According to NFPA 731 Section 4.4.3.4(3), the secondary supply is permitted to be an emergency generating system as defined in NFPA 70, Article 700.

NEC Article 700 is all about emergency systems and applies to the electrical safety of the installation, operation, and maintenance of emergency systems consisting of circuits and equipment intended to supply, distribute, and control electricity for illumination, power, or both, to required facilities when the normal electrical supply or system is interrupted.

Emergency generating systems are usually batteries or generators that provide emergency power in the event that the normal utility power fails.

A common mistake is made when an alarm technician wires the alarm system secondary supply to an optional standby generator thinking that all generators are considered an “emergency generating system”. Many houses and buildings have an optional standby generator for providing convenient power when normal power is lost. This doesn’t automatically mean that the generator is an emergency system. Optional standby systems are covered in NEC^® Article 702 and are permitted to have a manual transfer switch for switching between normal power and the optional standby source.

A true emergency generator provides actual emergency power essential for safety to human life and must use an automatic transfer switch listed for emergency use according to NEC^® 700.5(C).

If an onsite generator supplies a transfer switch not marked for emergency systems, then the generating system is not truly an emergency generator and the secondary supply for the security system cannot be connected to it.

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.

4.4.3 Secondary Supply.

4.4.3.4 The secondary supply shall consist of one of the following:

(1) A storage battery dedicated to the premises security system arranged in accordance with 4.4.4

(2) An individual branch circuit of an automatic-starting engine-driven generator arranged in accordance with 4.4.5 and storage batteries dedicated to the premises security system with 15 minutes of capacity under maximum alarm load

(3) An emergency generating system as defined in NFPA 70, Article 700

Below is a sample of the NEC^®. See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the latest NEC^® edition of NFPA 70.

700.2 Emergency Systems. Those systems legally required and classed as emergency by municipal, state, federal, or other codes, or by any governmental agency having jurisdiction. These systems are intended to automatically supply illumination, power, or both, to designated areas and equipment in the event of failure of the normal supply or in the event of accident to elements of a system intended to supply, distribute, and control power and illumination essential for safety to human life.

700.5 Transfer Equipment.

(A) General. Transfer equipment shall be automatic, listed, and marked for emergency use, and approved by the authority having jurisdiction…

(C) Automatic Transfer Switches. Automatic transfer switches shall be electrically operated and mechanically held. Automatic transfer switches shall not be permitted to be reconditioned.