fusing to obtain Class 2

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SceneryDriver

Senior Member
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NJ
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Electrical and Automation Designer
I have a power supply that outputs just over 13A @ 24VDC - decidedly not CL2.

My desire is to break that 24VDC output into (5) 2A circuits, each protected with a 2A fast-acting fuse (48W of available power per circuit). Would this approach allow a "non-inherently limited" PS to supply Class 2 circuits? The fuses would of course be immediately adjacent to the power supply, and in the same enclosure.

My research seems to indicate that fusing is an acceptable approach, but at the same time it's unclear. Article 725 is also strangely silent on that approach.

I unfortunately don't have the room to use a separate CL2 rated supply for each run, otherwise I'd consider that approach.

I need to draw approximately 1.1A per circuit, and each run will be done with CL2 rated 16AWG cable.

Thoughts?


SceneryDriver
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
III. Class 2 and Class 3 Circuits
725.121 Power Sources for Class 2 and Class 3 Circuits.
(A) Power Source. The power source for a Class 2 or a
Class 3 circuit shall be as specified in 725.121(A)(1), (A)(2),
(A)(3), (A)(4), or (A)(5):

Read carefully the 5 options you have for a class 2 power source.
 

GoldDigger

Moderator
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Retired PV System Designer
The discussion in this thread seems to indicate a fuse isn't sufficient:

http://forums.mikeholt.com/showthread.php?t=169223

Is that correct?



SceneryDriver
Yes, IMHO. You are not qualified to "list" the combination of power supply and fuses as being tested to meet Class 2 requirements. And that label is mandatory for a Class 2 power source.
Instead look for a power supply which already has multiple current limited outputs and has Class 2 certification as part of its design and manufacturing process.
 

SceneryDriver

Senior Member
Location
NJ
Occupation
Electrical and Automation Designer
All,
I just heard back that the powers-that-be have OK'd our proposed use of fuses to protect the circuits. While it doesn't meet CL2 requirements because the power supply isn't listed as CL2, the AHJ feels that we're meeting "the spirit" (his words) of the intention.

He feels that fusing 16ga wire at 2A, 24VDC won't create an unsafe condition. He did ask that we downsize the power supply so that there is no way at all to supply more current than the wire is capable of safely carrying (regardless of what fuses might ever be installed). We originally had spec-ed a 300W supply (12.5A @ 24VDC) and we downsized to a 150W PS (6.25@ @ 24VDC). That wattage still gives us enough power run the design, albeit with less headroom. We need absolutely no more than 125W @ 24VDC, so it's all good.

I'm counting this as a win. The AHJ is happy, and so are the client and architect.



SceneryDriver
 

gadfly56

Senior Member
Location
New Jersey
Occupation
Professional Engineer, Fire & Life Safety
All,
I just heard back that the powers-that-be have OK'd our proposed use of fuses to protect the circuits. While it doesn't meet CL2 requirements because the power supply isn't listed as CL2, the AHJ feels that we're meeting "the spirit" (his words) of the intention.

He feels that fusing 16ga wire at 2A, 24VDC won't create an unsafe condition. He did ask that we downsize the power supply so that there is no way at all to supply more current than the wire is capable of safely carrying (regardless of what fuses might ever be installed). We originally had spec-ed a 300W supply (12.5A @ 24VDC) and we downsized to a 150W PS (6.25@ @ 24VDC). That wattage still gives us enough power run the design, albeit with less headroom. We need absolutely no more than 125W @ 24VDC, so it's all good.

I'm counting this as a win. The AHJ is happy, and so are the client and architect.



SceneryDriver

You could also have done this. We use them all the time. Keep it in mind for next time.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
You can buy 2A 24VDC class 2 powers supplies for less than $40. You spent more than that in time trying to avoid doing so and came up with a non-compliant solution.
 

SceneryDriver

Senior Member
Location
NJ
Occupation
Electrical and Automation Designer
You can buy 2A 24VDC class 2 powers supplies for less than $40. You spent more than that in time trying to avoid doing so and came up with a non-compliant solution.

