Different class ckts same conduit

Status
Not open for further replies.
H

housemoney

Member
Location
Midwest
Occupation
Engr
I'm reading art 725 and going in circles: Can I run (2) shielded belden cables (24VDC) to high level and level ind. instruments at the top of vessel in the same conduit as 120VAC to an adjacent motor operated valve? Seems like it would be easy to induce a voltage from the line 120v onto the signal?
 
housemoney said:
I'm reading art 725 and going in circles: Can I run (2) shielded belden cables (24VDC) to high level and level ind. instruments at the top of vessel in the same conduit as 120VAC to an adjacent motor operated valve? Seems like it would be easy to induce a voltage from the line 120v onto the signal?
Click to expand...
I believe all the wires would have to be insulated to same level (Say 600V) to be in the same raceway or cable.
 
Its commonly said that you can do this if all conductors are insulated for the same voltage, 300.3(C). But most miss the note at the bottom that sends you to Article 725, which says you can't.
The NEC does not care if you induce noise on your Belden. The concern in the NEC is the danger if the insulation was damaged then you would have 120 Volts on the limited energy terminals or equipment.
Yes art 725 is confusing, but there are three classes of circuits
Class 1 is equipment that is wired with a chapter 3 wiring method, and can run in the same raceway if functionally related. There are two types of class I circuits, power limited and non power limited.
Class 2 is a circuit where the power and voltage is limited, think doorbells
Class 3 is a circuit with higher power levels than a class 2, think nurse call.
Art 725 gives us an alternative wiring method that is safe due to the lower power levels
If your instruments are a class 1 circuit then you can have in the same raceway if functionally related. you need to look at the circuit power and voltage, or equipment.
Most PLC circuits are Class 2.
Now, practically, you won't get any induction from 120V, as the belden is shielded and twisted pairs.
With 277/480, you might get some induction.
I have a nice chart on Art 725 I will post it later today
 
Thats a nice sum up thank you.
tom baker said:
Its commonly said that you can do this if all conductors are insulated for the same voltage, 300.3(C). But most miss the note at the bottom that sends you to Article 725, which says you can't.
Click to expand...
Thats a good reminder, the 300.3(C) FPN sends points us to 725.136(A), and then the 8 other paragraphs (B-I) which indeed boil down to you cant call it a class 2 or 3 circuit if you pull some 18 AWG 600V THHN.

So to hopefully clarify my earlier post my understanding is we can re-reclassify any class 2 or 3 circuit to class 1 as per 725.130 (A). That would nix the Beldan cable unless it is 18AWG and is one of the listed types in 725.49 (B).
Correct me if I am wrong here.
 
Yes you can reclassify but you have to remark all the power supplies, and then its all a chapter 3 wiring method. I wonder what the AHJ would say to changing the power supply marking?
 
See section 725.136. I thought there was an exception for the power limited circuits that are directly associated with their power circuits, however I cannot seem to find it. Generally, the only way you can run them in the same Raceway is if they are separated by innerduct, or in the same outlet box unless separated by fix barrier, or in some cases 0.25 in separation. You also cannot strap the power limited cables to the Raceway; see 300.11(B).
 
infinity said:
If your power supply is labelled Class 2 Only you cannot re-classify.
Click to expand...
Actually, IMHO, the relevant limitation is that if the load fed by the supply is only listed as Class 2 you cannot reclassify the circuit and the power supply.
If the load (such as a 0-10V controlled dimmer) is not dual listed as Class 1/Class 2 then you cannot remark the power supply and reclassify it.

What removing the Class 2 marking on the power supply does is indicate that it can no longer be safely used to drive a load whose safety requires that it be driven by a Class 2 supply. In the same way, any part of the circuit whose wiring method is listed for Class 2 only must be replaced with a Chapter 3 wiring method, not just the portion which will be sharing a raceway with a power circuit.
 
JFletcher said:
See section 725.136. I thought there was an exception for the power limited circuits that are directly associated with their power circuits, however I cannot seem to find it. Generally, the only way you can run them in the same Raceway is if they are separated by innerduct, or in the same outlet box unless separated by fix barrier, or in some cases 0.25 in separation. You also cannot strap the power limited cables to the Raceway; see 300.11(B).
Click to expand...
You can attach a thermostat cable that is class 2 circuit to the power supply conduit to say a furnace if the two are associated. You can not run the class 2 circuit inside the power supply conduit though, even if you used 600 volt conductors for the class 2 circuit.

You could pull a class 2 cable and a NM, MC, AC, etc through the same sleeve provided for physical protection - they are two separate stand alone wiring methods sharing the same space in that application.
 
Status
Not open for further replies.
Top