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

Many power supplies for security systems are Class 2 power sources. The supply side (line side) of a Class 2 power source is generally fed from a 120-volt branch circuit. The load side of the Class 2 power source supplies Class 2 security system cables.

NEC Article 725 provides rules for the proper installation and use of Class 1, Class 2, and Class 3 Remote Control, Signaling, and Power-Limited Circuits. This article also provides rules on the power supplies used to supply such circuits.

Below is a sample of NFPA 731. For the complete section, see the actual NFPA 731 text at NFPA.ORG. Once there, click on the free access link to the latest edition of NFPA 731.

4.4 Power.

4.4.1 Power Supplies.

4.4.1.1 Power supplies shall be installed in conformity with the requirements of NFPA 70.

4.4.1.2 Power supplies shall be reliable and have the capacity to service the intended load.

4.4.1.3 At least two independent power supplies shall be required, one primary and one secondary, for the following premises security systems:

(1) Intrusion detection systems

(2) Holdup, duress, and ambush systems

(3) Power over Ethernet (PoE) power source equipment (PSE)

4.4.1.4 When installed, secondary power supplies that are not required by 4.4.1.3 shall conform to the requirements of this standard.