Monday, September 16, 2019

Fire Alarm Wiring Based on NEC Article 760

A common topic for discussion in the fire alarm industry involves fire alarm wiring. This article will cover all aspects of fire alarm wiring including but not limited to separation, conduit fill, strapping, mechanical protection and marking.

Fire Alarm Circuits


The definition of a fire alarm circuit is as follows: "The portion of the wiring system and connected equipment powered and controlled by the fire alarm system. Fire alarm circuits are classified as either nonpower-limited or power-limited."

I'm sure you have heard these two terms in the industry before so let's break them down.

Non-Power Limited Fire Alarm Circuits

A non-power-limited fire alarm circuit commonly referred to as NPLFA, can operate at up to 600V and the power output isn't limited.

Power-Limited Fire Alarm Circuits

A power-limited fire alarm circuit commonly referred to as PLFA, must have the voltage and power limited by a listed power supply that complies with NEC 760.121. Based on this section, a power source can be either (1) a listed PLFA or Class 3 transformer, (2) a listed PLFA or Class 3 power supply or (3) listed equipment marked to identify the PLFA power source. A few examples of listed equipment would be fire alarm control panels with integral power sources and circuit cards listed for use with PLFA sources.

The two tables below provide the listing requirements for power-limited fire alarm circuit sources:

NEC Table 12a and 12b Power Source Limitations

Power Sources for Power-Limited Fire Alarm Circuits

Power-Limited fire alarm equipment must be supplied by a branch circuit that supplies no other load and is NOT GFCI or AFCI protected. The branch circuit overcurrent device (breaker) must be identified in red, accessible only to qualified personnel, and identified as "FIRE ALARM CIRCUIT". The red markings cannot damage the overcurrent protective device or cover any manufacturer's markings. The lock pictured below is available from Space-Age Electronics.

Fire Alarm Circuit Breaker Lock


Equipment Marking for Power-Limited Fire Alarm Circuits

The fire alarm equipment that supplies power-limited fire alarm cable circuits must be marked to indicate each circuit that is a power-limited fire alarm circuit. Per NEC article 760.30, the fire alarm circuits must be marked at terminal and junction locations.

Wiring Methods for Power-Limited Fire Alarm Circuits


Power-limited fire alarm circuits shall be installed in accordance with NEC article 760.46 and conductors shall be solid or stranded copper.

Cable splices or terminations shall be made in listed fittings, boxes, enclosures, fire alarm devices, or utilization equipment. If the circuits are installed exposed, the cables shall be adequately supported and installed in such a manner that maximum protection against physical damage is afforded by building construction. The thought here is that nails from baseboards, door frames, drywall, etc. may penetrate deep enough to damage the wire. To avoid this, make sure to install your fire alarm cables no closer than 1 1/4" from the edge or the framing.  If this is not possible, use 1/16" thick steel plate for protection [NEC 760.24(A)]. Where cables are installed within 7 feet of the floor, said cables shall be fastened in an approved manner at intervals of not more than 18 inches.

steel plate to protect cables in framing


Power-limited fire alarm cables are NOT permitted to be strapped to the exterior of any raceway as a means of support. Exposed cables must be supported by the structural components of a building so that the cable will not be damaged by normal building use. Cables must be supported by straps, staples, hangers, cable ties, or similar fittings designed and installed in a manner that will not dame said cable. If the calves or raceways are installed above a suspended ceiling, they must be supported by independent support wires attached to the suspended ceiling.

Cables passing through a wall or floor. Both Power-Limited and Non Power-Limited Fire Alarm Cables shall be installed in metal raceways or rigid nonmetallic conduit where passing through a floor or wall to a height of 7' above the floor, unless adequate protection an be afforded by building construction. Keep in mind if the cables pass through a fire barrier, you must provide fire caulking to insure the integrity of the barrier.

fire caulk penetration with metal raceway
Fire Caulk Plugs for Cables


Power-Limited Fire Alarm Circuit Separation


This is a topic that a lot of designers and technicians constantly go back and forth on.  To better understand the separation requirements, I believe it is important to know what the 3 different circuit classification are.

Class 1 Circuits. 

Class 1 remote-control and signaling circuits typically operate at 120V, but the NEC permits them to operate at up to 600V [725.21(B)]. You must install these circuits within a wiring method listed in Chapter 3 of the NEC, which includes raceways, cables, and enclosures for splices and terminations [725.25]. Remote-control circuit. These circuits, which control other circuits through relays or equivalent devices, are commonly used to operate motor controllers in moving equipment, mechanical processes, elevators, and conveyors.

