With the advent of power over ethernet in the 2017 National Electrical Code (NEC) , in Article 393 of the 2014 NEC covering low-voltage ceiling grid power, and other applications dealing with power-limited circuits, there have been suggestions to increase the permissible current and voltage for power-limited circuits. The existing power limitations in various locations within the NEC have been long-established safety levels dating back to the early days of the Code . Should these existing levels be changed due to the increased use of low-voltage and current levels for various loads being introduced into the electrical industry, or should these long-tested and enduring safety levels be maintained at the present levels? Careful study and understanding of the existing power-limited circuits in both the NEC and NFPA 70E, the Standard for Electrical Safety in the Workplace, should help explain these existing levels and dispel any urges to increase them.
Chapter 34 in the 1940 Code was the earliest edition that I could find that covered Class 1, Class 2 and Class 3 power-limited circuits. The application text in Section 3401 states, “The provisions of Chapter 34 shall apply to circuits for transmitting intelligence, including telephone, telegraph, district messenger, call bell and similar circuits. Wiring carrying small currents at low voltage within the limits described in this chapter used for other than signaling purposes, as, for example, for temperature or other control devices shall be considered as coming within the scope of this chapter.”
Chapter 34 also provides classifications for these primarily low voltage and low current circuits by providing details of Class 1, 2, 3 and 4 circuits. A Class 1 circuit was used for protective purposes such as fire alarms, burglar alarms and similar systems. A Class 2 circuit was a system where the transformer power output was limited to no higher than 100 volt-amperes (VA) with 0–15 volts (V) at 5 amperes (A) maximum, 15–30V at 3A maximum, 30–60V at 1.33A maximum, and over 60V to 1A and 150 watts maximum. A Class 3 circuit could not exceed 30V, but the current was unlimited. A Class 4 circuit applied to all other signaling and control circuits without any limitation.
In the latter part of the 1950s and early to mid-1960s, the NEC Correlating Committee Panel 16 and associated other industry experts initiated a study on remote control, signaling and power-limited circuits. During that time, further limitations were applied to Class 1 power-limited circuit transformers to not exceed 1,000 VA and a maximum of 30V, thus limiting the ampere level to not exceed 33.3A. This Class 1 power-limited application is the same as in the 2017 NEC . For Class 2 circuits, the 100-VA maximum transformer size and the voltage and ampere level of Class 2 circuits were relatively unchanged between the 1940 NEC and the 1965 NEC .
In the 1968 NEC , a new requirement was added for “circuits in which the open circuit voltage does not exceed 150 volts where the circuit is protected with a current limiting means (now called inherently limited, such as an internal link in the transformer), other than overcurrent protection, which are approved for the purpose and which will limit the current as a result of a fault to not exceed 5 milliamperes.”
Open circuit voltage is where the voltage peak occurs immediately before any current flow. All of the information on Class 2 and 3 alternating current (AC) and direct current (DC) was put into two tables: Table 725.31(a) for AC and Table 725.31(b) in the 1978 NEC . These two tables and the five notes to the tables were inserted to provide ease of use and to consolidate the information required for use of Class 2 and 3 circuits. The tables provided the 100-VA maximum nameplate rating of the power supplies and provided the limitation of 5 milliamperes of current as the voltage increased in the circuits above a certain level. The 5 milliamperes level for these circuits provided levels of current that would not be a shock hazard under normal circumstances. The power supplies for Class 2 and 3 circuits were not required to be specifically listed. In the 1996 NEC , these two tables and the accompanying notes were moved to Tables 11(a) for AC and 11(b) for DC circuits and 725.41 required Class 2 and 3 power supplies to be specifically listed based upon the requirements in the two tables.
As can be seen in this brief historical explanation of the background of Class 2 and 3 circuits, the power-limited values of these circuits should be maintained, unless appropriate substantiation of testing is provided.