Instrument Cable Type and Cable Size

Do you have any instruments?

What is your training?

Instrumentation Cable Selection Guide

You should really strive to learn these things before you need to use them.

Instrument manufacturers would be a good source of knowledge.

Or, a book. Or, a class. Or, searching on the internet, like I just did.

All of this should have been learned in school, before you joined the "work" force.

Or, are you still in school?

I was not into wiring before that's why i do not have any idea.

Control and Instrumentation Cables Information on GlobalSpec

Size & no. of cores or Pair is depend upon where you want to use the Cable. e.g. If you want to select the cable from Field Junction Box to the Cabinet in The building, then No. of core depends upon the no. of Terminal Blocks you are installing in the JB.

Core size is depends upon the 1.5 mm2 or 2.5 mm2 is depend upon the distance. To avoild the voltage drop higher core size cable is selected. Normally as per my experience 1.5mm2 cable is most widely used for analog signals.

Pair cable to be used for Analog Singals. & Core cable is used for Digital Signals.

British Standard 7671 will give cable sizes that are undersized mechanically on instruments. That's why many companies will specify a minimum cable size for this task, simply for the mechanical resiliance it offers in an industrial environment. "Min 1.5mm²" is a frequently-encountered requirement, as it makes the cable robust. It also becomes easy to fit spades, bootlaces and lugs where it is held by terminals.

As to the number of pairs, well, that is installation-design-specific, and should be obvious by looking at the cable schedule that the C&I Engineer will have called-up at design stage for the project.

Hi Arman,

Most of the time up to client requirement for the cable psecification, however for my current project we are using 1Px1.5 for AI/AO and DI/DO 2.5mm2, FR cable

Regard

Sir,

Don't be off-put just because you are new to the area. Learn from every resource you can find, such as the links given by others in their posts and the manufacturers' literature. In many instances the instrument will be quite forgiving with regard to the cable that is connected to it. In a few, however, the results will be very troublesome until you have exactly the proper cable.

Here are a few thoughts:

• Make sure that the voltage level of the outer insulation (the cable jacket) is higher than the voltages on the wires of the equipment it is near, unless it is properly and permanently separated from them by an insulating barrier or the required distance.
• Make sure that the cable is of a type that meets the specified voltage and power levels for the instrument (in the USA these might be given as a class number, for example).
• Make sure that the cable will not receive unwanted signal interference from nearby cables nor radiate interference to nearby cables or equipment. In some cases this will require that the cable have an overall shield that is grounded (at one end). In one application an unshielded cable may work well, but in another one it may fail, just because the nearby equipment and cables to them are different.
• Make sure the cable is designed for the type(s) of signals it is to carry. If it is an analog signal such as 0-10V or 4-20mA, then it must have a size large enough to carry that signal from its source to its destination without excessive voltage drop. If it is a digital signal, then the frequency will determine other limits such as capacitance per unit of length, twisting, etc.
• Make sure that the cable is rated for use in the environment it will occupy, such as temperature, approval for a cable tray or exposure to sunlight, resistance to particular chemicals or solvents, etc. Example--in a lead-acid battery plant, no portion of the cable's construction can use nylon because acid exposure will turn nylon into syrup within a few minutes.
• Determine if you need spare capacity, such as extra conductors or a higher frequency capability for future growth.
• If the cable is to carry multiple signals, do they need to be separated from each other by individual shielding? Such cable types are available.

Once you have evaluated the answers based on these types of questions, you are ready to choose or recommend a specified cable type and construction (or to call a reputable vendor or manufacturer with these answers and let that person's expertise and experience help you). You might want to call more than one manufacturer or vendor. You might want to include in your specifications the limits you determined from the questions above, or a manufacturer's catalog number and the phrase "or equal". You can then search for price and availability for the desired cable in the required length.

If the cost is outside your budget, then you have three options--revise the budget, review the selections to see if any specifications can be changed to a lower cost option, or review the proposed installation to see if any changes can be made there to permit a lower-cost cable.

--John M.

The proverbial "two bits of wet string" will support 4-20mA.

Concentrate on the mechanical resilience of the cable, as this will be the sizing factor for <...instrument cables...>, rather than anything else.