# NEC Article 725. Class 2. Help...



## chevywaldo (Oct 25, 2013)

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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## BBQ (Nov 16, 2010)

chevywaldo said:


> 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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## Chris1971 (Dec 27, 2010)

Might be time for a code update class.


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## don_resqcapt19 (Jul 18, 2010)

Chapter 9, Tables 11A and 11B.


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## chevywaldo (Oct 25, 2013)

Chris1971 said:


> Might be time for a code update class.


Yes - I agree.................

I feel like I've been cryogenically frozen for 20 years in terms of not keeping up with the NEC code. time to get to work and get caught up.


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## chevywaldo (Oct 25, 2013)

*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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## seabee41 (Dec 21, 2010)

Waldo do you work in Chicago or the outlying area ?


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## chevywaldo (Oct 25, 2013)

seabee41 said:


> Waldo do you work in Chicago or the outlying area ?


 
both. We cover 5 counties including Cook. How bout you?


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## seabee41 (Dec 21, 2010)

chevywaldo said:


> both. We cover 5 counties including Cook. How bout you?


Chicago


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## bkmichael65 (Mar 25, 2013)

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


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## chevywaldo (Oct 25, 2013)

bkmichael65 said:


> 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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## bkmichael65 (Mar 25, 2013)

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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## chevywaldo (Oct 25, 2013)

bkmichael65 said:


> 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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## bkmichael65 (Mar 25, 2013)

It doesn't have to be listed for class 2, but as long as it meets the power limitations it can be used for class 2 power supply


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## chevywaldo (Oct 25, 2013)

bkmichael65 said:


> It doesn't have to be listed for class 2, but as long as it meets the power limitations it can be used for class 2 power supply


 
ok - lets go with that.

Is it "inherently limited" or "not inherently limited" ?


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## bkmichael65 (Mar 25, 2013)

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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## chevywaldo (Oct 25, 2013)

bkmichael65 said:


> 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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## bkmichael65 (Mar 25, 2013)

chevywaldo said:


> 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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## don_resqcapt19 (Jul 18, 2010)

In general the power source must be listed and marked as a Class 2 power supply. See 725.121.


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