How to analysis the copper wires' cross-section


Thread Starter

Yang Lin

I'm working for Instrument installation in NPP construction Project.
During the site work, one thing is puzzled me.

For all level 0 signals are transmitted by copper wires which cross-section is 1.0 square milimeter even 1.5 square milimeter. So, one problem is appeared, the Cable Tray is full. In our Chinese national standard, the minimum of copper wires' cross-section is 0.5 suqare milimeter. I have collect the part of level 0 signals, some of them is 4-20mA, 12-36VDC, so I think it's too big. I hope someone can help me how to analysis and design the wires' cross-section according to the signal specification.

If you have any suggestion, please contact me. E-mail address: [email protected]

Your any information would be appreciated.


The first consideration, cable current carrying capacity, can almost certainly be forgotten for all your signal cables- they will probably be operating at low current, and heating effects will be negligible. Don't assume it, check. Beware of resistance in bridge circuits, (especially if they've forgotten to bring separate bridge voltage sense wires back) and in long current loops. Actuators might well operate at appreciable currents, so check those, and supplies to field- mounted instruments also need to be checked.

Next consideration is mechanical- wires need to be properly supported- and consider vibrations- small wires have much lower inertia, so they'll flex at higher frequencies. Using multiway cable counteracts this due to mutual support and vibration damping. The last few centimetres from the cable break- out to the terminations can be particularly vulnerable, as it's out of the multiway and the trunking and often has nothing but free space to support it.

Using multiple twisted pair cable intelligently can improve the system by cutting out crosstalk in higher- frequency signals.

Terminating fine cables also needs more thought, as it can be pretty fragile and copper work- hardens and becomes brittle. The point where a cable enters a terminal is a favourite place for a break to develop, bootlace ferrules and suchlike support helps here.

Make sure the cable is rated for the temperatures you'll get on the plant- both hot and cold- and watch out for runs alongside pipework etc.

Fine cable has thinner insulation, so watch voltage ratings (to adjacent circuits too)- segregate properly. Insulation can get worn away where the cable rubs against structures including trunking and trays- proper support and bend radii must be observed.

And don't forget the maltreatment the cable will almost certainly get as installers in a hurry pull it through inadequate gaps.