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Discussion Starter #1
I've been using my very old NEC book for 20 years or more. Now I just got a hold of a 2005 NEC book. Looking at Article 725, it completely changed, or so it appears. There used to be a table designating the maximum power, volts, amps, etc for each of the classes. that table is now gone. I read most of the article and I can't find anywhere where it states the criteria for Class 2 Circuits (volts, amps, inherently limited power, etc) Where does it explain what a Class 2 circuit is?

Again, I'm talking about the 2005 NEC book right now. I do not have the newer NEC books.
 

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I've been using my very old NEC book for 20 years or more. Now I just got a hold of a 2005 NEC book. Looking at Article 725, it completely changed, or so it appears. There used to be a table designating the maximum power, volts, amps, etc for each of the classes. that table is now gone. I read most of the article and I can't find anywhere where it states the criteria for Class 2 Circuits (volts, amps, inherently limited power, etc) Where does it explain what a Class 2 circuit is?

Again, I'm talking about the 2005 NEC book right now. I do not have the newer NEC books.
It is on page 70–639 (or at least it is in my pdf of it)

I think it was moved to the back with the tables as it is really not part of the code.
 

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Discussion Starter #6
Class 2 XFMR Question

1) So if a transformer meets the requirements of Class 2, but is NOT listed as "Class 2", is it a Class 2 device even though it carries no listing from the mfr?

2) Is a transformer with no built in circuit breaker or overcurrent protection considered an "inherently limited power source" ? I would think the answer is no.

3) If the answer to #2 above is "NO", then per table 11(A), if a transformer is 120/24 VAC, then per table 11(A) the maximum overcurrent protection is 100/Vmax = .83 amps (primary side). Is this correct?

4) If #3 above is correct, is "secondary" overcurrent protection required? See table 450.3(B). I think the answer is NO.

5) Can anybody answer this question: When is "primary and secondary overcurrent" protection required for a transformer, and where in the code does it state this?
 

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Discussion Starter #11
Any 24 volt control transformer rated at 100 VA or less is considered an inherently limited power source and no ocpd is required

thanks bkmichael65. May I ask where in the code book can I find that piece of information. I will need it for a presentation.
 

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I'm going to have to open up my code book.....Ok, chapter 9, table 12(A) inherently limited power source, middle column for over 20 through 30 volts, power source maximum nameplate ratings in VA
 

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Discussion Starter #13
I'm going to have to open up my code book.....Ok, chapter 9, table 12(A) inherently limited power source, middle column for over 20 through 30 volts, power source maximum nameplate ratings in VA

well, there is where I am confused. So I myself, can designate whether a power source in "inherently limited" just because of the fact that it's less than 100VA per table 11(A) ?

For example. I go to a supply house, buy a generic transformer rated at 100VA. It has a 120V primary and 24V secondary. It does NOT say Class 2 anywhere on the product or on the instruction sheet. Is this a Class 2 transformer or not? Is it considered a "Inherently Limited Power Source" even if it does not say it's Class 2 or "inherently limited" on the product or data sheet?
 

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It's inherently limited. I guess the thinking is that the primary windings are so small that the xfmr will destroy itself before allowing a dangerous overcurrent to happen
 

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Discussion Starter #17
It's inherently limited. I guess the thinking is that the primary windings are so small that the xfmr will destroy itself before allowing a dangerous overcurrent to happen
I basically agree with you on that, and it would likely be the case, but........does it actually say that somewhere in the code? That's what I'm having trouble with. I don't like to assume it's inherently limited just because I think it probably is. For example, I don't know for sure that the windings in the transformer won't start a fire if they are experiencing overcurrent with regards to the rating of the transformer, yet not enough overcurrent to trip the breaker feeding the wire to the transformer.

Say the transformer is 100VA and receiving 120VAC. The breaker feeding the circuit is 20 amp. The wire is #12. Let's say the transformer partially shorts or has a partial high resistance ground fault and the wire is drawing 15 amps. The breaker won't trip. How do I know the transformer will open it's windings before it heats up and starts itself or something laying on top of it on fire?
 

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I basically agree with you on that, and it would likely be the case, but........does it actually say that somewhere in the code? That's what I'm having trouble with. I don't like to assume it's inherently limited just because I think it probably is. For example, I don't know for sure that the windings in the transformer won't start a fire if they are experiencing overcurrent with regards to the rating of the transformer, yet not enough overcurrent to trip the breaker feeding the wire to the transformer.

Say the transformer is 100VA and receiving 120VAC. The breaker feeding the circuit is 20 amp. The wire is #12. Let's say the transformer partially shorts or has a partial high resistance ground fault and the wire is drawing 15 amps. The breaker won't trip. How do I know the transformer will open it's windings before it heats up and starts itself or something laying on top of it on fire?
The table gives the standard for whether it meets the standard for inherently limited or not and whether ocpd are required per NEC. That just means you aren't required to install ocpd, not that you can't. I always put an inline fuse on the secondary just in case
 
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