24 vdc and power circuits in same conduit

[realolman]

I have a situation where I have an existing 110vac piece of equipment that has to be changed to a new one. All that is available is 24vdc

I have several existing wires in a conduit with other 110 vac circuits, and a class 2 24vdc power supply in a panel with a PLC

It is my understanding if the power supply is class 1, I can run wires from it, in the same conduit as power circuits if the conductor insulation is high enough for the highest voltage

Can someone suggest a replacement for the Class 2 power supply to a class 1 so that I can use the existing wiring... the only thing the dc power supply is running right now is the PLC controller itself.


thank you

 

[don_resqcapt19]

You might want to take a look at Exception #2 to 725.130(A).

 

[realolman]

You might want to take a look at Exception #2 to 725.130(A).

thank you very much... I believe that'll do it.

Would you be interested in explaining why this "rule" exists in the first place.
What are we trying to relieve, prevent ?

If you're not interested that's OK too... you saved me a bunch o' work... thanks:thumbsup:

 

[don_resqcapt19]

I really don't have any idea why the code permits this. Changing the marking on the power supply doesn't change anything. Of course you have to use chapter 3 wiring methods but the devices are still class 2.

 

[realolman]

I really don't have any idea why the code permits this. Changing the marking on the power supply doesn't change anything. Of course you have to use chapter 3 wiring methods but the devices are still class 2.

I guess I can see not using thermostat wire or Cat 5 in a conduit with power conductors, but there's already something in there about all the conductors being insulated for the highest voltage. Actually I don't know if I can even see that.. the power wires are insulated well enough from the conduit... why aren't they insulated well enough from the 24 vdc cable? one of the mysteries of life I guess.

any way thanks.

 

[tom baker]

A similar rule was in fire alarm article 760 and the CMP added it to 725 based on it being in 760. Don't know why it was added to 760, this was maybe in 2005 NEC?

 

[realolman]

Seems like this might be a good article to revisit, clarify and perhaps downsize
probably happens a lot these days and i know i dont understand the rules or the reasons behind them very well

 

[kwired]

I really don't have any idea why the code permits this. Changing the marking on the power supply doesn't change anything. Of course you have to use chapter 3 wiring methods but the devices are still class 2.

The entire system becomes class 1. That means items like a class 2 only thermostat or class 2 only doorbell pushbutton would have to be replaced with devices suitable for use with a class 1 circuit, and of course they must use class 1 wiring methods as well. Some items possibly could still be usable if they can be located within an appropriate enclosure.

At least that is how I always understood this rule.

 

[don_resqcapt19]

The entire system becomes class 1. That means items like a class 2 only thermostat or class 2 only doorbell pushbutton would have to be replaced with devices suitable for use with a class 1 circuit, and of course they must use class 1 wiring methods as well. Some items possibly could still be usable if they can be located within an appropriate enclosure.

At least that is how I always understood this rule.

I don't see any requirement to change anything other than the marking on the power supply and to use Chapter 3 wiring methods. The chapter 3 wiring methods could be an issue for something like a doorbell button because of the requirement for an enclosure.

 

[kwired]

I don't see any requirement to change anything other than the marking on the power supply and to use Chapter 3 wiring methods. The chapter 3 wiring methods could be an issue for something like a doorbell button because of the requirement for an enclosure.

That is kind of what I was getting at, most class 2 wiring doesn't always use items that would meet chapter 3 requirements, like doorbell buttons, low voltage HVAC thermostats often wouldn't comply with chapter 3 either, and there are many items that have open wiring connections that would be otherwise be required to be enclosed, I have LV solenoid valves on my heating system that are open wiring connected, if they were class 1 circuits they would have to have enclosures for those connections.

 

[realolman]

Why couldn't / shouldn't the code be written in such a way that the chapter 3 wiring methods be used in the portion of the circuit where the class 2 conductors are in the same conduit as the power conductors and class 2 everywhere else?

I think that's what most everybody who has questions about this situation wants to do anyway.

My case , I have to replace a 115vac photoswitch with a 24 vdc model. There already exists a conduit with a bunch of 115vac and 230v 3ph conductors. If I'm getting this correctly, I can rub a black sharpie over the class 2 marking on the 24vdc power supply and use those conductors. Not a thing has been changed electrically. But then I am supposed to use those wiring methods everywhere in the circuit.

Suppose one of those were a doorbell circuit. What would be the increased fire or safety hazard if I did NOT obliterate the class 2 marking, used the existing conductors, and then when I got OUT of the common conduit, use whatever wiring method was appropriate for the class 2 circuit? (doorbell switches with no box etc.) I'm not suggesting that's how it's written now, but that's how it SHOULD be written... I think there is a lot of confusion in this particular area for no good reason.

I'm going to have to do that anyway, because the 24vdc photoswitch comes with a little 12mm cordset for 24 vdc... sometimes I think I should just do whatever suits me and don't worry about the code... Oh yeah... the best and only place for this photoswitch and its cord is above the dropped ceiling... another great rule.

 

[pjholguin]

What happens when there is a short circuit? Now you have class 1 voltages on a class 2 system...:'( I have pondered article 725 for a great amount of time, and have come up with this thought process...mixing classes is not good, whether it be a short circuit or induced voltages into a low voltage system. Not good leads to bad...real bad things happening. IMO it's not good idea!

Why couldn't / shouldn't the code be written in such a way that the chapter 3 wiring methods be used in the portion of the circuit where the class 2 conductors are in the same conduit as the power conductors and class 2 everywhere else?

I think that's what most everybody who has questions about this situation wants to do anyway.

