Douglas Krantz - Technical Writer - Describing How It Works

Can There be More than One EOL Resistor?

By Douglas Krantz | Descriptions | 6 minutes of reading

End of Line Resistors are installed in a conventional fire alarm system as circuit-completers. To constantly make sure the fire alarm devices are connected, the resistors are at the end of line so the panel can perform a continuity check of the wiring. Extra resistors cause problems.

Can There be More than One EOL Resistor?

Greetings Douglas,

Good morning. Can the fire alarm cable be looped into different directions in just one zone?

For example. Can it be split in the middle of an office so it will go three different directions with End of Line Resistors located at each end of line? as to serve 3 ends meaning that the EOLRs will be installed in 4 places on 1 zone? Example: the 1st end, the 2nd end, 3rd end, and the break glass end.

Will it work perfectly?

Thank you, TR

  • From an electrical point of view, there is a problem with adding extra End of Line Resistors
  • From a life-safety point of view, there is a problem with more than one End of Line Resistor
  • From a legal point of view, there is a problem with installing the system differently than what is shown in the installation manual


The End of Line Resistor (EOL Resistor) allows a small amount of electrical current to pass through it. When allowing the small amount of current to pass through it, the EOL Resistor is "Pulling Down" the voltage of the loop.

The fire alarm panel has a "voltmeter" inside it to read the voltage on the zone.
  • If there is no EOL Resistor, the voltage on the loop won't be pulled down at all. The panel's voltmeter sees that the voltage isn't being pulled down and turns on the trouble light and buzzer in the panel.

  • If there is a single EOL Resistor, the voltage on the loop will be pulled down a little. The panel's voltmeter sees that slight pulling down of voltage as "Normal" and won't turn on any light or buzzer.

  • If a second EOL resistor is installed on the loop, the voltage on the loop will be pulled down a little further. The panel's voltmeter may see this as a fire alarm, turning on all the horns and strobes in the building. If a third of fourth EOL Resistor is installed on the loop, the panel will always be in the "Fire Alarm Sounding Mode".

Electrically, don't use any extra EOL Resistors. Extra EOL Resistors will only cause electrical problems and will probably sound the fire alarms in the building.


Download and save the Class B Wiring PDF -

A fire alarm system Detects Fire and Warns People of Fire. In order to detect fires, the smoke detectors, pull stations and Manual Call Points (MCP) have to be connected to the fire alarm system. Unless the panel can detect that a wire is broken or has come loose, the detectors and MCPs (pull stations) that aren't connected to the panel are useless.

For the fire alarm panel to confirm that all the devices in the building are connected, a small current is passed through all the red wires, one at a time, through the EOL Resistor, and back to the panel through all the black wires, one at a time. This is an electrical pathway.

If the electrical pathway is broken, if any wire in the pathway breaks or comes loose from a connection, the electrical current stops. Then the EOL Resistor quits pulling down on the voltage, and the panel's voltmeter turns on the trouble light and buzzer. That way the building's owner will call for service.

If any wires are not in this electrical pathway, the wires not in the pathway can come loose or be broken, and then no one will figure out there is a problem... at least until a fire is not detected. When a fire is not detected, the fire becomes a life-safety issue for all the people in the building.

Splitting a loop is often called T-tapping. On a conventional fire alarm panel, never split the loop or T-tap a loop so there is more than one end of the circuit.


According to the NFPA, all fire alarm equipment, devices, and wiring methods have to be "Listed". Listing means that a third party, nationally known testing laboratory has tested the equipment, device, or wiring method, and found it to work. Then they place it on their list of equipment, devices, or wiring methods that work. The testing laboratory can be something like UL, ULC, CE, FM, CCC, etc.

The manufacturer has paid the testing laboratory to perform the tests. The manufacturer will show in their installation manual for the fire alarm system the "Listed" method of wiring. Often, there will be on the inside door of the panel a copy of that "Listed" method to use for the wiring.

Any wiring method that is not shown in the manufacturer's installation manual is not "Listed" (in other words not tested) and should never be used.

Reason for the "Laws"

The reason for the life-safety laws shown in the NFPA is not to make lawyers or governmental officials happy; the reason for the life-safety guidelines shown in the NFPA is to make sure that the fire alarm systems will be installed to work, and will save lives.

The NFPA says to use equipment and methods that have been that have been tested by a third party, nationally known testing laboratory, and then placed on their "List" of things that work. Even the wiring diagrams shown in the installation manuals and installation sheets have been tested and listed. Use the method of wiring shown in the manual.

Douglas Krantz
Life Safety
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