Except in this case, size limitations mean there isn't room for five separate power supplies. In the entertainment industry, we're more often than not left to the whims of the designer / architect, and what is seen by the audience or public trumps technical considerations. In this case, I've had to fit a closet's worth of equipment into a cubby the size of a steamer trunk. No one would budge on real estate, so I had to make the electrical fit the available space. Five power supplies wouldn't have fit. The one power supply plus the fuse block barely fits as it is. So, in this case, the time spent was necessary and worth it.



SceneryDriver
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Except in this case, size limitations mean there isn't room for five separate power supplies. In the entertainment industry, we're more often than not left to the whims of the designer / architect, and what is seen by the audience or public trumps technical considerations. In this case, I've had to fit a closet's worth of equipment into a cubby the size of a steamer trunk. No one would budge on real estate, so I had to make the electrical fit the available space. Five power supplies wouldn't have fit. The one power supply plus the fuse block barely fits as it is. So, in this case, the time spent was necessary and worth it.



SceneryDriver

I would bet that five power supplies of that size take up no more room than the large power supply and fuse blocks you are supplying.

In any case, convenience for the designer is not something that I can recall seeing anywhere in the code as an exception to the rules.
 

gadfly56

Senior Member
Location
New Jersey
Occupation
Professional Engineer, Fire & Life Safety
Except in this case, size limitations mean there isn't room for five separate power supplies. In the entertainment industry, we're more often than not left to the whims of the designer / architect, and what is seen by the audience or public trumps technical considerations. In this case, I've had to fit a closet's worth of equipment into a cubby the size of a steamer trunk. No one would budge on real estate, so I had to make the electrical fit the available space. Five power supplies wouldn't have fit. The one power supply plus the fuse block barely fits as it is. So, in this case, the time spent was necessary and worth it.



SceneryDriver

The solution I suggested would fit in a space 15.5" x 12.25" x 2.5". Was your final solution that small? Oh, and that's with battery backup.
 

SceneryDriver

Senior Member
Location
NJ
Occupation
Electrical and Automation Designer
The solution I suggested would fit in a space 15.5" x 12.25" x 2.5". Was your final solution that small? Oh, and that's with battery backup.

The solution fits in a footprint of approximately 4" x 10" x 1.75" fusing included. Sealed, "brick-style" power supply, and a small die-cast enclosure for the fuse holders. That's all the room I had. That's the entertainment industry; you place equipment where it will fit, and not necessarily where or how it would be nice to work on. Sometimes it can be frustrating, but I like it as it forces me to get creative.

I don't need battery backup, as this will be remote low voltage power for operating an LED light box for a large sign.



SceneryDriver
 

hbiss

EC, Westchester, New York NEC: 2014
Location
Hawthorne, New York NEC: 2014
Occupation
EC
As long as the NEC persists in including tables 11A and B in chapter 9 you are going to have this kind of second guessing and wiggling. It should be sufficient to say that a power supply be listed as a class 2,3 or 1 device and leave it at that. Then all somebody has to do is read the label. Leave the design to the manufacturers and UL. Without those tables the OP would have had to dig a little deeper and find something already suitable.

Something also struck me as strange here. How does the AHJ get involved with scenery design? It's temporary and not part of the structure, no?

-Hal
 

SceneryDriver

Senior Member
Location
NJ
Occupation
Electrical and Automation Designer
As long as the NEC persists in including tables 11A and B in chapter 9 you are going to have this kind of second guessing and wiggling. It should be sufficient to say that a power supply be listed as a class 2,3 or 1 device and leave it at that. Then all somebody has to do is read the label. Leave the design to the manufacturers and UL. Without those tables the OP would have had to dig a little deeper and find something already suitable.

Something also struck me as strange here. How does the AHJ get involved with scenery design? It's temporary and not part of the structure, no?

-Hal

Normally, you're right. The AHJ doesn't care about scenery. In this case though, this is for a permanently installed setup in a store. Anyway, as the AHJ said, the bottom line is that there's nothing here to create an unsafe condition, so he is willing to sign off.


SceneryDriver
 

iwire

Moderator
Staff member
Location
Massachusetts
The possibly open question is whether it is building wiring or not. But the separation between the supply/control "trunk" and the controlled devices seems to settle that.

Ok so you are saying the AHJ has nothing to say about oh lets say a fair or carnival?

Not part of a building at all, entirely temporary in nature.
 
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