So why do we use Class 2 Circuits?  The reason is obvious; we can use even smaller wire sizes than the No. 16 and 18 used for Class 1 circuits.  Additionally, because the power supplies are required to be no larger than a listed 100 Volt-ampere transformer that will not seriously arc or spark even when we short the output wires together, these circuits are very safe and can be handled hot without wearing protective gear.  Of course, this is in a normal dry and non hazardous environment.  Hazardous and wet environments are quite another story.  Class 2 power supplies are used to supply micro processor boards, analog and discrete circuits, damper motors for HVAC systems, control valves for glycol and steam, resistive temperature devices, thermocouples, all types of transmitters for level, flow, temperature, and pressure, and zone valves.  An entire HVAC control system can be powered from one or more Class 2 - 95 volt-ampere transformers. 

The down side for using Class 2 circuits is, of course, they must be kept separate from power and lighting and Class 1 circuits.  This means when control panels are built, the 120 volt supply has to come in on its own raceway and be kept in its own gutter away from the Class 2 circuits.  When cables and wires are installed in the field care must be taken to keep the Class 2 circuits away from other types of circuits.  This can become a tedious task when performing rework on old systems that were installed before the NEC was adopted or by an installer that paid little or no attention to separation requirements.

Another problem is using the correctly listed cables for the application.  If a cable passes through more than one floor a riser cable with a r suffix is required.  If a cable is installed in a space where environmental air passes such as a duct or plenum or above a ceiling then a plenum rated cable is required.  The listing of cables is very important factor that is usually checked by adept inspectors.  Using the wrong cable can be a very expensive mistake!  These listing requirements are throughout the NEC for coaxial, communication, and fire alarm cables.

Some important Code requirements for Class 2 and 3 circuits follow:

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	Power Limitations Per Tables 11(A) and 11(B) located in Chapter 9. Maximum Power Supply size for Class 2 Circuits is 100 VA.
	There are extensive requirements to keep Class 2 and Class 3 open circuit conductors separated from Power and    Lighting and Class 1 Circuit conductors.
	Power Source must be from a LISTED Class 2, Class 3 power supply, or from LISTED Information Technology Equipment, or from a small battery.
	Cables located outdoors in cable trays must be type PLTC. In Hazardous Locations type PLTC, or any of the Types listed in Section 501.4 that includes 300 volt Type ITC cable.
	Cables in indoor cable trays shall be types PLTC, MPP, MPR, MPG, MP, CMP, CMR, CMG, CM, CL3P, CL3R, CL3, CL2P, CL2R, and CL2.
	Wiring in Hazardous Locations must comply with Articles 501 through 517.
	Where run outdoors and above ground Class 2 and Class 3 circuits that are subject to accidental contact with power and lighting conductors over 300 volts to ground or that are subject to lightning must be grounded with a grounding electrode the same way communication circuits are grounded.
	Abandoned cables that are not terminated at equipment and not tagged shall be removed.
	Work must be performed in a neat and workmanlike manner.