Class 2 Circuits.

Class 2 circuits typically include wiring for low-energy (100VA or less), low-voltage (under 30V) loads such as low-voltage lighting, thermostats, PLCs, security systems, and limited-energy voice, intercom, sound, and public address systems. You can also use them for twisted-pair or coaxial local area networks (LAN) [725.41(A)(4)]. Class 2 circuits protect against electrical fires by limiting the power to 100VA for circuits that operate at 30V or less, and 0.5VA for circuits between 30V and 150V.

Class 3 Circuits. 

Class 3 circuits are used when the power demand for circuits over 30V exceeds 0.5VA, but is not more than 100VA [Chapter 9, Table 11]. We often see Class 3 signaling circuits for security systems and public address systems; voice, intercom, and sound systems; and some nurse call systems.
Higher levels of voltage and current are permitted for Class 3 circuits (in contrast to Class 2 circuits).

Fire Alarm Cable Separation based on Circuit Classifications


PLFA with Class 1 Circuits

NEC 760.136 (A) Power-limited fire alarm circuits must not be placed in any enclosure, raceway or cable with conductors of electric light, power or class 1 circuits.

NEC 760.136 (B) If the circuits are separated by a barrier, power-limited fire alarm circuits are permitted with electric power conductors.

NEC 760.136 (D) Power-limited fire alarm circuits can be mixed with electric light, power and class 1 circuits in enclosures where these other conductors are introduced solely for connection to the same equipment and a minimum of 1/4" separation is maintained from the power-limited fire alarm cables.

Power-limited fire alarm circuits shall be separated by not less than 2" from insulated conductors of electric light, power or Class 1 circuits. Exception: If the electric light, power, class 1 circuit or power-limited fire alarm circuits are installed in a raceway, metal-sheathed, metal-clad, nonmetallic-sheathed or underground feeders.

PLFA with Class 2 and Class 3 Circuits

NEC 760.139 (A) Two or more PLFA Circuits. Power-limited fire alarm circuits, communications circuits or Class 3 circuits can be installed in the same cable enclosure, cable tray, raceway or cable routing assembly.

NEC 760.139 (B) PLFA and Class 2 Circuits. Power-limited fire alarm circuits and Class 2 circuits can be within the same cable, cable tray, cable routing assembly, enclosure, or raceway provided the Class 2 circuit insulation is not less than that required for the power-limited fire alarm circuits.

NEC.139 (C) PLFA and Low Power Network Communication. Low-powered network powered broadband communication circuits hall be permitted in the same enclosure, raceway, cable assembly, or cable tray.

NEC 760.139 (D) PLFA and Audio System Circuits. Power-limited fire alarm circuits and audio system circuits using Class 2 and Class 3 wiring methods shall not be installed in the same raceway, enclosure, cable routing assembly or cable tray. Please not this does not apply to voice evacuation and mass notification speaker circuits controlled by a fire alarm control unit or amplifier.

Fire Alarm Cable Substitutions


NEC 760.154(A) The following fire alarm cable substitutions are permitted as long as the wiring requirements of NEC Article 760 Parts I and III apply.

FPLP (Fire Power-Limited Plenum) ------------> CMP
FPLR (Fire Power-Limited Riser) --------------> CMP, FPLP, CMR
FPL (Fire Power-Limited) -----------------------> CMP, FPLP, CMR, FPLR, CMG, CM

Fire Alarm Conductor Size


NEC 760.142. Conductors of 26 AWG shall be permitted only where spliced with a conductor listed as suitable for 26 AWG to 24 AWG or larger conductors that are terminated on equipment or where the 26 AWG conductors are terminated on equipment listed as suitable for 26 AWG conductors.

Single conductors shall NOT be smaller than 18 AWG.

How to Figure Conduit Fill


Conduit fill requirements can be found in the NEC Annex Table C.  This is toward the back of the book and is broken up into different sections based on the type of raceway being used.  In this example, we will use table C.1 for EMT (Electrical Metallic Tubing).  Take a look at the table below and try to locate the maximum number of 14 AWG THHN conductors permitted in 2 1/2" EMT raceway. The answer is 241.








No comments:

Post a Comment