24v control and 208v power wire in control cabinet

[mavrck]

I have a question about installing different voltage in wire duct in a control cabinet.
I’m building a couple control cabinets that were mainly supposed to 120 /208 volt operation. Well the equipment showed and some now need 24 volt control and they added 3 drives with 24 input. There is no PLC everything is push button. Unfortunately there was no plan for this 24 volt application and I’m having to run 24 control wire beside 120/208 in wire duct. Can this cause EMI issues. Shielded cable aren’t a option due to routing the wire.

 

[tortuga]

This is just in the control cabinet right?
You'll have to refer to the documentation in the drive or the 24V source.
On one hand you can (re)classify the 24V as class 1 and use chapter 3 wire (TFFN).
While that might be allowed, it might not work reliably unless you can get some separation.

 

[Jraef]

Assuming you mean 24VDC?
There will be induction happening, but no way to tell if it will be enough to cause you troubles. I don’t understand the inability to use shielded wire though. It’s barely larger than unshielded.

 

[mavrck]

I can’t use shielded because I have to go form the control cabinet to a separate hmi cabinet using 24 vdc relay then then to my drive. So in order to make this work as they want I would have to remove the shielding and jacket and split the wires to the 24volt and to the common , basically making the shielded cable useless

 

[tortuga]

Hmmm thats not going to work as a class 1. Can you use a different 2 wire shielded cable for the relay?
Please post a diagram or a picture.

 

[JR Martin]

I have a question about installing different voltage in wire duct in a control cabinet.
I’m building a couple control cabinets that were mainly supposed to 120 /208 volt operation. Well the equipment showed and some now need 24 volt control and they added 3 drives with 24 input. There is no PLC everything is push button. Unfortunately there was no plan for this 24 volt application and I’m having to run 24 control wire beside 120/208 in wire duct. Can this cause EMI issues. Shielded cable aren’t a option due to routing the wire.

 

[JR Martin]

Code requires separate raceways for different voltages. Any kind of control wiring such as 4 to 20 ma must be in its own conduit. I would also check code as their is exceptions to this article.

 

[petersonra]

Code requires separate raceways for different voltages.

in what paragraph of the code will you find this requirement?

 

[Eddie702]

What about a factory built roof top heating and ac unit? 480v contactors with 24 volt controls it's done all the time

 

[BillyMac59]

It's hard NOT to do in a typical panel, especially with "add-ons" and "improvements" after the original is built. Isolate the voltages as best you can. If they must cross, keeping the individual conductors 90' from one another will minimize noise/induction.

 

[petersonra]

What about a factory built roof top heating and ac unit? 480v contactors with 24 volt controls it's done all the time

as long as the 24V circuits are not class 2 circuits it does not matter much.

 

[hbiss]

What about a factory built roof top heating and ac unit? 480v contactors with 24 volt controls it's done all the time

That would be a factory assembled control panel or equipment that has been listed. And thermostat circuits are CL2. The manufacturer may provide a divider or compartment for the 24 volt control field wiring. However, what wiring they do external to that wouldn't necessarily have to be according to the NEC.

-Hal

 

[JR Martin]

in what paragraph of the code will you find this requirement?

I do not know the location of the installation therefore I will recommend that you check Article 720, article 725 and article 727. Depending on the location will determine the installation requirements.

 

[Macbeth]

The basics is the difference Voltages don't matter, as long as the insulation rating on all wire/cable is greater than the highest voltage any wire/cable comes within 2" of. For a bunch of years now, we have needed to install 600v rated Cat5/6 cable in VFD cabinets, due the the VFD gland plate holes being less than 2" apparat.
If your class2 cables come within 2" of your 208vac power wires or any voltage greater than 42.8v, then your class2 cables must be changed to class1 300vac wire/cable.

