Power/Control Wiring Separation

[nhee]

I'm wondering what most specifications call for regarding separation/spacing between power and DC instrumentation cabling in an industrial environment (for noise/interference reduction). For instance, what would most consider adequate separation between one tray with 480 VAC 3 ph. motor leads, and a second with 24 VDC transmitter wiring?

My thought is that using twisted, shielded cable, these circuits would not require all that large a separation distance. Most of our installations are steel conduit, and we simply run separate AC/DC conduits. Now we're using tray, so we don't have the shielding benefits of the conduit.

 

[jim dungar]

Re: Power/Control Wiring Separation

Most shielded instrumentation wiring insulation is rated for only 300V so it must be seperated from 480V conductors.

 

[tom baker]

Re: Power/Control Wiring Separation

Tary Cable is rated 600V, and is available in shielded twisted pair configuration. Power Limited Tray Cable is 300 V rated.

 

[eesac]

Re: Power/Control Wiring Separation

If you are asking separation needed for interference (not Code), we usually use 12" separation between LV "signal" cabling and line voltage cabling as a general rule. I also see this same distance in other specs and publications.

 

[ray94553]

Re: Power/Control Wiring Separation

In the 2002 NEC Article 392.6(E) "Multi Conductor Cables Rated 600V Or Less", there is no requirement for the separation of cables rated 600 volts or less.
Then in Article 392.9(A) "Any Mixture of Cables", it says "...or any mixture of multi conductor power,lighting,control and signal cables, the maximum number of cables shall...", which seems to indicate that there is no requirement for separation.
If cables rated over 600V are in the same tray as those rated less then 600V then by Article 392.6(F) the requires that a solid metal barrier of compatible material with the cable tray be installed.
However I have only had one occasion in 25 years where low voltage signal cables were installed in the same tray without at least 12 inches of separation. On the one occasions that I did mix the power and low voltage control cables I argued against it and I could not find anything in the NEC to support my argument.
The separation of low voltage control, and signal cables from other mixed voltage power and or lighting cables rated 600v or less is only by engineering standards and recommendations by the suppliers of the instrumentation and control equipment. I have read requirements like this in the Honeywell installation specifications for their DCS systems.

 

[nhee]

Re: Power/Control Wiring Separation

Thanks for comments.

Agreed that other than the separation of 300 V cables from the 480 power wiring, code does not specify a minimum distance. Assuming separate instrument and power cable trays, I was looking for the normal separation distance adhered to. It sounds like 12" is common design spec.

I'm just wondering if 12" is really necessary for a properly installed shielded twisted pair.

 

[petersonra]

Re: Power/Control Wiring Separation

Originally posted by nhee:
I'm just wondering if 12" is really necessary for a properly installed shielded twisted pair.

It sort of depends. As noted in other posts, you have to make sure all the insulation ratings are adequate if you plan to mix them.

The other issue is whether you will get noise induced onto the signal wires. Usually it is more practical to just require seperation rather than figure out what might be OK and what won't.

 

[pierre]

Re: Power/Control Wiring Separation

See
725.55(H) Cable Trays. Class 2 and Class 3 conductors shall be permitted to be installed in cable trays, where the conductors of the electric light, Class 1, and non-power-limited fire alarm circuits are separated by a solid fixed barrier of a material compatible with the cable tray or where the Class 2 or Class 3 circuits are installed in Type MC cable.