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#23763 - 03/26/03 11:00 PM Please help me settle another argument  
CTwireman  Offline
Joined: Feb 2002
Posts: 840
Connecticut, USA
The argument is about the common practice of installing the power and low-voltage control wires in the same raceway, such as the seal-tight to feed an AC unit.

I say this practice is wrong, based on 725-52(a). ('99 NEC)

Those I have argued with say it's ok, as long "the wires have the same insulation." (no code reference provided.)

[Linked Image]


[This message has been edited by CTwireman (edited 03-27-2003).]


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#23764 - 03/27/03 12:08 AM Re: Please help me settle another argument  
txsparky  Offline
Joined: May 2001
Posts: 552
Magnolia,.Texas U.S.A.
I personally have never seen this before and don't like the sound of it.How are they gaining entry into the sealtite?

Hmmm.....So....An a/c seviceman could possibly be troubleshooting what he believes to be 24 volts,that could possiby be 277v(3ph.480v unit) to ground due to a short.Don't sound very safe to me.


#23765 - 03/27/03 07:50 AM Re: Please help me settle another argument  
spyder  Offline
Joined: Oct 2002
Posts: 210
I do not like this practice either. However my understanding is if the the low voltage conductors have an insulation of equal to or better than the line voltage conductors than it is a legal installation. I am going to try and hunt down a code reference. I hope the person doing this type of installation has the sense to use some odd colors to more easily identify low voltage contol wiring form line voltage.

#23766 - 03/27/03 08:12 AM Re: Please help me settle another argument  
Joe Tedesco  Offline
Joined: Oct 2000
Posts: 2,749
Boston, Massachusetts USA

[This message has been edited by Joe Tedesco (edited 03-27-2003).]

Joe Tedesco, NEC Consultant

#23767 - 03/27/03 12:03 PM Re: Please help me settle another argument  
resqcapt19  Offline
Joined: Nov 2000
Posts: 2,148
How do you read 725.52(A)Exception #2? This is a new an confusing section and looks like it undermines the rules in 725.55. It doesn't exactly say this, but it sure implies it. Even looking at the ROP and ROC, I don't get a clear understanding of what the new exception means.


#23768 - 03/27/03 12:22 PM Re: Please help me settle another argument  
Joe Tedesco  Offline
Joined: Oct 2000
Posts: 2,749
Boston, Massachusetts USA

[This message has been edited by Joe Tedesco (edited 03-27-2003).]

Joe Tedesco, NEC Consultant

#23769 - 03/27/03 01:06 PM Re: Please help me settle another argument  
SolarBear  Offline
Junior Member
Joined: Aug 2002
Posts: 2
St. Louis, MO USA
This is a great and timely topic that others and myself have been kicking around in the office for over a month. Please bear with me for a minute and let me know if you think this flies - I welcome all opinions and interpretations!

Here's my POV: If I'm interpreting the NEC correctly, as long as the control circuits can be classified as Class I circuits, then they can be run in the same raceway with the power circuit ONLY if they are functionally associated.

There is a discrepancy that is raised from this because 300-3(c)(1) apparently lets you mix ANY cables with the same insulation rating, with the exception of the FPN covering Class II and III cables. That section does not make mention of Class I cables having to be separate according to 725-26 (unless functionally associated).

There was an article in last month's EC&M confirming this - "Making Sense of the NEC's Rules on Industrial Controls." According to the author, all Class I circuits and power circuits must be totally separate according to the NEC (unless functionally associated). This apparently spells the end to the very common industrial tradition (still being practiced as I write this) to mix 600V insulated power conductors with 120V/125VDC/24VDC 600V insulated discrete control conductors all together in tray and conduit.

My opinion is this: I see the NEC's point in separating the controls from the power circuits. In the case of insulation breakdown, accident, whatever - you get 277 into a PLC discrete input or something like that and fry the equipment and/or the guy troubleshooting the controls (as pointed out also in an earlier post). But - at the same time, the NEC allows you to run power and controls that are functionally associated together where I believe you would have the greater risk of an accident - such as mixing up the power and control terminations at a starter, where you have a couple of multiconductors coming in the same conduit or some scenario like that.

So finally, from all this I would gather, CTwireman, the installation you referred to is kosher with the NEC if the A/C Unit's control circuits are Class I because they can be considered functionally associated.

[This message has been edited by SolarBear (edited 03-27-2003).]

#23770 - 03/27/03 01:38 PM Re: Please help me settle another argument  
resqcapt19  Offline
Joined: Nov 2000
Posts: 2,148
725.52(A) ... Exception No. 2: Class 2 and Class 3 circuits shall be permitted to be reclassified and installed as Class 1 circuits if the Class 2 and Class 3 markings required in 725.42 are eliminated and the entire circuit is installed using the wiring methods and materials in accordance with Part II, Class 1 circuits.
FPN: Class 2 and Class 3 circuits reclassified and installed as Class 1 circuits are no longer Class 2 or Class 3 circuits, regardless of the continued connection to a Class 2 or Class 3 power source.

I don't like the idea of Class 2 conductors in with the power or Class 1 conductors and agree that 725.55(A) was very clear before the exception was added to 725.52 in the 2002 code, but this new exception really confuses the issue. After the circuit is reclassified from Class 1 to Class 2 using 725.52(A) Ex.#2, 725.55 no longer applies as the circuit is now a Class 1 circuit and not a Class 2 circuit. The rules in 725.26 would now apply to this circuit.

[This message has been edited by resqcapt19 (edited 03-27-2003).]


#23771 - 03/27/03 07:57 PM Re: Please help me settle another argument  
John Steinke  Offline
Joined: Apr 2001
Posts: 518
Reno,Nv., USA
From an academic point of view:
UL standards typically require all wires that are in proximity to have insulation capable of withstanding voltages based upon the highest nominal voltage present. Typically, this is not a hard thing to accomplish.
As for control circuits- I believe that the key is to understand what will happen when the power circuit induces a voltage onto the control wires. Will this fool the controls into operating? While the induced voltage isn't about to actuate a starter coil, it could very well fool a PLC into thinking a button had been pressed.
Since a thermostat is often a mechanically operated switch, we've been able to get away with slipping the t-stat wire into the seal-tite. With longer runs, and electronic controls, this could become a problem.
I should also note that the absence of a separate conduit is NOT proof of control cables being run together with the power. I've seen the tistat wire run together with the A/C hose set, and I've seen the compressor triggered by a pressure drop, caused when a remote valve is actuated.
Technically, I agree that T-stat wires should not be run within the power conduit.

#23772 - 03/28/03 01:32 AM Re: Please help me settle another argument  
nesparky  Offline
Joined: Jun 2001
Posts: 642
I for one do not recall seeing 600V rated thermostat wire for over 5 years. The only thing I have seen is rated for 300V. Since the insulation is not the same, it cannot be in the same raceway. 300.3(C)(1).
If some one ran 600V rated wire for the T-stat, it may be ok by code but IMO not very smart.


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