According to NFPA 731, Power supplies used in security systems must be installed in compliance with the requirements in NFPA 70 (the NEC).

Most power supplies used in a security alarm system will include a transformer to take the branch circuit from 120 volts down to 12 or 24 volts AC or DC. Most of the requirements for these power-limited sources and circuits are located in NEC^® Article 725 which covers Class 1, Class 2, and Class 3 Remote- Control, Signaling, and Power-Limited Circuits.

A big problem in the electrical industry is the proper use of Listed products. Many alarm installers purchase power supplies that are not Listed by a Nationally Recognized Testing Laboratory (NRTL) such as UL and cannot pass final inspection because the power supply is not Listed. Many alarm installers use unlisted power supplies and get away with it because they never get a permit or have their installation inspected. Sometimes they might get an inspection, but the inspector may allow an unlisted power supply because the inspector may not truly know the code requirements.

NEC 725.41(A) requires Class 1 Power-Limited Circuits to be supplied from a source that has a rated output of not more than 30 volts and 1000 volt-amperes.

NEC 725.121 requires the power source for a class 2 or 3 circuit to be one of the following:

• A listed Class 2 or Class 3 transformer
• A listed Class 2 or Class 3 power supply
• Other listed equipment marked to identify the Class 2 or Class 3 power source
• Listed audio/video, information technology (computer), communications, and industrial equipment limited-power circuits
• A battery source or battery source system that is listed and identified as Class 2

NEC Chapter 9 Table 11(A) and 11(B) states the following:

“As part of the listing, the Class 2 or Class 3 power source shall be durably marked where plainly visible to indicate the class of supply and its electrical rating”.

In a nutshell, if a power supply is not marked Class 2 or 3 then it is either not listed, or is a Class 1 power supply and the NEC^® allowable wiring methods permitted on the load side of a Class 2 or 3 power supply cannot be used even if the power supply has the same voltage output as a class 2 or 3 power supply.