UL508a
28.4 Separation of circuits
28.4.1 An industrial control panel shall be constructed so that a field-installed conductor of any circuit is
segregated as specified in 28.4.2 or separated by a barrier from:
a) A field-installed conductor connected to any other circuit unless:
1) Both circuits are Class 2 or both circuits are other than Class 2; and
2) The conductors of both circuits are intended to be insulated for the maximum voltage of
either circuit.
b) An uninsulated live part of any other circuit.
c) A factory-installed conductor connected to any other circuit, unless the conductors of both
circuits will be insulated for the maximum voltage of either circuit.

29.1.2 All internal wiring shall have insulation rated for the maximum voltage involved.

I have never seen a problem with discrete signals having EMI interference with power induced voltages.

 

[paulengr]

The basics is the difference Voltages don't matter, as long as the insulation rating on all wire/cable is greater than the highest voltage any wire/cable comes within 2" of. For a bunch of years now, we have needed to install 600v rated Cat5/6 cable in VFD cabinets, due the the VFD gland plate holes being less than 2" apparat.
If your class2 cables come within 2" of your 208vac power wires or any voltage greater than 42.8v, then your class2 cables must be changed to class1 300vac wire/cable.

UL508a
28.4 Separation of circuits
28.4.1 An industrial control panel shall be constructed so that a field-installed conductor of any circuit is
segregated as specified in 28.4.2 or separated by a barrier from:
a) A field-installed conductor connected to any other circuit unless:
1) Both circuits are Class 2 or both circuits are other than Class 2; and
2) The conductors of both circuits are intended to be insulated for the maximum voltage of
either circuit.
b) An uninsulated live part of any other circuit.
c) A factory-installed conductor connected to any other circuit, unless the conductors of both
circuits will be insulated for the maximum voltage of either circuit.

29.1.2 All internal wiring shall have insulation rated for the maximum voltage involved.

I have never seen a problem with discrete signals having EMI interference with power induced voltages.

I have definitely seen it. Allen Bradley analog cards are 100% total crap. Everyone else generally uses optoisolated inputs but not them. +/-10 V is really bad but even 4-20 mA which by nature should be pretty immune is a problem. Unless you want to buy signal conditioners on every input my suggestion is to wire all the returns (DC negative) together into a common DC neutral, possibly grounded to an isolated analog ground bus. This will at least prevent your 4 wire transmitters from floating on an induced 60 Hz which drives AB analog inputs crazy.

I know it sounds ridiculous but if you’ve ever fought with process equipment with long signal cables mixed with power conductors you will learn what the issue is.

Moving up to 600 V AWM CAT 5 is simply not an option outside an industrial control panel. By definition AWM is manufacturer specific and like RU not recognized as a Listed assembly but merely a component. Worse yet AWM is a pass through. It only becomes Listed as part of a tested assembly. So I don’t get how you can be using an AWM rating on a CAT 5 cable. It would have to be on the list if acceptable components and Listed under UL as an assembly or RU as a component. AWM cable is neither. So the 600 V rating goes away and it is only communication cable.

 

[tom baker]

What about a factory built roof top heating and ac unit? 480v contactors with 24 volt controls it's done all the time

Are you referring to the internal wiring? Is the 24 Volt Class 1 or Class 2?

 

[tom baker]

In control panels per UL 508A, the rules are different that what we use for different voltage systems (Re 300.3 C 1, please see the note)
I agree with PaulEng that 4-20 MA signals are very immune to induced noise.
Almost every job I installed had wireway and would often put discreet and 4-20 in the wireway with 120 volt, never 480.
It can be done to shielded wire, but you need to run the cable to each device, open jacket, make connections and then go to next device
If you equipment is just relay logic it may work just fine.

 

[tom baker]

When we talk about separation, Its for noise and fire/shock prevention
Induced noise is not a concern of the NEC.
Class 2 and 3 circuits are kept separate from power wiring as the equipment that the class 2/3 connects will not withstand 120 V if there is a short between the Class 2/3 and power wiring. (I don't like the term low voltage)
Most electricians will say its OK if insulated for the same voltage as thats what 300.3 C 1 states, ignoring the IN at the end.
Class 1 is different and can be mixed with power as the class 1 is wiring with a chapter 3 method, but its power limited, example, a thermostat control on an electric heater.