earth path. Steel wire armour is the general standard for buried and industrial power cables; aluminium wire armour is used on single-core AC circuits to avoid the circulating currents that steel would induce.
Control, instrumentation and screening
Control cables carry switching signals and are specified largely on core count and flexibility. Instrumentation cables carry low-level analogue and data signals and are far more sensitive to electrical noise — which is what screening addresses.
- Individually screened pairs prevent crosstalk between signals within the same cable
- Overall screening protects the whole bundle from external interference
- Both individual and overall screening is specified where signal integrity is critical
- Earth the screen at one end only unless the system design specifies otherwise — earthing both ends can create a circulating current
- Route instrumentation cable away from power cable, and cross at right angles where crossing is unavoidable
Voltage grade and glanding
Cable voltage grade is expressed as two figures — the phase-to-earth and phase-to-phase voltages the insulation is designed for. The cable grade must equal or exceed the system voltage. Terminating hardware matters too: the gland must suit the cable construction and the enclosure’s ingress protection rating, and armoured cable needs a gland that terminates the armour correctly for earth continuity.
What to include in a cable enquiry
- Number of cores and conductor cross-sectional area
- Conductor material — copper or aluminium
- Insulation and sheath material, and voltage grade
- Armoured or unarmoured, and screening requirement for control and instrumentation
- Installation method — buried, tray, conduit, free air — and ambient temperature
- Total length required per drum, plus glands and accessories
- Delivery city and required date