Reliable, dependable fire detection depends on electrical contractors (ECs) installing the correct cable. Quality is just as important as wire gauge. The internal and external characteristics of the product must fit the application.
There are three methods used to transport supervisory, trouble and alarm signals from one point to another. The first method involves the use of metallic wire. The second method involves fiber optic cable, and the third relies on radio technology. Each one is extremely important when installing fire alarm systems in new buildings.
NEC regulates cable use
Installers that work primarily with low-voltage systems, such as teledata, fire and intrusion alarm systems, should always keep in mind that the National Electrical Code (NEC) regulates the application of high--voltage and low-voltage cables.
And according to Section 184.108.40.206 of NFPA 72, 2007 edition, “The installation of all wiring, cable, and equipment shall be in accordance with NFPA 70, National Electrical Code, and specifically with Articles 760, 770, and 800, where applicable. Optical fiber cables shall be protected against mechanical injury in accordance with Article 760.”
Article 300, for example, offers an enormous amount of direction to installers’ ring methods. Although it’s code oriented, the information it offers is helpful in knowing which type of cable to use in certain applications.
Probably one of the most important concerns involves techniques designed to prevent the migration of smoke from one space to another. For example, Article 300.21, 2008 edition, instructs that fire stopping is required when a cable(s) penetrates a hollow space, vertical shaft, or ventilation or air-handling duct. This consists of special putty or sealant that seals openings between cables and surrounding surfaces where they penetrate.
Article 760.53(B)(3), 2008, also calls for riser-rated cable for vertical runs to minimize the likelihood of fire migration in a vertical direction. “Cables installed in vertical runs and penetrating more than one floor or cables installed in vertical runs in a shaft shall be Type NPLFR (non-power-limited fire riser). Floor penetrations requiring Type NPLFR shall contain only cables suitable for riser or plenum use.”
A cable’s outer sheath must be noncombustible. In addition, when exposed to heat, it must not emit a toxic gas, which can injure or kill people still in the structure. Toxic smoke will kill long before the heat of the fire reaches the victim.
According to Section 220.127.116.11.3, NFPA 72, 2007, “Conductors and fiber optic cables shall have an approved insulation. The insulation or other outer covering shall be flame retardant and moisture resistant.”
Conductor path and wire gauge
Another important consideration when installing a fire alarm system is how the wires should be routed through a structure. When using Class A wiring, initiating circuits are routed in such a manner that the feed and return cables travel by completely different paths within the building (see Section 18.104.22.168, NFPA 72, 2007).
Choosing the signal path for conductors relies on several issues. According to Section 22.214.171.124, NFPA 72, 2007, integrity and reliability is dependent on several factors, such as conductor length, total building space, the nature of the fire hazard and the functional requirements relating to the level of protection required.
Other aspects associated with choosing the correct wire for the job include wire gauge, the way the conductors are positioned in the cable sheath, whether the equipment requires a shield, the type of dielectric material used to separate the internal conductors, and the manner in which the installers apply them.
On the power supply side, primary power usually is brought to the panel using 14- or 12-AWG conductors. These wires usually connect directly to a stationary power transformer within the fire alarm panel. By the Code, the circuit used for primary power must be equipped with an overcurrent device no greater than 20 amps.
The wire gauge of the conductors connecting house power to the transformer’s primary can be less than 14 AWG, assuming the transformer’s flying leads do not exceed 12 inches in length. Most manufacturers incorporate this requirement into their design, but installers should know what the Code calls for in this regard.
Where alarm panels include a separate plug-in transformer, as with combination burglar/fire panels, the Code requires that the transformer be secured. This most often is performed using a shroud that prevents it from being easily removed. The shroud and transformer fasten to the center screw tying the receptacle cover to the receptacle.
Power-limited fire alarm circuits
There are two basic circuit classifications that determine the type of cable used in a fire alarm system. They are power limited and nonpower limited. Power-limited circuits are typified by some means of limiting the amount of power that can flow through an attached cable.
The Code calls for the use of Class 2 and 3 circuits when using power-limited cable. These most often are the same low-voltage circuits that comprise the detection, notification and power circuits on the load side of the fire alarm panel and accompanying power supplies.
According to Section 126.96.36.199, NFPA 72, 2007, these circuits must be supervised by the alarm panel. They also must be marked at the point of termination. The code states that “unless otherwise permitted or required by 188.8.131.52 through 184.108.40.206.13, all means of interconnecting equipment, devices, and appliances and wiring connections shall be monitored for the integrity of the interconnecting conductors or equivalent path so that the occurrence of a single open or a single ground-fault condition in the installation conductors or other signaling channels and their restoration to normal shall be automatically indicated within 200 seconds.”
Power-limited cabling used in fire signaling circuits should be listed for use in fire alarm systems, and it must be labeled accordingly (see sidebar). Conductors should be no smaller than 16 AWG when using single strands, 19 AWG when there are two or three conductors in a single cable, 22 AWG with four or five, and 24 AWG when there are six or more.
The insulation on these conductors should be rated for at least 300 volts, and the jacket should be constructed so it can withstand the normal rigors of placement and use. An appropriate sheath also must be used to ensure the integrity of the electrical connection between detector/sensor and the fire alarm panel, which is especially critical during a fire.
In a nonpower-limited circuit, an overcurrent device, such as a fuse or breaker, limits current. Article 760, titled Fire Alarm Systems, calls for a rating of 7 amps when working with 18-AWG conductors and 10 amps with 16-AWG conductors. There are exceptions to these specified wire gauges, so be sure to review the relevant articles in the NEC.
In some systems, fiber optic cable is used to connect addressable hubs within a large facility. This is a growing trend in the fire alarm field, and we’ll see more of it as the market turns to network-based fire detection technology.
In closing, because there is such a volume of information and requirements found in NFPA code sets, it would be impractical to expect that we could cover them all in a single article. I suggest that ECs whose focus is on fire alarm systems obtain NFPA 72, National Fire Alarm Code, in addition to NFPA 70 (NEC).
COLOMBO is a 32-year veteran in the security and life safety markets. He currently is director with FireNetOnline.com and a nationally recognized trade journalist located in East Canton, Ohio.