Control wiring practices

[RayRay]

Can anyone advise on correct segregation of wiring/tubing..I am installing older process system with PLC-5 system that uses penumatic tubing, 24VDC and 4-20ma signals...can these all go in same conduit or is that bad practice (I assume need to segregate 110V power)...Thanks in advance for your help

 

[OkiePC]

Can anyone advise on correct segregation of wiring/tubing..I am installing older process system with PLC-5 system that uses penumatic tubing, 24VDC and 4-20ma signals...can these all go in same conduit or is that bad practice (I assume need to segregate 110V power)...Thanks in advance for your help

When I have had to put air hoses and wires in the same pipe, I figure the cross sectional area of the air hoses separately and add them to the size results for the electrical, or subtract it from an existing pipe to see what else may fit. "Pneumatic conductors" are almost always less flexible in joints and bends so going strictly by the math is no good for a long run with many bends/joints, then common sense has to take over.

I know of no code reason you can't do this, although I am certainly not a code expert, others here will be more qualified to comment on the legalities. Definitely avoid putting anything over 50v in the same pipe with signals and 24vdc. Doing so can change your arc flash safety requirements and open you up to noise problems.

And, just because it all installs okay and looks good on paper, make sure you can service it (pull one cable, tube, or hose back out) when it is all said and done.

 

[The Plc Kid]

I would keep them separate if you could. The 24 VDC and analogs would be ok together but I would put any pneumatics by itself.

What happens when the air system starts making water and that get on your wiring. It may never happen with proper systems in place but I would not chance it myself IMHO.

 

[OkiePC]

I would keep them separate if you could. The 24 VDC and analogs would be ok together but I would put any pneumatics by itself.

What happens when the air system starts making water and that get on your wiring. It may never happen with proper systems in place but I would not chance it myself IMHO.

I agree with this. In our cases where it's been done, the control panel is two rooms and 150' away from the air powered valves that are being controlled, the wiring has to be water resistant anyway. To separate them making extra penetrations through some of these insulated walls requires multiple approvals and special attention to sealing.

In other words, it might be way more trouble to separate them, but if not, go ahead and do that.

 

[Lancie1]

Installation of Conductors with Other Systems: NEC Article 300.8

Can anyone advise on correct segregation of wiring/tubing..I am installing older process system with PLC-5 system that uses penumatic tubing, 24VDC and 4-20ma signals...can these all go in same conduit or is that bad practice..?

It is bad practice. In general the National Electric Code prohibts non-electrical conductors in electrical raceways. Electrical raceways are defined in Article 100 Raceways as "an enclosed channel of metal or non-metallic materials designed expressly for holding wires, cables, or bus-bars, with additional functions as permitted in this Code.

Raceways include, but are not limited to, rigid metal conduit, rigid non-mnetallic conduict, intermediate metal conduit, liquidtight flewible conduit, flexible metallic tubing, flexible metal conduit, eleectrical nonmetallic tubing, electrical metallic tubing, underfloor raceways, cellular concrete floor raceways, cellular metal floor raceways, surface raceways, wireways, and busways."

Although cable trays are not considered to be raceways, the permitted uses for cable trays do not include non-electrical items.

EDIT: No, I was wrong. There is a specific prohibition in NEC Article 300.8 Wiring Methods. Installation of Conductors with Other Systems:

"Raceways or cable trays containing electric conductors shall not contain any pipe, tube, or equal for steam, water, air, gas, drainage, or any type of service other than electrical".
-

 

[OkiePC]

Thanks Lancie. I should have known that it would be against code no matter how harmless and innocent it might appear in some cases.

 

[Lancie1]

I think a lot of the stuff in the NEC is nothing other than electrical "turf-protection" thinking that is also seen in many other places. If we could get rid of that paranoid thinking and instead help each other to get things done, instead or "hey, that's my ball park and stay out", then the world would be better off.

I doubt if violating this article will have any serious results.

Still, the NEC contains the rules such as they are, and you wouldn't have wanted me to say it wasn't in there, would you?

 

[Kent Hostetler]

Pneumatic tubing, and any other example of items listed in 300.8 don't belong in an electrical conduit, no matter how benign you believe the combination will be.

As Lancie stated in his first post, it is a violation of the NEC and just a plain bad idea.

If the tubing needs to be run to the same area, you can install it by itself in it's own conduit if you want. That's because a conduit is not a raceway by definition until it has an electrical conductor, fiber optic cable, etc. inside of it. Unless it does contain a conductor, it isn't covered by the NEC.