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I can't agree with that.

Here's why: Look to any UL standard, and you'll find that were circuits are required to be class 2 or class 3, there are many other requirements. After all, it's possible to use too large of a class 2 transformer - that is, to use a transformer that is capable of providing enough power to create the hazard you're trying to avoid. It's kind of like saying an ordinary circuit is 'limited' simply because there's a fuse on it.

Which, incidentaly, is the real reason for 'power limited' transformers. By having the transformers' impedence limit the power available, the circuit designers are able to avoid using a fuse that UL would otherwise require. This is a matter completely apart from whether the power is limited so as to prevent another hazard (for example, an intrinsically safe circuit).

Now, I don't expect anyone to take my explanation here at face value. Chat with your UL guys, review a few UL standards - I might suggest the ones describing fire alarm control panels would be a good place to start - and you'll see what I mean.

Here's the short version of the testing procedure:
The transformer is shorted out.
If it has a fuse, the fuse has to blow- and, btw, the transformer is not allowed to get hot.
If there is no fuse, the transformer has to exceed a certain temperature, to 'prove' that it is preventing excess power from entering the circuit. It HAS to get hot.

I think we can all see the many reasons a manufacturer might want to do away with the fuse - and most of them have nothing to do with the nature of the control circuit.
 
renosteinke said:
I can't agree with that.

Here's why: Look to any UL standard, and you'll find that were circuits are required to be class 2 or class 3, there are many other requirements. After all, it's possible to use too large of a class 2 transformer - that is, to use a transformer that is capable of providing enough power to create the hazard you're trying to avoid.
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You are mistaken, if a transformer, or any supply is marked class 2 it means it is impossible for it to supply more power than class 2 standards allow.
 
renosteinke,

Your comment: "I might even have to cut a hole in a partition within the unit to pass the thermostat wire from the wiring compartment" in post #19, doesn't that lend credence to the idea that this is a violation? The fact that the manfacturer has gone to the trouble to create a seperate compartment for the class 2 and the power?
 
renosteinke said:
I can't agree with that.

Here's why: Look to any UL standard, and you'll find that were circuits are required to be class 2 or class 3, there are many other requirements. After all, it's possible to use too large of a class 2 transformer - that is, to use a transformer that is capable of providing enough power to create the hazard you're trying to avoid. It's kind of like saying an ordinary circuit is 'limited' simply because there's a fuse on it. ...
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If it is a listed Class 2 transformer or power supply, it cannot supply more current than is permitted for a Class 2 circuit. There is a range of sizes and the size would be selected based on the circuit requirements, but even the largest listed Class 2 transformer or power supply cannot not source more power than is permitted for a Class 2 circuit.
 
Texie, I was asked about my own experiences; that is, the implied question was whether inspectors had ever objected to my running the thermostat cable along with the power. My response was that I have never had the situation come up, so I don't know what an inspector might say. I cannot speak for all equipment and all installations.

Otherwise, I would have to say that any 'implied' conclusions would be trumped by the plain language of the code- which has already been the matter of much discussion. This is hardly the first time the code has actually said something quite different than what many folks thought it said.

Don, Iwire ... maybe you're right ... my detailed familiarity with power limited circuits is focused on the actual testing lab procedures, and I can make the blanket statement that the paperwork never asks for the size of the transformer. I don't know what "class 2 standards" are; the only difference I saw in the testing was the requirement the transformer got real hot when shorted. Some of those transformers put out a fair amount of power - far more than would be required to ignite a hazardous atmosphere, so there has to be somethig more to "intrinsically safe' than a 'class 2' rating on the transformer. (Otherwise, EVERY doorbell would be hazardous location rated). I've also seen -first hand- manufacturers get around fuse-related listing requirements by simply substituting the transformer. That's why I have trouble accepting the simple precence of a class 2 transformer as proof that I have a class 2 circuit.

I've referred to 'intrinsically safe' circuits only for the purpose of illustration. There are other circuits with different concerns; for example, one would not want the doorbell circuit to fool the alarm system into thinking there was a robbery in progress. (Alarm circuits being another area where many 'power limited' circuits are used). It is for such very limited situations that I believe 725 was wriiten, rather than as a general wiring instruction.

Now, if someone has a link to "Class 2" standards I'd probably find them a fascinating read.
 
renosteinke said:
...Don, Iwire ... maybe you're right ... my detailed familiarity with power limited circuits is focused on the actual testing lab procedures, and I can make the blanket statement that the paperwork never asks for the size of the transformer. I don't know what "class 2 standards" are; the only difference I saw in the testing was the requirement the transformer got real hot when shorted. Some of those transformers put out a fair amount of power - far more than would be required to ignite a hazardous atmosphere, so there has to be somethig more to "intrinsically safe' than a 'class 2' rating on the transformer. (Otherwise, EVERY doorbell would be hazardous location rated). I've also seen -first hand- manufacturers get around fuse-related listing requirements by simply substituting the transformer. That's why I have trouble accepting the simple precence of a class 2 transformer as proof that I have a class 2 circuit.
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Class 2 has nothing to do with intrinsically safe circuits.

I've referred to 'intrinsically safe' circuits only for the purpose of illustration. There are other circuits with different concerns; for example, one would not want the doorbell circuit to fool the alarm system into thinking there was a robbery in progress. (Alarm circuits being another area where many 'power limited' circuits are used). It is for such very limited situations that I believe 725 was wriiten, rather than as a general wiring instruction.
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725 was written to pemit the use of a standard of wiring less than what is required in Chaper 3 for control and signaling circuits, some of which are power limited and some are not. You can have a 480 volt Class 1 motor control circuit.

Now, if someone has a link to "Class 2" standards I'd probably find them a fascinating read.
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For a start, look at Chapter 9, Table 11(A) and (B).
 
From 725.2 definitions:
Class 2 Circuit. The portion of the wiring system between the load side of a Class 2 power source and the connected equipment. Due to its power limitations, a Class 2 circuit considers safety from a fire initiation standpoint and provides acceptable protection from electric shock.


I don't know that the actual operating temperature of the transformer is what is considered here.

Limited available fault current is likely an important factor. Short circuit a class 2 control circuit and the supply transformer very well may get hot, however the amount of current flowing in the conductors will not be enough to cause excessive heating in the conductors.

JMO.
 
Well, there you're on the right track.

It's sort of backwards .... ordinarily the temperature tests have a maximum temp that something is allowed to achieve- yet, with these transformers it's a minimum; for all they care, the transformer can get red-hot, even melt down into a puddle of slag, when it's shorted.
 
renosteinke said:
Well, there you're on the right track.

It's sort of backwards .... ordinarily the temperature tests have a maximum temp that something is allowed to achieve- yet, with these transformers it's a minimum; for all they care, the transformer can get red-hot, even melt down into a puddle of slag, when it's shorted.
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What?

The fact it is class 2 means it will not overheat when shorted.
 
It's the transformer that's required to get hot when shorted ... this is seen as proof thet it's not letting excess electricity 'out.'

Yea, it sure does seem backwards .... all those other requirements that various things never become hot enough to start a fire .... yet, here we are, desiging something to get hot when there's a fault.

I will agee, that the definition given a few posts up does say 'the transformer makes the circuit.' I had not been willing to assume that. I suppose that we still need to remember that we still need to replace a bad transformer with one of similar ratings, and not just use anything marked 'class 2.' That, of course, is another topic.
 
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