Proper Wiring Method:Instrumentation Cables

[mcollings]

We are setting up a C1, D2, group B industrial research area within a building. Addressing our instrumentation needs, I am looking at the code for circuits utilizing low-voltage (24 VDC) current loops. This has lead me to Article 725. It appears that Type PLTC cable should be used in trays if the wiring could be incendive, or type PLTC-ER if outside of cable trays more than 6 feet and incendive. However for non-incendive runs the shaded area sounds as though it contradicts itself.

1) Is there an error in the wording of the shaded comment following NEC 2008 725.154(D)(2) "Type PLTC cable is permitted and required ...if the circuits are non-incendive according..."? Should it read "not non-incendive" like below where it says "not intrinsically safe" for the analogous D1 technique?

2) If the circuit is non-incendive, then can any acceptable cable be used for this circuit ?

3) Can this wire also be used outside of the tray like PLTC-ER ?

 

[bobgorno]

The shaded area does contradict 501.10.(B)(3). Since the shaded area comments are not code, I would stick with the requirements as noted in 501.10(B)(3) and 725.154. There are special requirments to meeting non-incendive and Class 2 circuits. Be aware of all of the them.

It might be easier to use Article 727 and ITC-ER cable rule. The PLTC cable may already have an ITC rating.

 

[mmorancpe]

Non-incendive circuits will have a nonincendive rated power supply. Even though the code provides for this, there are very few nonincendive rated power supplies. Try and buy one. It's impossible. The few that exist are specific to the equipment they power.
Forget that.

501.1 Scope tells you that it covers all wiring and all voltages in hazardous locations. That is where you should have looked first. 725.3 (D) also refers you back to the 500 sections when installing in hazardous locations.

501.10 Wiring Methods, (B) Class 1, Div 2 list acceptable wiring methods for class 1 div 2. In that section (4) PTLC cable is permitted with some restrictions.

Hope this helps.

 

[justdavemamm]

Non-incendive circuits will have a nonincendive rated power supply.

Please provide where in the Code it says this. Seriously. I can't find it.

Even though the code provides for this, there are very few nonincendive rated power supplies. Try and buy one. It's impossible. The few that exist are specific to the equipment they power.
Forget that.

I can buy many Class I Division 2 rated P/S's. Are they rated nonincendive ? I don't know. What I do know is that somehow the many hundreds of nonincendive instruments that I buy each year are installed in Class I Division 2 areas are powered by something.

 

[mcollings]

I am in a similar situation with respect to creating a nonincendive circuit. Unlike intrinsically safe circuits, it appears that there are very few options for making a circuit nonincendive. You can buy a Class 1/Div 2 power supply, but to the best of my understanding, that doesn't make the complete circuit (supply, wiring, field device) nonincendive because you have no documentation referencing capacitance, inductance, current and voltage limitations under normal operating conditions. With instrinsically safe systems there is a barrier between the supply and the field device for this purpose, and a paper trail to document the limits of the installation. But this is expensive and overkill for Div 2. You can limit the current thru fusing, and the voltage thru a Class 2 supply, but how can you assure C/L are within safe limits? The requirement for a control drawing in 501.10(B)(3) seems to make the method difficult to comply with and removes the cost benefit for trying.