In security system, consideration must be given to acceptable circuit types for primary power. According to NFPA 731, the primary power supply used in a security system must be supplied from either a dedicated branch circuit, or the unswitched portion of a specific branch circuit if the branch circuit is not a dedicated branch circuit.

Neither NFPA 731 or the NEC^® has a definition of a “dedicated branch circuit” but the NEC^® does have the following definition of an “individual branch circuit”:

NEC Article 100: Branch Circuit, Individual. A branch circuit that supplies only one utilization equipment.

According to NFPA 731, if a dedicated branch circuit is not used, the primary power supply can be connected to the unswitched portion of a branch circuit serving commercial lighting or power, certain engine driven generators or a specific alternative energy source.

Connecting the primary power source ahead of any switches (on the line side) ensures continuity of power and continued operation. This requirement helps to avoid a situation where the primary power for the security system is inadvertently interrupted by the opening of a switch.

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.

4.4.2 Primary Supply.

4.4.2.1 Primary (main) power shall be supplied from either a dedicated branch circuit or the unswitched portion of a branch circuit by one of the following means:

(1) Commercial light and power

(2) An engine-driven generator or equivalent in accordance with 4.4.5, where a person specifically trained in its operation is on duty at all times

(3) An engine-driven generator or equivalent arranged for cogeneration with commercial light and power in accordance with 4.4.5, where a person specifically trained in its operation is on duty at all times

(4) An alternative energy source with capacity required by the security vulnerability assessment (SVA)