Today most new fire alarm systems are power-limited. Power limiting is accomplished by the equipment manufacturer by limiting the amount of energy available to a level that is incapable of electrocution or igniting structure fires in ordinary installations. When a testing laboratory such as UL® verifies that the power available on a fire alarm control panel terminal is limited in accordance with NFPA 70-2002 Chapter 9 Tables 12A and 12B, then the equipment is listed and labeled as being power-limited – inherently safe.
The most common wiring method used on power-limited systems includes using power-limited cable marked “FPL” (general purpose), “FPLR” (riser), and “FPLP” (plenum ceilings). What many people overlook is that this is but one of three wiring methods permitted by NFPA 70 to be used on power-limited fire alarm systems.
The three permitted power-limited wiring methods are:
- Use NFPA 70 Chapters 1-3. For example, THHN conductors installed in EMT indoors and THWN conductors installed in ENT outdoors. Conductors insulated to 600V and conductor size as small as 14AWG, (18AWG and 16AWG are permitted, with restrictions).
- Use non- power-limited cable marked “NPLF” (general purpose), “NPLFR” (riser), or “NPFLP” (plenum ceiling), with an overall insulation of 600V and conductor size as small as 18AWG. Imagine Romex® cable.
- Use power-limited cable marked “FPL”, “FPLR”, or “FPLP” with an overall insulation of 300V and conductor size as small as 26AWG. Imagine CAT 5 cable.
Here’s the source material:
- NFPA 70-2002 760.52 Wiring Methods and Materials on Load Side of the PLFA Power Source.
- Fire alarm circuits on the load side of the power source shall be permitted to be installed using wiring methods and materials in accordance with either 760.52(A) or (B).
(A) NPLFA Wiring Methods and Materials.
Installation shall be in accordance with 760.25, and conductors shall be solid or stranded copper.
Exception No. 1: The derating factors given in 310.15(B)(2)(a) shall not apply.
Exception No. 2: Conductors and multi conductor cables described in and installed in accordance with 760.27 and 760.30 shall be permitted.
Exception No. 3: Power-limited circuits shall be permitted to be reclassified and installed as non- power-limited circuits if the power-limited fire alarm circuit markings required by 760.42 are eliminated and the entire circuit is installed using the wiring methods and materials in accordance with Part II, Non- Power-Limited Fire Alarm Circuits.
FPN: Power-limited circuits reclassified and installed as non- power-limited circuits are no longer power-limited circuits, regardless of the continued connection to a power-limited source.
(B) PLFA Wiring Methods and Materials.
Power-limited fire alarm conductors and cables described in 760.71 shall be installed as detailed in 760.52(B)(1), (2), or (3) of this section. Devices shall be installed in accordance with 110.3(B), 300.11(A), and 300.15.
Here’s the key:
NFPA 70-2002 Article 760.52(A) is the thread back to choice number 1) and choice number 2).
NFPA 70-2002 Article 760.52(B) is the thread back to choice number 3).
Additional references to other articles include:
- Article 110.3(B) allows a maximum 4.4V volt-drop versus the 0.6V volt-drop found in Chapter 2.
- Article 300.15 requires a box for splices and terminations, unless an integral box is provided as part of the device. This article also requires a bushing to protect cables as they enter and exit boxes and raceway.
- Article 300.22(C) restricts the type of wire used within “other spaces used for environmental air”, which is commonly referred to as a plenum ceiling. The wiring material shall be low smoke producing such as cables marked “NPLFP” or “FPLP” or to enclose cables and conductors in the space within metal raceway.
- Article 310.11(A) requires securely fastening cables and conductors to the structure, and not to use the hung ceiling as support.