jssmit@monsanto.com
Member
- Location
- Boise, Idaho
Would a flow switch (24VDC dry contact at DCS) that is powered by 120VAC, be able to have both sets of wires in the same conduit?
I don't think it is very common to have a non-power limited 24 volt fire alarm circuit. Power limited fire alarm conductors cannot be mixed with 120 volt conductors. 760.136(A).Most flow switches that are NRTL have a Normally Open, Common, and Normally Closed set of contacts (Form C). If you're monitoring a flow switch, why not use the Normally Closed Contacts and make the circuit "fail safe" and save the money of cable and a relay?
Article 760 for "Fire Alarm Systems" and a DCS is not an NRTL fire alarm system, nor is it permitted by OSHA to be used as a fire alarm system, no matter what the applications. Having said that, 760.48 permits the mixing with 120 VAC circuits, stating:
(A) Class 1 with NPLFA Circuits. Class 1 and non?power limited fire alarm circuits shall be permitted to occupy the same cable, enclosure, or raceway without regard to whether the individual circuits are alternating current or direct current, provided all conductors are insulated for the maximum voltage of any conductor in the enclosure or raceway.The question now becomes what is the power supply?
. . . Having said that, 760.48 permits the mixing with 120 VAC circuits, stating:(A) Class 1 with NPLFA Circuits. Class 1 and non?power limited fire alarm circuits shall be permitted to occupy the same cable, enclosure, or raceway . . .
Just having 600V insulation is not enough to allow mixing under the NEC.In the industrial world they're not rare at all. While the equipment may support the use of Power Limited Circuits, the wiring typically uses 600 volt insulation thereby allowing the circuit to be considered "Non-Power Limited"
Can you cite the section in Article 760 that says that?In the industrial world they're not rare at all. While the equipment may support the use of Power Limited Circuits, the wiring typically uses 600 volt insulation thereby allowing the circuit to be considered "Non-Power Limited"
Not if the 24 V conductors are part of a class 2 circuit.The flow switch isn't for a fire alarm circuit. It is just a 24V process control run through a normally closed contact on
a flow switch that also needs 120VAC power. Can I run the 120VAC and the 24VDC for the NC contact in the same conduit?
760.124 Circuit Marking. The equipment supplying PLFA circuits shall be durably marked where plainly visible to indicate each circuit that is a power-limited fire alarm circuit.Can you cite the section in Article 760 that says that?
Not if the 24 V conductors are part of a class 2 circuit.
A lot of potential conditions in that chart - especially on right side where there are variables that can be entered, I think you can have a lot more then 8 amps in some instances - but also depends on voltage and power abilities of the source.Can you explain how one goes about defining if the circuit is class 2 or not?
This is something that I've never been able get my arms around. I've looked at Table 11(B) and If I understand this correctly (aside from the marking on the power supply), if the output of the power supply is above 8 Amps, it isn't a Class 2 power supply?
Dan,760.124 Circuit Marking. The equipment supplying PLFA circuits shall be durably marked where plainly visible to indicate each circuit that is a power-limited fire alarm circuit.Informational Note: See 760.130(A), Exception No. 3, where a power-limited circuit
760.130(A)Exception No. 2: Conductors and multiconductor cables described in and installed in accordance with 760.49 and 760.53 shall be permitted.
Exception No. 3: Power-limited circuits shall be permitted to be reclassified and installed as non?power-limited circuits if the power-limited fire alarm circuit markings required by 760.124 are eliminated and the entire circuit is installed using the wiring methods and materials in accordance with Part II, Non?Power-Limited Fire Alarm Circuits.
In the industrial world they're not rare at all. While the equipment may support the use of Power Limited Circuits, the wiring typically uses 600 volt insulation thereby allowing the circuit to be considered "Non-Power Limited"