Class A (Style 6) wires the circuit as a loop that returns to the panel. One break does not cut off any device.
Class B (Style 4) is a single run that ends in an EOL resistor. Devices past a break lose contact with the panel.
The class applies per circuit: detection circuits (IDC) and sounder circuits (NAC) can differ.
A fire alarm specification may say “Class A” or “Class B”. It may also say “Style 6” or “Style 4”. These terms describe how the wire runs from the panel to each detector. That routing decides what still works after one break in the line.
Conventional zoned panels such as FireSmart’s FCP and FSP use one of these patterns. Knowing the difference keeps a circuit working after a fault.
Both names describe the same idea in two code traditions. NFPA 72 uses “Class A” and “Class B” for initiating device circuits (IDC) and notification appliance circuits (NAC). The older UL 864 numbering uses “Style 6” for a Class A-equivalent circuit and “Style 4” for a Class B-equivalent circuit. Older documents and some markets still use the style numbers.
Both answer one question. If the wire is cut or shorted at one point, how much of the circuit keeps working?
Class B (Style 4)
Class A (Style 6)
Wire route
One run from the panel to the last device, ending in an EOL resistor
The wire returns to the panel a second time and forms a loop
After one break
Devices past the break lose contact with the panel until repair
Each device is reachable from two directions, so the panel can usually still reach all of them
Panel report
Flags an open or short on the zone wire
Reports the break as a trouble condition
Cable
One run out
Roughly double the cable of the same Class B run
Typical use
Small zones, such as a few devices in one room
Life-safety circuits in larger, higher-occupancy or higher-risk buildings
Class B / Style 4 — a single run, supervised end to end
A Class B circuit runs from the panel to the last device and stops there. An end-of-line (EOL) resistor lets the panel supervise the wire for an open or short.
FireSmart’s conventional zone panels, FCP and FSP, use this pattern. Each zone is a supervised run that ends at the last device. It is not a full loop back to the panel.
The panel flags a break or short anywhere on the zone wire. Devices past the break stay cut off until the fault is repaired.
NOTE
A zone that spans a long corridor or several floors carries more risk. One accidental cut can silence every detector downstream. Renovation work, rodent damage or a drill through a wall can cause it.
Class A / Style 6 — a loop that survives a single break
A Class A circuit serves the same string of devices. The wire returns to the panel a second time, so it forms a loop and not a dead-end run. Each device is reachable from two directions.
If the loop is cut at one point, the panel can usually still reach every device from the other end. It reports the break as a trouble condition and keeps coverage.
Specifiers choose this wiring for life-safety circuits in larger, higher-occupancy or higher-risk buildings. Losing a whole zone to one fault is not acceptable there. The tradeoff is cable. A Class A loop uses roughly double the cable of the same Class B run. The wire goes out and back.
What this means for a conventional zoned system
FCP and FSP supervise each zone for opens and shorts with EOL resistors. This is the Class B/Style 4 pattern, and the zone cards are built around it.
Wiring an installation as a true Class A loop depends on two things: the cabling layout and the zone card capability for that project. Do not assume it from a datasheet alone.
TIP
Need Class A/Style 6 survivability for a corridor, a stairwell or another critical run? Raise it with the manufacturer or system integrator at the design stage. Do it before the wire is in the wall.
Some projects need full loop redundancy across the whole system. An addressable architecture handles this differently. Each device reports individually over its own radio link. A single point of failure does not take a whole group of detectors offline, as a cut Class B wire can.
Does the class apply to detectors, sounders, or both?
“Class A wiring” is not one setting for the whole system. The classification applies per circuit, not per building.
Circuit
What it carries
Class
Initiating device circuit (IDC)
Signals from detectors and manual call points to the panel
Class A or Class B, set on its own
Notification appliance circuit (NAC)
Panel output to sounders, strobes and bells
Class A or Class B, set on its own
A project can mix classes if the applicable code and risk assessment allow it. For example, the detection circuits for a critical stairwell can be Class A while the sounder circuit on the same floor stays Class B.
NOTE
A drawing that says “Class A throughout” is not specific enough. A quote that does not separate IDC from NAC is also unclear. Ask before wiring begins. Adding a loop return after the conduit is closed costs far more than specifying it first.
Questions to ask a supplier before specifying wiring class
Get specific answers before a wiring class goes on a drawing. Do not assume a default.
Does the local fire code or the building’s occupancy classification require Class A/Style 6 for any circuits, or is it a design choice?
How many devices sit on each zone? What happens to the rest of the zone if the wire is cut at the far end?
Does the panel’s zone card support a genuine loop return, or only a single supervised run?
For a zone that needs loop redundancy, is the extra cable run in the budget? Is the extra conduit path back to the panel in the budget too?
Quick answers: Class A and Class B wiring
What is a Class A circuit?
A Class A circuit routes the wire from the panel through every device and back to the panel. It forms a loop. If the loop breaks at one point, the panel can still reach every device and reports the break as a trouble condition.
Class A is the NFPA 72 term. Style 6 is the older UL 864 style number for a Class A-equivalent circuit.
What happens on a Class B circuit if the wire is cut?
The panel flags the open or short through the end-of-line (EOL) resistor. Devices past the break are cut off from the panel until the fault is repaired. A single cut can silence every detector downstream of that point.
Are Class B and Style 4 the same thing?
Both describe the same single-run circuit supervised by an end-of-line resistor. Class B is the NFPA 72 term. Style 4 is the older UL 864 style number for a Class B-equivalent circuit.
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