Douglas Krantz - Technical Writer - Describing How It Works

Do We have to Install a Separate Circuit to Power the Speaker-Strobes?

By Douglas Krantz | Maintenance

Do We have to Install a Separate Circuit to Power the Speaker-Strobes?

Do We have to Install a Separate Circuit to Power the Speaker-Strobes?

Greetings Douglas,

I'm working on a fire alarm project in Boston and there was an available MC Fire Alarm cable going from the suite where we've wired in 5 speaker strobes to the Notifier Fire Alarm panel approximately 200 feet away.

When we returned to the site to connect to the cable running to the panel the sprinkler vendor had connected a flow tamper switch to the cable.

The question is can we still connect our cable with the devices wired in to it to the cable that has the flow tamper switch on it, or do we need to run a separate cable back to the Fire Alarm Control Panel?

Can the existing cable support the flow tamper switch and the devices simultaneously or does the flow tamper switch require a dedicated fire alarm cable?

Your reply will be appreciated!

Thank You, JV

A single pair of conductors inside any cable can only be used for a single type of circuit. Connecting devices other devices than what a circuit is meant for can easily cause the circuit, and possibility the whole fire alarm system, to fail.

From your description, it sounds to me like there are actually three different types of circuits being used.

MC Cable

MC Fire Alarm Cable sounds to be Metal Clad Fire Alarm Cable. The Metal Clad is a description of the flexible metal case surrounding the inner conductors. Almost all of the time for fire alarm circuits, I've seen two conductors inside the MC Cable. Two conductors, though, are needed for each of the three circuits.

Fire Alarm Circuit - Strobes

A dedicated circuit is needed to provide power for the strobes. The first strobe on the circuit sounds like it will be 200 feet as-the-crow-flies from the fire alarm panel. The distance for the conductors in the cable will be greater because the path for the wires is longer.

This is not an electrical 110-volt system, this is a low-voltage fire alarm system. Being a fire alarm system that uses 24 volts nominal (20 volts to 27.5 volts), the added-up resistance in the wires is a real issue.

I can't tell from your description, but according to the National Fire Protection Association (NFPA), all the strobes have to be daisy-chained on a dedicated Notification Appliance Circuit (NAC). With the cable route through the building, the last strobe may be as far as 400 feet conductor-distance from the panel.

Remember, this is a 24-volt (nominal) circuit, so according to the NFPA, the voltage losses in the wires (voltage drop) needs to be calculated. The fire marshal may also require that you show your calculation-work on the voltage drop.

Yes, you probably can get away with using 14 AWG wire - at least while everything in the fire alarm system, including the power into the building, is working.

However, when there is a power blackout in the neighborhood, the fire alarm system switches to the backup battery. Because of the lower voltage of the battery, the system may fail. If the wire is too small for the lower voltage, that is not a good thing.

Fire Alarm Circuit - Speakers

The speakers are a separate circuit from the strobe circuit. While the strobes operate on DC voltage, the speakers use an AC Audio power. The speaker circuit also has to be daisy changed, like the strobe circuit.

Because of the distance for the speaker circuit is similar to the strobe power circuit, power loss calculations may be required for the speaker circuit.

DO NOT INCLUDE the wires for the speaker audio circuit in the same conduit or metal clad cable as the strobes, and especially any other fire alarm circuit or power circuit. Installing the speaker circuit next to other circuits causes crosstalk-interference. The interference between the different type of circuits may mean there will be unacceptable low-level sounds from the speakers.

Fire Alarm System - Waterflow Switch and Tamper Switch

I'm guessing that the fire alarm system is an addressable type of fire alarm system. Because it may be an addressable system, a circuit called a Signaling Line Circuit (SLC) is used to send low-power data signals between the fire alarm panel and addressable modules and detectors.

Then again, if the system is a conventional type of system, there will have to be more research involved before completely finishing the system.

No matter what kind of system you're working with, connecting strobes or speakers to the MC cable with the tamper switch will make the circuit useless. Don't connect the speakers or strobes to this circuit.

Follow the Instructions in the Control Panel's Installation Manual and Installation Sheets

Each manufacturer requires different wiring methods for their fire alarm system. Some of these differences don't seem very big, but they really are important. Just because another manufacturer uses a common method of wiring, doesn't mean that that common method will work here.

Read the installation manual and the installation sheets each time you are working on a different system. You'll install better systems, and you'll save headaches for yourself, and headaches later for technicians.

Douglas Krantz
Life Safety
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