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Service Maintenance Company, Inc. Electrical Contractors |
| NEC Article 760 - Fire Alarm Signaling Systems | 6/9/2004 |
| Mike Holt for EC&M Magazine A fire alarm signaling system must not be an early casualty of a fire. Interest in emergency systems rose sharply after September 11 and remains high. Owners no longer install fire alarm signaling systems just to satisfy their insurance company. Today, the motivation is different and that means much more work involving this kind of equipment. The work includes new installations, retrofits, upgrades, testing, and maintenance. Article 760 covers the installation of wiring and equipment for fire alarm systems, including all circuits the fire alarm system controls and powers (see. Figure 760-1 un760-01 760-01.cdr). NFPA 72-National Fire Alarm Code provides other fire alarm system requirements. Fire alarm systems include fire detection and alarm notification, voice communications, guard's tour, and sprinkler supervision. The fire alarm system might control or power other systems, too. These include elevator capture, elevator shutdown, door release, smoke doors and damper control, fire doors and damper control, and fan shutdown. Building control circuits (e.g., elevator capture and fan shutdown) associated with the fire alarm system must comply with Article 725. Article 760 applies only where the fire alarm system directly powers and controls these circuits. Relation to other articles Article 760 doesn't stand in isolation. For example, 300.21 requires you to use approved methods to provide firestops for fire alarm circuits installed through fire-resistant-rated walls, partitions, floors or ceilings-to maintain the fire resistance rating. You must install cables through fire-rated assemblies with an approved fire-stop material per the specific instructions supplied by the manufacturer of the specific type of cable and construction material. When you make any opening in fire-resistant walls, floors or ceilings, you must do so in a way that does not substantially increase the possible spread of fire or products of combustion. Special installation requirements apply to: [300.22] Fire alarm circuits in ducts, plenums and other air-handling spaces. [Articles 500 - 516] Fire alarm circuits in hazardous (classified) locations. [Article 725] Building control circuits associated with the fire alarm system (but not powered by it or controlled by it). [Article 770]. Optical fiber cables used in fire alarm circuits. General installation requirements Part I gives general requirements, and in 760.2 we find definitions particular to fire alarms systems. For example, what is a Nonpower-Limited Fire Alarm Circuit (NPLFA)? These circuits cannot operate at more than 600V, but there is no power or current limitation for them (see Figure 760-2 un760-02 760-02 01 def NPLFA.cdr). An NPFLA circuit is similar to a Class 1 circuit. The main difference is an NPLFA circuit is powered and controlled by the fire alarm system-frequently at 120V. You cannot supply these circuits, or those of Power Limited Fire Alarm (PLFA) circuits, through GFCIs-except as noted in the FPNs that refer to 210.8(A)(5). [760.5] Locate cables so they don't prevent the removal of ceiling panels for access to electrical equipment. [760.6] Install equipment and cabling in a neat and workmanlike manner, and support them. If you install cables on the exposed on the surface of ceilings and sidewalls, support them by the structural components of the building in a manner that prevents damage from normal use. You can secure the cables to structural components by straps, staples, hangers or similar fittings designed and installed so as not to damage the cable. If you install cables next to framing members, you must protect them against physical damage from penetration by screws or nails by 1 1/4 in. separation from the face of the framing member or by a suitable metal plate per 300.4(D). [760.7] Where twisted pair power-limited fire alarm circuit conductors extend beyond a building, install them per Article 800 or use a Chapter 3 wiring method. [760.9] System grounding is not required for fire alarm circuits that operate at 50V or less. Fire alarm circuits that operate at over 50V must follow the grounding and bonding requirements in 250.30. [760.10] Identify fire alarm circuits at all terminal and junction locations, in a manner that will prevent unintentional interference with the fire alarm circuits during testing and servicing. After giving the general requirements in Part I, Article 760 gives the