20 A DC power supply feeds multiple class 2 circuits using 4 amp breakers?
When 725.121 states that a Class 2 power source must be a listed class 2 power supply, does that mean that one cannot take a 20 amp (non-class 2 listed) power supply and then protect one or more 24 volt DC class 2 circuits by simply limiting each circuit to less than 100 VA by placing a 4 A breaker (4 A x 24 V = Less than 100 VA) in each circuit? Or is it saying that the power supply ITSELF must be class 2 listed?
I posit that the answer is NO, otherwise why would there be listed Class 2 power supplies if one could simply use any old 24 VDC power supply and simply stick a 4 amp breaker in the circuit. I am trying to prove this point to my customer who wants to use Class 2 wiring methods but not limit the power supply to 100VA, but rather limit the individual circuits to 100 VA with an OCPD.
The individual power supply must be NRTL listed and clearly marked as Class 2.
Originally Posted by richwaskowitz
There are a few manufactured units that consist of a single large DC supply with multiple sets of output terminals, each marked as Class 2. But that is a listed unit, subject to testing on each individual output, and not a field wired fused splitter.
If you wanted to pay thousands of dollars and looked very carefully in to the UL testing you might be able to get something field certified by UL or another NRTL, but the cost would be prohibitive.
Go around 725.121 (A) and still call it a class 2 circuit?
That's what I said. I cited 725.121 (A) where the only power supplies allowed are listed Class 2 type. But I just got an opinion from a certified electrical inspector from a field evaluation service we use sometimes and he muddied-up the waters by alluding (he was in the field at the time and not able to cite specifics) to some exception or other set of conditions which would allow one to limit the current outside of the 20 A power supply and meet the requirements of a class 2 CIRCUIT. Does anyone know of something that would allow me to go around 725.121 (A) and still call it a class 2 circuit?
You can work with almost anything in the NEC that requires UL (actually any NRTL) listing by getting a one of a kind product field listed.
Originally Posted by richwaskowitz
I am not aware of any exceptions that would allow use of locally created current limiting without actually getting a field listing inspection done.
In other areas of the NEC there are exceptions for equipment or wiring done under engineering supervision (typically a PE rather than just an EE?) and where only qualified personnel have access to and maintain the equipment. But AFAIK not where Class 2 and 3 circuits are involved.
The NEC section not only requires listing it requires that the Class 2 status be clearly labelled on the product.
Now instrument and control wiring may also a bit different where it is not ever carrying power as its primary purpose.
seems to me that there used to be a device that did just this. wonder what happened to it.
I seem to remember that also showing up in an earlier thread.
Originally Posted by petersonra
It was a listed assembly that had a single input and individually labelled Class 2 outputs. It would have been NRTL tested for Class 2 compliance at the outputs with an unlimited input supply.
Since Class 2 circuits can sometimes be reclassified as Class 1 and wired with Chapter 3 methods, anybody is allowed to obscure or deface the Class 2 marking. Only an NRTL is allowed to authorize it to be applied.
I found out that there are several products available which will allow one to take a non-Class 2 power supply and then use these modules to create individual Class 2 sources. One of them is By Murrelektronik called a MICO.
It is clear to me by virtue of the fact that there was need for these modules to be made, that one cannot simply stick a 4 amp fuse after a non-Class 2 power supply and do the same thing.
"Murrelektronik's MICO protective modules for UL508A control cabinets are a proven solution for intelligent power distribution and efficient branch circuit’s protection. 8 different MICO modules are now NEC Class 2 approved, so you can efficiently design your power distribution scheme."