Field Control Wiring Method

J

Thread Starter

J Shah

I used 12 conductor cable to wire status and control signals from field equipment (138KV Circuit Breaker) to PLC Remote IO (Siemens ET200M). Out of 12 conductors, 4 wires are used for 24VDC signals and other 5 wires are used to carry 125VDC signals and rest of them are spares? Is this method OK to use or allowed as per NEC? Appreciate help.
 
It's not clear from the information provided how much current is being carried on the 125 VDC signal wires and how much current is being carried on the 24 VDC wires.

It's also not clear if the wires in the multi-conductor cable are individual conductors ("cores") or twisted, shielded pairs.

The industry practice is not to mix "high" and "low" voltage signals in the same cable, or even in the same conduit or cable tray or wireway. The loose "standard" is low voltage is anything below approximately 50 VDC, and high voltage is anything above that.

It's all really about the amount of current being carried in the conductors, and in wires and cables that are adjacent to or in close proximity to other cables and wires. If there is a "lot" of current being carried in wires at higher voltages, and/or if that current changes at a fast rate (50- or 60 Hz AC or higher frequency signals, for example) then it's possible for wires and conductors in close proximity to have unwanted voltages induced on them, which can affect the lower-voltage circuits.

A lot of sites "get away" with doing what you're doing, if the voltage and current levels of the signals being carried in the same cable are low enough so as not to affect the other signals. Also, the proximity of this cable to other cables carrying high voltage and/or current signals can cause unwanted induced voltages. The length of the cable and how much of it is in close proximity to higher current and voltage-carrying cables and conductors is also a factor in how much, if any, unwanted induced voltage may occur.

Based on the information provided, we can't be of much more help. You may be lucky, and you may not. The odds are evenly split, based on the information provided.
 
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