Fire & Life Safety: Outdoor Wiring

Dec. 13, 2018
While it isn’t in the traditional wheelhouse of a fire alarm installer, learning the nuances can be the key to interconnecting buildings on a campus

For the most part, fire alarm installers do not have to run wire outdoors; however, occasionally, overhead or underground wiring will end up being something you will have to learn. This may be the case if a property owner has more than one building under construction – you will probably want to weigh the benefits of having one system cover the entire property instead of installing separate complete fire alarm systems.

A school campus is a prime example. It could include portable classrooms, an elementary school, a middle school and a high school, as well as a separate bus garage, or detached shop class or portable library. In a small district, it might even include the Board of Education office building located on that same campus. Other examples include a factory with offices and a warehouse on the same property; or a construction and supply company with several office and storage buildings to protect.

In these instances, by leveraging the capability of a single, intelligent addressable fire alarm control panel (FACP), you can treat additional buildings as if they are no more than additional wings of the main building. All you need is to have your Signaling Line Circuit (SLC/addressable circuit) connect each separate building to the FACP. Each building will then have a NAC power booster installed in it, instead of another complete fire alarm system.

This configuration creates huge business benefits for the alarm installer. It could eliminate the costs associated with purchasing additional FACPs, drawing up separate plans with separate permit fees, communication equipment, and all the site-specific programming and points of failure these additional systems would entail.

After you make the decision to use one FACP to create a unified system for all the buildings, you will need to run a SLC between each building and the FACP. This presents two cabling options: either overhead or underground. To stay compliant, the NEC at 760.32 directs to Article 800, which outlines the rules for outdoor wiring between buildings. When deciding between the two, distance and terrain will be the most important conditions to consider.

Overhead Wiring: If you need to run wire between buildings that have a paved area between them, you should consider running the wire overhead. Use outdoor-rated and sunlight-resistant copper cable and a messenger wire for support. All the clips and supports are available at your local parts supplier.

You will also have to support these cables at least 10 feet above the ground. The cables must be placed at least 18 feet above the surface if they run across public streets, alleys, roads or parking areas subject to truck traffic; above driveways on other than residential property; and other land such as cultivated, grazing, forest and orchard vehicle traffic. Be sure to consult the NEC at 230.24, where it is all spelled out in detail.

Underground Cables (UF): If not running long cables overhead, then direct burial of the circuit may be the easiest installation route. In this case, using conduit can provide a layer of protection against physical damage coming from things such as rocks and future digging. If conduit is not used, plan to use a 24-inch-deep trench. If installed in PVC conduit, then the trench may only need to be 18 inches deep.

UF stands for Underground Feeder and may be as small as 14 AWG, although you may not be able to find that size locally, and it may need to be special-ordered. PVC is easy to work with and not that expensive; in fact, you might save even more by letting the electrical contractor bury an empty conduit (with a pull string in it) for you. Most engineers will include this in the specifications for the electrical contractor when they know low voltage systems will be needed.

Using conduit will also enable you to pull additional conductors if they are needed in the future. If the empty conduit is at least one inch, you will be prepared for just about any future fire/security projects.

If you hate to trench and no conduit has been provided for you, then rigid metal conduit will require a 6-inch deep trench. Simply make sure to follow the table in the beginning of NEC Article 300.

Lightning Arrestors

At the point your SLC leaves one building and enters the other building, you will need to provide a splice box on each end of the conduit or cable that connects the buildings – in order to install required lightning arrestors. This is not a regular surge protector – it must bear the specific UL Listing of 497B, which designates a protector for “Data Communications and Fire-Alarm Circuits.” If it is not listed to UL 497B, it is the wrong device (see Informational Note at 760.32 in NEC).

Sometimes you can eliminate using these $100 protectors if the cable run is “140 feet or less, directly buried or in underground conduit, where a continuous metallic cable shield or a continuous metallic conduit containing the cable is connected to each building grounding electrode system” (which is already an Article 800 requirement).

According to NEC article 800.90 “Protective Devices,” there are two more options to eliminate the cost of these 497B protectors. The first is where buildings are close together and “sufficiently high to intercept lightning.” The second is in areas “having an average of five or fewer thunderstorm days per year and earth resistivity of less than 100 ohmmeters,” such as along the Pacific coast (NEC Article 800.90).

Fault Isolation Modules

NFPA 72 (2016) at 23.6.1 states that “a single fault on a pathway connected to the addressable devices shall not cause the loss of the devices in more than one zone.” This relatively new rule will require much more frequent use of Fault Isolation Modules (FIM). Even if you were able to use one of the three NEC “Exceptions” mentioned above to eliminate using 497B lightning arrestors, you will always need a FIM on an outdoor section of the SLC.

As a fire alarm installer, your depth of knowledge regarding “other than typical” installations will increase as your experience grows. Although you may have thought you would never need to know what the requirements were for burying wire underground, sometimes special installs require you to learn a new trick of the trade. You will be a more valuable employee if you can brush up on the nuanced rules for these special applications and become competent in a new skill.

Greg Kessinger has been SD&I’s fire alarm and codes expert and a regular contributor for more than 15 years. Please email him your fire & life safety questions for potential inclusion in this column at [email protected].