My case , I have to replace a 115vac photoswitch with a 24 vdc model. There already exists a conduit with a bunch of 115vac and 230v 3ph conductors. If I'm getting this correctly, I can rub a black sharpie over the class 2 marking on the 24vdc power supply and use those conductors. Not a thing has been changed electrically. But then I am supposed to use those wiring methods everywhere in the circuit.

Suppose one of those were a doorbell circuit. What would be the increased fire or safety hazard if I did NOT obliterate the class 2 marking, used the existing conductors, and then when I got OUT of the common conduit, use whatever wiring method was appropriate for the class 2 circuit? (doorbell switches with no box etc.) I'm not suggesting that's how it's written now, but that's how it SHOULD be written... I think there is a lot of confusion in this particular area for no good reason.

I'm going to have to do that anyway, because the 24vdc photoswitch comes with a little 12mm cordset for 24 vdc... sometimes I think I should just do whatever suits me and don't worry about the code... Oh yeah... the best and only place for this photoswitch and its cord is above the dropped ceiling... another great rule.

 

[kwired]

Why couldn't / shouldn't the code be written in such a way that the chapter 3 wiring methods be used in the portion of the circuit where the class 2 conductors are in the same conduit as the power conductors and class 2 everywhere else?

I think that's what most everybody who has questions about this situation wants to do anyway.

My case , I have to replace a 115vac photoswitch with a 24 vdc model. There already exists a conduit with a bunch of 115vac and 230v 3ph conductors. If I'm getting this correctly, I can rub a black sharpie over the class 2 marking on the 24vdc power supply and use those conductors. Not a thing has been changed electrically. But then I am supposed to use those wiring methods everywhere in the circuit.

Suppose one of those were a doorbell circuit. What would be the increased fire or safety hazard if I did NOT obliterate the class 2 marking, used the existing conductors, and then when I got OUT of the common conduit, use whatever wiring method was appropriate for the class 2 circuit? (doorbell switches with no box etc.) I'm not suggesting that's how it's written now, but that's how it SHOULD be written... I think there is a lot of confusion in this particular area for no good reason.

I'm going to have to do that anyway, because the 24vdc photoswitch comes with a little 12mm cordset for 24 vdc... sometimes I think I should just do whatever suits me and don't worry about the code... Oh yeah... the best and only place for this photoswitch and its cord is above the dropped ceiling... another great rule.

What happens when there is a short circuit? Now you have class 1 voltages on a class 2 system...:'( I have pondered article 725 for a great amount of time, and have come up with this thought process...mixing classes is not good, whether it be a short circuit or induced voltages into a low voltage system. Not good leads to bad...real bad things happening. IMO it's not good idea!

Short circuit isn't quite the term you are looking for, what happens if there is crossover connection between the class 1 or power wiring and the class 2 wiring? If you reclassify the class 2 circuit and contain the entire circuit in methods acceptable for class 1 you eliminate the hazards that could be present if you continued to use class 2 methods and materials for portions of the circuit. Example of common (incorrect) applications I have seen is running the class 2 24 volt control to a AC condensing unit inside a raceway to the unit with the power conductors. If the power conductors would somehow get cross connected to the class 2 circuit, what kind of possible problems are present at a typical class 2 thermostat. Now if that entire system is wired using chapter 3 methods and items like the thermostat are changed to something rated for at least 240 volts, those risks are really non existent.

 

[pjholguin]

Maybe short circuit was to vague...what if there is a breach of the insulation between the two.

Short circuit isn't quite the term you are looking for, what happens if there is crossover connection between the class 1 or power wiring and the class 2 wiring? If you reclassify the class 2 circuit and contain the entire circuit in methods acceptable for class 1 you eliminate the hazards that could be present if you continued to use class 2 methods and materials for portions of the circuit. Example of common (incorrect) applications I have seen is running the class 2 24 volt control to a AC condensing unit inside a raceway to the unit with the power conductors. If the power conductors would somehow get cross connected to the class 2 circuit, what kind of possible problems are present at a typical class 2 thermostat. Now if that entire system is wired using chapter 3 methods and items like the thermostat are changed to something rated for at least 240 volts, those risks are really non existent.

 

[ActionDave]

Maybe short circuit was to vague...what if there is a breach of the insulation between the two.

That's why the insulation for all the conductors have to be the same min rating.

Sure something could go wrong. Something always can go wrong no doubt. At some point you have to get in your truck, put your seat belt on and go to work.

 

[pjholguin]

Sure something can always go wrong, but planning for it is why we have a code in place. There are several places that we are required to maintain separation i.e. 1/4" rule or a barrier in a cable tray or control cabinet...JMHO

That's why the insulation for all the conductors have to be the same min rating.

Sure something could go wrong. Something always can go wrong no doubt. At some point you have to get in your truck, put your seat belt on and go to work.

 

[kwired]

Maybe short circuit was to vague...what if there is a breach of the insulation between the two.

And that does happen even in higher voltage components.

A breach of insulation between the two would be exactly one condition I was trying to get at in my earlier post. Say you had a class 2 control circuit that you "reclassified" as class 1 as permitted but only "reclassified" the supply and only used 600 volt conductors in the places where you shared a raceway with a 480/277 volt power circuit. What do you suppose could happen to say a low voltage wall thermostat should one of the control conductors become 277 volts to ground because of an insulation breach and there is a grounded "C" conductor within that thermostat.

Now it may not go all that well should same thing happen in a thermostat designed for at least 277 volts, but chances are it does a much better job of containing the results, limiting external damages, and limiting users from shock or burn hazards from that thermostat.

ETA: in that example - you likely run a class 2 cable outside raceways away from class 1 or power conductors to get to the thermostat, but chances are if you have the situation I described there - that class 2 cable experiences a meltdown in the incident.