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How many T-Taps are allowed on a Signaling Line Circuit (SLC)?

Every device on a Signaling Device Circuit is actually a T-tap
Every device on a Signaling Line Circuit is a T-Tap for the Panel


By Douglas Krantz

The answer to the question of how many T-Taps is allowed --- is "It Depends". It depends on the manufacturer and it depends on how easy the installer wants to make the system for the technician when servicing the system for the coming years.

Style 4 wiring is Class B wiring, which is the data-loop or signaling line circuit (SLC), allows an unlimited number of T-Taps, at least for most manufacturers.

Long Star-Taps

Most fire alarm systems that use Style 4 wiring for the SLC are electronically connecting every device directly to the terminals of the fire alarm panel. Being wire nutted at all junctions, electrically the wire loop is a single pair of copper conductors.

For all practical purposes, all devices on the loop are connected directly to the panel, and the panel can't tell the difference between home runs to each device and a single daisy chain.

Yes, the installed wiring may daisy chain to 75 devices in a straight line, but electrically, every last one of them is connected directly to the terminals of the fire alarm panel.

For these types of systems, the wires can be thought of as having a long star-tap.

Later Servicing the System

The real concern with the T-Taps is with the later servicing of the system. When trying to find a ground fault or bad device. A limited number of T-Taps makes it easy to divide up the system.

Not enough T-Taps and the technician has to guess where the wires run through the building. Too many T-Taps and the technician has to pull a lot of T-Taps apart to get an idea of where to find the faulty device or wiring.

Manufacturer's T-Tap Limit

The manufacturers that limit the number of T-Taps have an input side and an output side to each device on the SLC. Here the panel itself is creating a map of the SLC wiring system. If there are too many t-taps, or if the ins and outs of the devices are not wired according to the installation sheets, the panel's created maps become useless for later servicing.

How Many T-Taps

When trying to determine how many T-Taps are allowed, consult the manufacturer's installation sheets, and then decide on how easy the servicing of the system should be in the coming years.
Based on his electronics training, and his understanding of Life Safety, Douglas Krantz has compiled his knowledge of Conventional Fire Alarm Systems into the book Make It Work - Conventional Fire Alarms. The book covers the basics of the Conventional Fire Alarm System, and shows how Life Safety and internal supervision affects the fire alarm system.

Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

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