Control cable distribution standarts

[bigbb]

Hi all, I am new in US so it was suprise for me to see control signal cables pulled together (Analog, digital) in the same trays, and also see the multiconuctor cable with different voltages iside (48VDC, 120VAC)Personally I don not like it.
Question is if there are codes and standarts for control cables. Thanks in advance.

 

[itsmoked]

Personally no one likes it! But it is far less expensive, if it works.

An experienced EE can usually look at the signals and equipment involved and make a good call on whether or not the expense of separation is warranted.

There is no code or standard I know of that forces isolation of just analog and digital signals.

Code is primarily to prevent fire and peripherally to prevent blatant electrocution. It cares not whether the system works. There are some codes and standards that do, like the rules governing fire equipment.

A company may have internal standards that state some sort of separation. But just constant separation really makes little technical and economic sense.

The overriding code involved is just for separation based on voltages and the individual insulation ratings of the various conductors.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[dpc]

In the US, the National Electrical Code says any combination of voltage levels, ac or dc, UP TO 600 V can share a conduit or cable as long as all of the conductors are insulated for the highest voltage present. But the NEC is just concerned with safety. The fact that it is allowed by code doesn't mean it will actually work.

It used to be fairly common to run 120 V ac control wiring in the same conduit with the 480 V motor power, provided 600 V insulation was used for the 120 V circuits.

As Keith says, most companies have internal rules about signal and power segregation.

 

[bigbb]

Thanks a lot, just kinda unusial to see 3 phase 480v 200HP motor power cable in the same tray with 4-20ma control cables, hope it will work.

 

[controlsdude]

we do not mix low level signal (24vdc, 0-10vdc, or communications) in the same conduit or wireway. This has caused all sorts of problems. Now VFD power wiring causes the most problems and these should be in seperate conduit from all other signals.

Now I have seen where VFDs were run in the same conduit but I believe the reason this was ok was that all the equipment is basically running together. If you have different motors that run at different speeds and such running them seperate from each other is probably necessary. As far as shielded cable being necessary for VFDs I have not seen where replacement of individual wire with shielded cable has made a difference.

Now on wireway you can get the type of wireway that has a metal divider to seperate low level from 480vac.

 

[dpc]

That is not a normal practice, by any means, and I would not recommend it. The signal circuit cable must have 600 V insulation.

 

[itsmoked]

I'm with dpc. No excuse for 460V to share low level signal space. Where did they find 600V analog cables?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[roydm]

It often makes sense to run signal cables in the same tray as power cables, for example if there are only a few of each but there should be a metal barrier between the two.
Roy

 

[dpc]

roydm,

I'm not sure it ever makes sense to run a 4-20 mA circuit in a tray with 480 V power, even with a barrier. If there are only a few of each, I'd run conduit.

 

[roydm]

DPC,
We do projects all over the globe so we see lots of different wiring methods. My least favorite is the heavy conduit used South of the border.
There are lots of occasions where splitting a tray with a metal barrier makes sense, I have even seen 2 barriers on occasion with MV, LV & 4-20 on one tray.
In Canada you often see motor and control "Teck" armored cables on the same tray without a barrier.
Regards
Roy

 

[dpc]

I don't have a big problem with 120 V running with 480 V in a tray, although I try to avoid it. Analog circuits are another matter. I would never do that.

You're right - I've seen a lot of things installed in many different ways. But that doesn't mean I would design it that way myself.

Cheers,

Dave