Art 725 Class 3 Systems

[tom baker]

I would like to see an example of an NEC Article 725 System, can some one post a link to a typical system.
There are lots of Class 2 systems
Also I found out that the typical 480V electrical unit heater (Markel) with a low voltage contactor, uses a Class 1 24 VAC control circuit. this means the thermostat can share the same raceway as the power, if the thermostat is wired with a Chapter 3 wiring method.

 

[ryan_618]

Nurse call is a common Class 3 installation. Really, the rules are the same as class 2, other than the cable and the power supply (as least, as far as I can remember without looking things up).

 

[Pierre C Belarge]

Tom
Class 3 circuits are less common than Class 2.
The Class 3 circuits are defined by their source, just as a Class 2 ckt is.

As Ryan has stated, a nurse call is a typical C3 ckt. So are some home theater systems. Commercia sound systems and public address systems like we hear at ball parks are generally C3 ckts.

Class 3 ckts are also considered safe as to fire initiation, but because of higher voltage and current permitted, the shock hazard exists.

 

[ryan_618]

Pierre, wouldn't sound systems be governed by Article 640, not 725?

 

[Pierre C Belarge]

Typically one will follow the requirements of Art 640, but there is reference in 640.9(B), 640.9(C) and 640.21(C) to Art 725.

As may be typical with other areas of the code, there may be some cross referencing.

 

[don_resqcapt19]

Tom

Also I found out that the typical 480V electrical unit heater (Markel) with a low voltage contactor, uses a Class 1 24 VAC control circuit. this means the thermostat can share the same raceway as the power, if the thermostat is wired with a Chapter 3 wiring method.

I just installed a Trane that was like that. One thing that I noticed was that the 24v system was not a grounded system as required by 250.20(A)(1).
Don

 

[tom baker]

Consider this:
The two articles in the NEC that deal with "low Voltage"
720 Circuits operating at less than 50 volts
411 Lighting systems operating at less than 50 volts
Do not have the term low voltage in the title.
720 is for farm lighting systems. Do a google for Delco Lighting Plants, this article has been in the NEC for a long time
411 was added to the 96 or 99 nec.
If we could just use the term limited energy it would go a long way to eliminating confusion on "low voltage"