Difference between control and instrumentation cable

The difference between control and instrumentation cable is mainly in use. Either can be solid or stranded. Back in the “good old days” control cables (for motors) needed to have larger wire than instrumentation cables in order to handle the larger currents required for the motor starters. They were typically terminated under screw terminals, and solid wire makes this termination easier. The instrumentation cables were smaller diameter and frequently made of stranded wire which is more flexible. Today, with greater use of electronic starter controls, it is mainly switchgear (breaker) control wiring that needs the larger diameter wire.

The other difference between the 2 types is that the instrumentation cable is typically a shielded (screened), twisted pair. This construction serves to minimize “crosstalk” (inductive coupling) that causes erroneous readings for the instrumentation. The control cables, whose circuits operated at 125 VDC, 110 VAC or 220 VAC levels were generally immune to this, and so did not require the shielding.

Today, when the control signals are run at 24 VDC, the shielded twisted pair construction is advised for them as well.

You still need to be careful about level separation, but as long as you are dealing with low DC voltages (28 V maximum), resistive loads, and using shielded twisted pair cables, you can combine the control and instrumentation cores into a single cable where needed. (It is still better practice to keep them separate.) You also should keep inductive loads (like solenoids and relays) separate from the instrumentation, since they can create high inductive spike voltages when they are de-energized.

You still need to choose a wire size sufficient to handle the maximum current and insulated for the maximum voltage. For the control wiring, be careful about using a single common wire for multiple devices - it will need to be sized for the total current.

1 Like