Complying With Key Points of National Electric Code/NFPA 70
It never fails to amaze me how many alarm installers believe National Electric Code (NEC) is only for electrical wiring and devices. It would seem a relatively large percentage of them believe that because the systems they work on are classified as low voltage (12VDC to 24VDC) NEC has no jurisdiction over what they do. How wrong they are.
Savvy fire and burglar alarm installers know that NEC, also referred to as NFPA 70, has a lot to do with the physical aspects of their installations, whether it involves metallic wire or fiber-optic cable. NEC determines not only what kind of wire they have to use for fire alarm applications, but also how and where that wiring is installed — in particular Article 760.
Article 760, titled “Fire Alarm Systems,” covers most of the issues that fire alarm installers encounter during the course of their work. It encompasses anything and everything that involves fire detection, notification and supervision.
Of special importance are circuits related to the control of elevators, building safety systems, door release devices, smoke doors, damper control and fan shutdown. Article 760 covers all of these and more.
This month, we’ll discuss a few of its highlights. Because Article 760 is so comprehensive in scope, it is recommended those who do not have a copy of this code buy one. This will enable them to actually know what the building department expects them to do when they install their fire alarm systems.
NEC Covers Overlooked Aspects
First and foremost, NEC requires your work be conducted in a “workmanlike manner.” Support for the cable you install is especially critical as life and death can easily be determined by a lose wire that becomes damaged during the normal course of activity within a structure.
For example, something that low-voltage installers often overlook is grounding. Article 760.9 states that all fire alarm circuits and equipment should be tethered to earth ground in accordance with Article 250. Again and again in NEC you will find references to other sections of the same code, as well as references to other code sets, such as NFPA 72, National Fire Alarm Code (NFAC). To be sure your fire alarm installations comply, it’s important to zigzag your way through NFPA 70, 72 and 101, and NFPA 13 when working with sprinkler systems.
Another important issue is conductor identification. Just because a wire is associated with a particular initiating device circuit (IDC) does not mean you’ve successfully complied with NEC. Here, Article 760.10 distinctly says, “Fire alarm circuits shall be identified at terminal and junction locations, in a manner that will prevent unintentional interference with the signaling circuit during testing and servicing.”
In other words, don’t leave life safety to chance, for you never know who the next technician will be on the job. Number and letter labels are available that will make quick work of the cable identification issue, providing you keep accurate “as-built” drawings that show each and every cable identifier. Veteran fire technicians are usually accustomed to this practice, but many serious security dealers may not. Those who intend to install serious fire detection systems should study NEC.
Understanding 2 Basic Circuits
There are two types of fire alarm circuits in use in the fire alarm industry: nonpower-limited fire alarm (NPLFA) and power-limited fire alarm (PLFA). Not only must you understand the difference between the two, you must assure that the type of wiring you use conforms to the listing of the equipment.
Article 760.41 covers PLFA cable. The types of power sources specified by NEC for PLFA circuits include listed PLFA or Class 3 transformers, a PLFA or Class 3 listed power supply, and equipment listed and marked as a PLFA power supply.
Under the latter (Article 760.41(C)), NFPA provides examples of listed devices. Examples include fire alarm control panels equipped with an integral power supply listed for use as a source of power for PLFA circuits.
A current-limiting impedance, listed for use with or as part of a listed product, can also be used in conjunction with a nonpower-limited transformer or a stored energy source, such as a storage battery. For a listing of requirements for PLFA fire alarm circuit sources, refer to NFPA Chapter 9, Table 12(A) and 12(B).
Putting NPLFA Circuits to Good Use
NPLFA circuits are usually, but not always, powered by the power company’s electrical system. NEC distinctly references compliance with Chapters 1 through 4, which involve the installation of electrical wiring.
Power in a NPLFA circuit is commonly provided by a circuit breaker or similar device. Where conductors of 14 AWG and larger are used, Article 760.23 specifies the use of the ampacity charts contained in Article 310.15.
When smaller conductors are involved, the same section specifies overcurrent protection of 7A for 18 AWG and 10A for 16 AWG. An overcurrent device is a current-limiting mechanism designed to protect the fire alarm system, like a breaker.
Of special importance is conduit fill, which refers to the number of conductors that can be contained by a single conduit. Article 760.26 addresses the issue of conduit and NPLFA cables.
Although security dealers may not be involved in larger fire alarm projects where rigid or galvanized EMT is employed, they should be aware of these and similar issues. This will enable them to do a better job when called upon to do a large fire alarm job.
Differing Voltages Inside Conduit
Class 1 and NPLFA wiring can occupy the same conduit without regard to AC and DC voltage. However, the insulation on each conductor must be rated for the maximum voltage encountered by any conductor therein.
The other issue is combining the circuits of two different systems. Part (B) of Article 760.26 states that “Power-supply and fire alarm circuit conductors shall be permitted in the same cable, enclosure, or raceway only where connected to the same equipment.”
A good example of this recently surfaced on a job in a chemical plant where the electrician on the job intended to install the conductors for the explosion-proof 24VDC strobe-only devices and the 70V voice evacuation system in the same conduit. Because these two systems are separate and do not share the same power supply, Article 760.26 says they cannot be included in the same conduit.
Another issue that often confuses installers is conductor size. According to Article 760.27, NPLFA circuits must be served by 18 and 16 AWG conductors, provided they do not exceed the ampacities given in Table 402.5 of the same code. The same section provides direction on insulation and other aspects associated with NPLFA conductors.
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