What is A Low Voltage Circuit? he NEC does not consistently address low voltages.  In Section 110.26(A)(1)(b), voltages of not greater than 30 volts ac rms and 60 volts dc are the dividing line.  Article 720 is titled "Circuits and Equipment Operating at less than 50 volts." Article 725 uses tables which list voltages from 0 through 150 volts ac and dc.  Other Sections of the Code are divided based on whether or not the voltage is less than or equal to 600 volts or over 600 volts.  Still other sections use 150 and 250 volts as the dividing line.  "Low voltage circuit" just like "low voltage work" are slang terms that do not have precise definitions but are commonly used to describe what most electricians think of as work with voltages under 50 volts and using No. 16, No. 18  and smaller wire sizes.  Note: Most control work is in Section 15 of job bid packages and  since Section 15 is  the mechanical section, control work is often performed by employees of  the mechanical contractor and not by electricians employed by the electrical contractor. What is a signaling system of 48 volts or less? his term and the term low voltage control system are confusing and are not defined in the NEC.  "Voltage" is defined in Article 100 as the greatest root-mean-square (effective) difference of potential between any two conductors of the circuit concerned.   IEEE Definitions here are definitions of signal, remote-control, and power-limited circuits given in the IEEE Dictionary.  Also, "Circuit" is defined as a source of supply, a load, and the conductors which connect the source of supply to the load. A signal circuit is defined as any electrical circuit which supplies energy to an appliance or device that gives a visual and/or audible signal.  Such circuits include those for doorbells, buzzers, code calling systems, signal lights, enunciators, fire or smoke detection, fire or burglar alarm, and other detection indication or alarm devices. A remote-control circuit is any circuit which has as its load device the operating coil of a magnetic motor controller, a magnetic contactor, or a relay.  It is a circuit which exercises control over one or more other circuits.  Other circuits controlled by the control circuit may themselves be control circuits, or they may be 'load" circuits carrying utilization current to electrical equipment such as lights, motors or heating appliances. The elements of a control circuit include all the equipment and devices concerned with the function of the circuit: conductors, raceway, contactor operating coil, source of energy supply to the circuit, overcurrent protective devices, and all switching devices which govern energization of the operating coil.  Control circuits include wiring between solid-state control devices as well as between magnetically actuated components.  Low voltage relay switching of lighting and power loads is also classified as remote-control wiring.	A motor control circuit is a special type of control circuit and is defined in the NEC in section 430.71 as the circuit of a control apparatus or system that carries the electric signals directing the performance of the controller, but does not carry the main power current. Power-limited circuits are circuits used for functions other than signaling or remote-control - but which the source of the energy supply is limited in its power (volts times amps) to specified maximum levels.  Low voltage lighting, using 12 - V lamps in fixtures fed from 120/12 - V transformers, is a typical power-limited circuit application. 24 volt damper motors driven by a small 120/ 24 - V transformer is another application of a power-limited circuit.   How the NEC Defines low voltage circuits he Code does not attempt to define low voltage circuits in general terms.  Instead the Code delineates the rules for remote-control, signaling, power-limited and motor control circuits.  Furthermore, the Code states that remote-control, signaling, or power-limited circuit is the portion of the wiring system between the load side of the overcurrent device or the power-limited supply and all connected equipment. Class 1 circuits are divided into two types (1) power-limited, or (2) remote-control and signaling circuits.  Power limited circuits are limited to 30 volts and 1000 volt-amperes.  Class 1 remote-control and signaling circuits are limited to 600 volts, but there aren't any limitations on the power output of the source. Class 2 and Class 3 circuits are defined by two tables, one for alternating current and one for direct current.  In general, a Class 2 circuit using 24 volts with a power supply durably marked 'Class 2" and not exceeding 100 volt-amperes is the one most commonly used.     Class 2 Power Supplies Shall be Marked as "Class 2 Power Supplies"