requirements for NPLFA circuits in Part II and PLFA circuits in Part III. Part II is about half the size of Part III. Let's look at Part II first. NPLFA circuits The main thing to remember about NPLFA circuits is their power source cannot exceed 600V, nominal. The y differ from PFLA circuits in another way: they have circuit overcurrent circuit protection devices (OCPDs). OCPDs must not exceed 7A for 18AWG or 10A for 16 AWG conductors. You must locate the OCPDs at the point where the conductor receives its supply, unless your system meets one of three exceptions noted in 760.24. [720.6] Class 1 and NPFLA conductors can be in the same cable, enclosure, or raceway without regard to whether individual circuits are AC or DC, provided all the conductors are insulated for the maximum voltage of any conductor in the enclosure or raceway. Power supply and NPFLA conductors can be in the same cable, enclosure, or raceway only where connected to the same equipment. [720.27] You can use only copper conductors in fire alarm systems. Insulation must be suitable for 600V. Other conductor limitations apply. [760.28] If you have only NPFLA and Class 1 conductors in a raceway, fill it according to 300.17. You must use the conductor adjustment factors of 310.15(B)(2)(a) if the conductors carry a continuous load in excess of 10% of the ampacity of each conductor. If you mix power supply and NPFLA conductors in a raceway per 760.26, determine the wire fill per 300.17. If you install conductors in cable trays, apply 392.9 through 392.11. [760.30] You can use multi-conductor NPFLA cables, if the cables and installation methods meet a long list of requirements. Among those: The circuits must be 150V or less. You must provide support at intervals of not more than 18 in., if the cables are within 7 ft of the floor. You can't install Types NPLFP, NPLFR, or NPLF in plenums or ducts. You must use specific cable type for certain applications. For example, you must use Type NPLFR for vertical runs that penetrate more than one floor. The cables must be listed as Circuit Integrity (CI) cable. PLFA circuits PLFA circuits (load side) don't have OCPDs, but their power sources (supply side) do. You must protect transformers or other devices supplied from power-supply conductors by an OCPD rated not over 20A. A PLFA power source must be one of the following: A listed PLFA or Class 3 transformer A listed PLFA or Class 3 power supply Other power-limited fire alarm equipment listed for use. While you don't need to mark NPFLA circuits as NPLFA, 760.42 requires you to label PLFA circuits with durable and plainly visible markings on the equipment to indicate each circuit that is PLFA. [760.52] For PLFA, you can use the same wiring methods already discussed for NPLFA (see Figure 760-12 un760-12 760-52.cdr). As you might expect, exceptions apply. If you need to install PLFA circuits inside the same raceway or cable with power or Class 1 circuits, you can reclassify PLFA circuits. To do so, you must remove the PLFA marking on the equipment, provide OCPDs per 760.23, and ensure the conductors have insulation suitable for 600V. Another requirement: you can't install PLFA circuits reclassified as NPLFA circuits with other PLFA circuits that have not been reclassified as NPLFA. Separation is an important concept in PLFA. Per 760.55, you cannot place PLFA conductors in any enclosure, raceway or cable with conductors of electric light, power, Class 1 non-power-limited fire alarm circuits or medium power network-powered broadband communications cables (non-power-limited energy circuits). You can mix PLFA conductors with non-power-limited energy conductors if the non-power-limited energy circuit conductors are introduced solely to connect to the equipment and comply with either of the two points below: You maintain a minimum of 1/4 in. separation from the PLFA conductors The non-power-limited energy circuit conductors operate at 150V or less to ground and the PLFA conductors are installed as non-power-limited fire alarm circuits per 760.25 (see 760.52 Exception No. 3). Article 760 lays out rules for ensuring fire alarm system wiring lasts long enough for occupants to escape a fire. You need to understand the general requirements in Part I, no matter what kind of system you install. If the system is power-limited, follow the requirements of Part II. If the system is not power-limited, follow the more detailed requirements of Part III. And remember, your work with fire alarm systems does more than satisfy an insurance company requirement. |
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