Instrumentation cables sizing


Thread Starter

Ahmed Sami

Dear All,

I need you to help me to calculate cable cross sections for the signals from the instruments in the hazardous area to the control room. used Instruments are:

1. Pressure and temp. transmitters.
2. Limit switches.
3. proximity switches.
4. solenoid valves.
5. actuators.

Thanks in advance,
Ahmed Sami
Why do you sugget 1.5 mm2 cable? Does that follow any standards recommendation or calculations?

also why shall I use 3 cores or 6 cores? Taking into consideration that all the instruments I use are 2 wire.
Ahmed Sami,

Choosing wire/cable for a particular application is a very involved process.

Does the wire/cable need to be armor-shielded?

Does it need to be twisted and shielded (to prevent nuisance electrical noise (induced voltages) from affecting the signal(s) being carried on the wire(s)?

Does the wire/cable have to have special type of casing/insulation (PVC or non-PVC; fire retartardant or non-fire retardant; etc.)?

Some sites and locales/regions require special colors to be used for certain signals, such as wires/cables which carry trip signals have to have a certain color outer casing. So, are there any special considerations for your site or locale/region that have to be considered?

And, most importantly, how many amperes will the wire carry, and over what distance? Ampacity is probably the most important aspect when choosing a wire as the wire needs to be capable of carrying the maximum anticipated current without damaging the insulation. And, if the distance over which the wire must transmit the current is a long one the voltage drop over the length must also be considered, which means sometimes the cable has to be a little larger to reduce the expected voltage drop if the distance is "long."

These are just a few of the most important considerations to take into account when choosing wire/cable for a particular application. It's not just cross-sectional area, or even the number of conductors (sometimes called "cores") required, it's also about how much current over how much distance, and if armor-shielding is required, and if the cable is to be plenum-rated, outdoor rated (sunlight resistant), fire retardant, and if it has to have a particular color to meet site or local/reginal requirements and/or technical regulations/standards.

This kind of question is a very good one to ask of local wire/cable suppliers in your area. They are aware of local/regional codes/standards, and they will likely ask a few questions about amperage and distance and so forth, and they will usually make very quick recommendations if they are knowledgeable and experienced.

Asking this type of question on an Internet forum with almost no information (you didn't provide expected voltages/currents for any of the devices, and no expected distances, and nothing about shielding (armor or noise) or fire resistance/retardancy (is that even a word??), etc.

Please, get some more information and work with a local supplier in your area to get the correct wire for the application. And, see to it that the cable is routed properly into the control room (signal level separation is important with low-level control signals!).

Let us know how this turns out for you.
Dear Ahmed Sami,

Your question is incomplete I can say. Cable cross section and hazardous area rarely related to each other.

My advise is study on the instruments' technical information.

Signal cable is decided from which field protocol you use, twisted pair 1.3 mm2 is common.

Power cable is depends on your power used. Most information is available in instruments sheet.