· How does a nonpower-limited fire alarm circuit differ from a power-limited fire alarm circuit?
· Do the requirements differ for power-limited fire alarm circuits versus nonpower-limited? What type of wiring methods would be used for each different circuit?
· Can the wiring methods on the load side of the power-limited fire alarm circuit be changed from cabling to raceway and back to cabling, or is it necessary to stay with one particular wiring method for the entire circuit?
· What are the requirements for extending fire alarm circuits from one building to another?
· Can nonpower-limited circuits be installed in the same raceway with a power-limited circuit simply by reclassifying the entire power-limited circuit?
Noncompliance with any of the issues raised in these questions could result in a system that is "red tagged" and costly to correct. These questions must be answered before one can successfully design and install a fire alarm system based upon Article 760 in the NEC.
But before answering any of these questions, it is necessary to understand the differences between nonpower-limited fire alarm circuits and power-limited circuits.
For nonpower-limited fire alarm circuits, Section 760-21 provides text explaining that the power source of nonpower-limited fire alarm circuits must comply with Chapters 1 through 4 of the NEC like any other power circuit. The only limitation of a nonpower-limited circuit is the output voltage cannot exceed 600 volts.
Power-limited circuits are required by Section 760-41 to be supplied from a Listed power-limited fire alarm transformer or power supply, a Listed Class 3 transformer or power supply, or Listed equipment marked to identify the power-limited power source.
The design of these power sources is based upon the information provided in Tables 12(a) and 12(b) in Chapter 9 of the NEC. The voltage on a power-limited system can be from as little as zero volts to as much as 150 volts. The current may vary from as much as 8 amperes for the zero- to 20-volt system, to as little as 1 ampere for the 150-volt system.
The second question is answered very easily after realizing that a nonpower-limited circuit is only limited to a maximum of 600 volts with no limitation on the current of the system.
Nonpower-limited circuits are treated as power circuits requiring the same wiring methods as a power circuit.
There is some additional permission from Part B of Article 760 to use nonpower-limited fire alarm cable, different insulation, and smaller conductor sizes than normally allowed for power circuits. In other words, No. 18 AWG and No. 16 AWG conductors can be used for nonpower-limited fire alarm circuits in accordance with Section 760-27, while Section 310-5 restricts the size of normal building wire to no smaller than No. 14 AWG.
Power-limited fire alarm circuits, on the other hand, can either be installed using the nonpower-limited fire alarm wiring methods and materials, as mentioned previously, on the load side of the power limited source, or they can use power-limited wiring methods and materials.
Section 760-52 permits either method to be used for the fire alarm circuit but does not give permission to use both nonpower-limited wiring/materials and power-limited wiring methods on the same circuit. This means that once a wiring method has been chosen, either the nonpower-limited or the power-limited, the circuit wiring method cannot be changed from nonpower-limited to power-limited or vice versa.
For example, assume the power-limited circuit is installed using power-limited fire alarm cable from the fire alarm panel to a junction box on an outside wall of the building. It is not permissible to then switch to a conduit installed underground from one building to another using normal building wire, such as THWN. A power-limited cable Listed for direct burial must be installed.
The power-limited circuit may start out as a nonpower-limited wiring method, (such as a conduit system enclosing regular building wire) from the fire alarm panel through the building to the junction box at that same outside wall. The system can then be installed as an underground conduit containing THWN.
If a power-limited fire alarm circuit is extended beyond the building where it originates and is run outdoors, Section 760-7 requires the circuit to either meet the requirements in Parts B, C, and D of Article 800 for communications circuits or meet the installation requirements for Article 225.
Article 225 covers outside branch circuits and feeders. If the installer uses Article 225 for the installation, all pertinent requirements in Article 225 must be followed. This means that Section 225-31 would require that a disconnecting means be installed on the power-limited fire alarm circuit at a second building. Section 225-36 would require that the disconnecting means be suitable for use as service equipment.
If a nonpower-limited fire alarm circuit is extended beyond the building where it originates, it must meet the requirements of Article 225, including the requirements previously mentioned for the disconnecting means at a second building.
The disconnecting means for both the power-limited fire alarm circuit and the nonpower-limited fire alarm circuit can be locked in the closed position so that only qualified persons would have access to open the circuit. If either circuit were to be opened, the fire alarm panel would show a "trouble" and a trouble alarm would sound. This trouble alarm could be silenced until the disconnecting means is again closed, completing the fire circuit again.
The last question dealing with installing nonpower-limited circuits in the same raceway as power-limited fire alarm circuits is dealt with by an exception. Section 760-52(a), Exception No. 3 permits power-limited circuits to be reclassified as nonpower-limited circuits and installed as nonpower-limited circuits.
Any power-limited markings on the fire alarm equipment must be eliminated and the entire circuit must be installed as a nonpower-limited circuit. This reclassification of the power-limited circuit would permit these circuits to be installed in the same raceway as nonpower-limited circuits.
Following these few Code requirements can make a better fire alarm installation and help ensure that it is Code compliant.
ODE is a staff engineering associate at Underwriters Laboratories, Inc., in Research Triangle Park, N.C. He can be reached at (919) 549-1726 or by e-mail at email@example.com.