I recently found out that electrical engineers and electrical personnel do not always use the same thought process when classifying low-voltage systems, such as Class 1, 2 and 3 circuits. For example, those working on systems over 600 volts (V) stated they considered low-voltage systems to be 600V or less. While some folks, working on 600V or less, say that low-voltage includes systems and circuits fewer than 50V, and others simply call them limited-energy systems. Those attending a recent workshop wanted to understand the classifications and how the National Electrical Code (NEC) would accept and deal with these circuits once their outputs had been classified.
After listening to participants discuss these circuits, I understood their thought process, and I came to the conclusion that electronics engineers and instrumentation personnel work on one side of the decimal in which they deal with milli-amps (mA). On the other hand, power engineers and electricians work on the side of the decimal with systems that produce thousands of amps (A). No wonder they seem to have different points of view when regarding these systems and the manner in which they classify, ground and install them. Limited-energy designers and installers did not consider grounding and bonding the secondary side of these low-energy transformers as important as other folks did with grounding and bonding the secondary side of higher voltage transformers.
This article covers points that were discussed concerning Class 1, 2, and 3 circuits that have outputs of less than 50V. Note that this article does not discuss all low-energy circuits.
Article 725 of the 2008 NEC
Article 725 covers remote control, signaling and power-limited circuits that are not part of a device or appliance. It consists of Class 1, 2 or 3 circuits. Class 1 circuits can be power-limited with a maximum of 1,000 volt-amperes (VA). However, power-limited circuits will be discussed in this issue of Electrical Contractor. Note that a Class 1 circuit is considered both a possible shock and a fire hazard if not installed properly.
A Class 2 circuit is one that is not considered a shock or fire hazard because, as the voltage for the system goes up, the amperage of the circuit is reduced to a level that will not constitute a shock or fire hazard. For example, a Class 2 circuit operating from 0 to 20V has an output of 100 VA and 5A. However, when designing with a 21 to 30V power source, an output of 100 VA and 3.33A is available. Note: when a 31 to 150V system is used, the obtained output is 5 VA and 5 mA respectively. For determining the output levels of a Class 3 circuit, see the Table 11(A) for alternating current (AC) sources and Table 11(B) for direct current (DC) sources. Both tables are found in Chapter 9 of the NEC.
Students, in the workshop, finally agreed that a Class 3 circuit is one that is not a fire hazard. However, it still may be a shock hazard, since the current is permitted to be slightly higher than that in a Class 2 circuit when powered at the same voltage level.
Class 1, 2 and 3 circuits
Power-limited Class 1, 2 and 3 circuits are the most common low-voltage systems (less than 50V) in use today. These systems are used for doorbell circuits, motor-control circuits, low-voltage lighting and associated control devices, intercom systems, and burglar alarm systems, to name just a few. Since, Class 1 can be a shock and fire hazard, the wiring for these circuits is the same as would be used for a normal-power circuit of similar voltage and amperage levels. Class 2 and Class 3 circuits do not normally present the same types of dangers to the user, and the wiring methods can be relaxed somewhat from the methods used for Class 1 circuitry.
Minimum wire size
The minimum wire size for Class 1 circuits can be 18 and 16 AWG, or special cables can be used for limited-energy circuits as outlined in 402.5 and Table 725.154(G). The wiring methods and installation techniques permitted for Class 1 power-limited or nonpower-limited circuits should be reviewed very carefully.
Other sections of the NEC that should be reviewed when designing and installing systems of less than 50V are 725.46 for wiring methods, 725.49(B) and 725.179 for insulation, 725.51 for derating, 725.43 for overcurrent protection, 725.48 for mixing, and 725.24 and 725.143 for supporting. When reclassifying a Class 2 or Class 3 circuit to a Class 1 circuit, see the requirements of 725.130(A), Ex. 2.
As can be seen from the brief overview provided in this article for Class 1, 2 and 3 circuits, circuits of less than 50V are used in many applications throughout our electrical industry and are widely misunderstood.
STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the National Electrical Code and other standards, including those from OSHA. Contact him at 817.581.2206.