Class 2 Circuits Power Source

[Brian Z]

Hello everyone,

I have a quick question on class 2 circuits. I have seen a new product that has come out that has a single 120VAC/24VAC 500VA transformer with (5) 4Amp circuit breakers on the secondary side. Since the secondary side has 4Amp circuit breakers (essentially cutting the load side into (5) 100VA sections) the supplier is suggesting that this product can supply power for class 2 circuits. I have looked at Table 11(A) in Chapter 9 of the 2014 NEC and it states that a class 2 power source for 21VAC through 30VAC can only have a maximum nameplate rating of 100VA. I am thinking that this new product would not retain a class 2 circuit rating since the transformer is 500VA. Is my logic correct or am I missing something else in the code that allows a circuit breaker to trump the nameplate rating of the power source.

Thanks for any help,

Brian Z.

 

[don_resqcapt19]

There are electronic breakers that are listed as having a Class 2 output.
http://www.controleng.com/single-ar...ersions/8bdc17b886208ad4817260e92be5e649.html

When I read the article, it was not clear that these breakers can supply a Class 2 circuit in compliance with Article 725. I send a follow up question to the author and received a copy of the UL listing information that says these breaker are listed to provide a Class 2 circuit, even where the power supply that feeds the breakers is not a Class 2 circuit.

If this is the type of breaker you have, you should be good to go. If they are standard breakers that are not listed as Class 2 power sources, I don't think so.

 

[kwired]

The ability to use relaxed wiring methods for class 2 circuits is because the power source isn't able to deliver enough energy to cause some of the hazards that can exist with higher capacity power sources, this makes the source characteristics the number one factor in determining what is a class 2 circuit. Sure you can put low setting overcurrent devices in your output, but you still have a "let through" value before the overcurrent device opens the circuit.

A specially designed current limiting device possibly is acceptable - but pretty much needs to have a listing for the application as NEC doesn't specifically recognize such devices in general.

Take a look at the informational note after 725.121(A) Exception No. 2 to (3):

Informational Note: Examples of other listed equipment are as follows:


(1) A circuit card listed for use as a Class 2 or Class 3 power source where used as part of a listed assembly


(2) A current-limiting impedance, listed for the purpose, or part of a listed product, used in conjunction with a non?power-limited transformer or a stored energy source, for example, storage battery, to limit the output current


(3) A thermocouple


(4) Limited voltage/current or limited impedance secondary communications circuits of listed industrial control equipment

I would think the breaker listed for the purpose as mentioned earlier would something that is under the scope of item (2) in the list.

 

[don_resqcapt19]

Here is the reply from the manufacturer.

NEC2014 Article 725.121 (A) defines, that power sources for Class 2 or Class 3 shall comply with

1. A listed Class 2 or Class 3 transformer, or
2. A listed Class 2 or Class 3 power supply, or
3. Other listed equipment marked to identify the Class 2 or Class 3 power supply.

Per exception No 2 to (3) a current limiting impedance (per example 2) is also applicable as long as the limits per Chapter 9 Table 11(A) and (B) are not fulfilled.

Our Class 2 eBreakers are UL recognized per UL2367 as ?Solid-state Overcurrent Protector? in File E356250 and UL508 listed (E219022). In addition to UL2367 the units PM-9824-076-x and PM-9724-152-x were evaluated to comply with Limited Power Source requirements per NFPA 70 (NEC) and additionally for Class 2 Outputs.

 

[Brian Z]

Thank you for the guidance don_resqcapt19 and kwired

 

[hbiss]

NEC2014 Article 725.121 (A) defines, that power sources for Class 2 or Class 3 shall comply with

1. A listed Class 2 or Class 3 transformer, or
2. A listed Class 2 or Class 3 power supply, or
3. Other listed equipment marked to identify the Class 2 or Class 3 power supply.

Looks clear to me. It's not enough for a manufacturer to say that their power supply meets CL2 requirements. If indeed it does why is it not marked as such as required by the Code?

-Hal

 

[powersupplyguy@netzero.ne]

Maybe this would work for you

Maybe this would work for you

http://literature.rockwellautomation.com/idc/groups/literature/documents/pp/1692-pp001_-en-p.pdf

regards,

Powersupplyguy