Many contractors work on building tenant fit-outs or reconfigurations of existing spaces where an existing fire alarm system requires updating. Often, a fire alarm equipment salesperson will lure the contractor into thinking it is OK to reuse the existing system’s wiring and cable. That same salesperson may also tell the contractor that the least expensive option is a simple one-for-one replacement of all devices and appliances. Blindly following that advice will likely lead to a loss on the project.


Unfortunately, the codes remain silent regarding the reuse of existing cable. But, without further examination, do not blindly accept the idea that you may do so. 


NFPA 72 2016, National Fire Alarm and Signaling Code, requires every fire alarm system to have suitable written documentation. This documentation, if available, helps with evaluation. The evaluation should include battery capacity and de-rating calculations (where batteries are provided), voltage-drop calculations for notification appliance circuits and where occupant notification is required, and minimum sound-­pressure levels the system must produce by the audible notification appliances in applicable covered areas.


Generally, the as-built drawings would show pathway diagrams between the control unit, devices, notification appliances, control circuits and the supervising station and shared communications equipment, if any.


First, ensure the entire existing fire alarm system works properly, the wiring remains intact from device to device, and all notification appliances operate properly. Test the entire system before going to the next step in the evaluation. Charge for this test as a part of your services, because doing so will ultimately help save the customer money. 


Once you have verified the wiring is intact, determine each circuit’s electrical load. Start with the greatest load, which is usually the notification appliances.


To meet code requirements, any upgrade to the existing system will require more horns, speakers and/or strobes. The current notification appliance circuits may have already reached their maximum acceptable loads, which may require new cabling to serve additional appliances.


Depending on the age of the existing system, you may also need to use either twisted cable or shielded cable for additions to the existing control unit. If you intend to replace the entire system, verify that the type of cabling originally installed meets the code requirements and will work with the new system to be installed. Of course, for all newly installed cabling or equipment, NFPA 70 2017, National Electrical Code, comes into play here, in addition to NFPA 72 2016.


It should not be a surprise that the existing system may not meet the requirements of whichever code edition the jurisdiction currently enforces. If you replace the system, you will need to determine the audibility—and intelligibility for an in-building voice evacuation fire alarm system—and whether the system provides enough strobes to meet the current code. In the case of audibility and strobe placement, determine if the building layout has changed since the installation of the original fire alarm system and if the existing system performs any fire-safety-control functions, such as control of the heating, ventilating and air conditioning system or smoke doors.


Once you fully understand the condition of the cabling and the fire alarm system’s operational integrity, next comes the intent of the upgrade or replacement system. You are responsible for developing a written narrative that describes the system operation for changes. If you simply add smoke detectors in a specific area of the building, you may only need to write a rather simple narrative. The narrative must include how the operation of the new smoke detectors will affect the balance of the existing fire alarm system. Also, you must show a cabling diagram depicting how the new equipment is connected to the existing system. If you can locate the original as-built drawings, update those drawings to document all changes that have been made.


If you simply install a “subpanel” to connect and operate the new devices and notification appliances, make certain that you can properly interface the new subpanel with the existing system. 


Remember, once you work on the system, you assume responsibility for the entire system. You are accountable for ensuring that any fire alarm system you touch remains operational when called on. The litigation “rule” states that the last person to touch the system will be held liable if the system fails to operate in any way.


So, when can you reuse existing fire alarm system cable? It depends on the condition and viability of the existing cable as well as your understanding of any additions or changes you must make to the system. Only then can you use the existing cable.