Douglas Krantz - Technical Writer - Describing How It Works

How is Class B Converted to Class A?

By Douglas Krantz | Descriptions | 15 minutes of reading

How is Class B Converted to Class A?

How is Class B Converted to Class A?

Greetings Douglas,

What's involved in converting a Class B system to a Class A system?

Thank you, AA

Here is an outline of what's involved when converting from a Class B to a Class A fire alarm system:
  • NFPA Classifications
  • Decide on How Much Should be Changed
  • Control Panel
  • Initiating Device Circuit (IDC) or Input Circuit
  • Notification Appliance Circuit (NAC) or Output Circuit
  • Signaling Line Circuit (SLC) or Input and Output Circuit
  • Adding New Wires
  • Testing and Certifying

NFPA Classifications

This is a summary of what the National Fire Protection Association (NFPA) expects for Class A and Class B circuits. Notice that the NFPA is talking directly about the circuits in the building. The NFPA may be talking about the building wiring, but whatever control panel is being used, it requires its own capability to use the Class of wiring that is being connected.

Class A
  • This will include a redundant signal path - If the path is interrupted, the system feeds both ends of the paths so there are now two paths; the original outgoing path which is now cut shorter, and the return path which is now being used as an outgoing path (In other words, all devices are still connected to the panel.)
  • If wires are used, a wire-to-wire short may shut down the whole path
  • Both conventional and addressable systems fit into this
  • Both the IDC (Initiating Device Circuit) and the NAC (Notification Appliance Circuit) fit into this
  • The panel shows a trouble signal when there is a problem

Class B
  • There is no redundant path
  • Any device beyond a break won't work
  • If wires are used, a wire-to-wire short may shut down the whole path
  • Both conventional addressable systems fit into this
  • Both IDC and NAC fit into this
  • The panel shows a trouble signal when there is a problem

Keep in mind that no Class A circuits in the building will have a T-tap. If the circuit is a Conventional Class B circuit, the Class B circuit won't have T-taps either.


The first part of changing from a Class B fire alarm system to a Class A fire alarm system is deciding how much of the system is going to be converted.

Almost all fire alarm systems have conventional wiring. Waterflow switches, for instance, are conventional devices and are usually wired to a Conventional Class B Input Module or Control Panel Zone Input, but I have seen them wired as Conventional Class A Zone Inputs.

Horns and strobes, as another instance, are often wired to Class B Notification Appliance Circuit (NAC) Supervised Output Module or Control Panel NAC Output, but I have seen them wired as Conventional Class A NAC Outputs.

Signaling Line Circuits (SLCs), which have Detectors, Input Modules, and Output Modules, are often wired as Addressable Class B Circuits, with many T-taps along the SLC, but I have seen them wired as Addressable Class A Circuits.

To be Class A, none of the circuits can have T-taps, and all of the circuits have to start and end inside the control panel.

Control Panel and the Inputs and Outputs

To be used for controlling a Class A building circuit, the control panel requires separate Class B Output terminals and Class A Input terminals. The exact labeling may be different for each manufacturer, but for an input zone, there has to be 4 screw terminals; for a NAC output zone, there has to be 4 screw terminals; for each SLC, there has to be 4 terminals.

The control panel may have to be changed to be capable of handling Class A circuits. If so, all of the detectors, pull station, MCPs (Manual Call Points), horns, strobes, input modules, output modules, etc., have to be checked to make sure they are compatible with the panel. (Compatible means that the devices and panel actually work together, reliably. Check with Technical Support for the new panel's manufacturer to find out for sure.)

If any of the devices are not compatible with the panel, the incompatible devices have to be changed when the control panel is changed.

Conventional Initiating Device Circuit (IDC)

The IDC (Input Circuit) has all the Detectors, Pull Stations, Flow Switches, etc. To convert a Conventional Class B IDC to a Conventional Class A IDC, the end of line resistor has to be removed and wires have to connect the last device to the Class A Input inside the control panel. This is the "Redundant" part of the Class A wiring, the part of the circuit that connects devices to the panel even when a wire comes loose somewhere on the circuit.

The big consideration is that to be a true Class A circuit, this newly installed wire can't be run with any of the current Class B wiring for the circuit.

Conventional Notification Appliance Circuit (NAC)

The NAC (Output Circuit) has all the horns and strobes, or sometimes instead, it has speakers. To convert from a Conventional Class B NAC to a Conventional Class A NAC, everything that was done for the IDC has to be done.

But the NAC is a power supply circuit. The wires have to be much larger to carry the electrical current to the horns, strobes, or speakers. If the wire is too long, or too small, some of the horns and strobes won't work, or some of the speakers will be too quiet, especially during a utility power blackout.

To check to make sure, the added wire has to be included in the circuit, and the voltage loss calculations have to be done again on the circuit.

Usually, the original calculations are lost, so in reality, the whole NAC circuit should be rewired if it will be converted.

Of course, the whole synchronization system for the horns and strobes might not work, so more changes to the NAC power supplies and the horns and strobes may be required.

Addressable Signaling Line Circuit (SLC)

The SLC has addressable detectors, addressable input modules, and addressable output modules. If the SLC was wired as a Class B circuit, almost without exception, the SLC has T-taps. Class A circuits cannot have T-taps, so about the only way to remove all T-taps is to rewire the whole Addressable System.

New Wires

All three circuits, the IDC, the NAC, and the SLC, require that at least some new wiring is installed in the building. In many circumstances, lots of new wiring is required.

The building is finished, and the new wiring has to be installed behind the finished walls and the finished ceilings. Keep that cost in mind when considering upgrading a Class B system to a Class A system.

Test and Certify the Whole Fire Alarm System

Once the system is converted to Class A, the system has to be tested. Remember, people depend on the fire alarm system to warn them of a fire. Not being warned means possible injury or worse. To finish the job, you have to test the system, in detail, to make sure it will detect all fires, and warn everyone of fires.

To protect yourself, get someone in authority (the fire marshal) to agree with you that the newly converted system works.

Douglas Krantz
Life Safety
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