Construction at a glance
Power cables have few cores of large cross-section. Control cables have many cores of small, usually uniform cross-section — commonly 1.5 or 2.5 sq mm across anywhere from two to sixty-one cores.
| Feature | Power cable | Control cable |
|---|---|---|
| Core count | Typically 1 to 4 | Typically 2 to 61 |
| Cross-section | Large, load dependent | Small and uniform |
| Carries | Load current | Switching signals and status |
| Sized for | Ampacity, voltage drop, fault level | Voltage drop and mechanical strength |
| Screening | Usually none at LT | Frequently screened |
Different sizing problems
Power cable sizing is a thermal calculation. Current produces heat, heat must escape, and the conductor is sized so insulation temperature stays within limits after derating — then checked for voltage drop and short-circuit withstand.
Control cable carries negligible current, so thermal rating is rarely the constraint. Cores are sized instead for acceptable voltage drop over long runs and for enough mechanical strength to survive pulling and termination. A 0.5 sq mm core might carry the signal perfectly well and still snap during installation.
Screening and signal integrity
Power cables radiate electromagnetic interference. Control and instrumentation cables have to work in that environment without picking it up.
Screening is the defence. An aluminium mylar tape wrapped around the cores with a tinned copper drain wire gives induced interference a controlled path to earth. Instrumentation cables go further, twisting cores into pairs, triads or quads so interference induces equal and opposite voltages in each conductor that cancel at the receiving end.
- Overall screen — one shield around all cores
- Individual screens — each pair shielded separately against crosstalk
- Drain wire earthed at one end only, to avoid an earth loop
- Twisted formation to cancel induced interference
Earth the screen at one end only. Earthing both ends creates a circulating current path that injects the noise you installed the screen to keep out.
Instrumentation is a separate category
Instrumentation cable is often lumped in with control cable but solves a harder problem. Control cables switch contacts on and off, so a little induced noise is tolerable. Instrumentation cables carry analogue process signals — 4–20 mA loops, thermocouple millivolts — where induced noise becomes a wrong reading on the operator's screen.
These cables are built to BS 5308, with tighter control on capacitance, inductance and screen coverage than a general control cable requires.
Installation practice
Keep power and control cables physically separated in trays and trunking wherever the layout allows. Where they must cross, cross at right angles — this minimises the length over which the power cable can couple into the signal cable.
Running a shielded instrumentation cable in the same bundle as a motor feeder undoes the screening you paid for.
Applicable standards: IS 7098 and IS 1554 for power cables, IS 1554 Part 1 for PVC control cables and BS 5308 for instrumentation cables.