Can You Use Four Conductor Cables for Sounder Bases?

By Douglas Krantz | Maintenance

When using the Signaling Line Circuit (SLC) for smoke detectors that have low-frequency sounder bases, questions are asked if 4-conductor cable can be used for both the SLC and powering the sounder base.

Power Dependent

We'll start with powering the horns during a full-building fire. When there's a full-building fire, all the sounder bases in the units (apartments, condos, hotel rooms, etc.) sound off at once. The wiring has to carry enough electrical current to all of the low-frequency horns, at once.

These are low-frequency horns, about 520 Hz. They are power-dependent, not voltage dependent.

An incandescent light bulb is voltage dependent. As the voltage goes up, the current goes up and the light gets brighter; as the voltage goes down, the current goes down, and the light gets dimmer. Basically, the wattage goes up and down, following the voltage going up and down.

A low-frequency fire horn is power (wattage) dependent; a low frequency fire horn makes as much noise when the voltage used to power it is high as when the voltage is low.

Example: System Sensor B200S-LF Low Frequency Intelligent Sounder Base

That means they use roughly as much power at 33.0 volts (current of 70 mA times voltage of 33 V) as they use at 16.0 volts (current of 140 mA times voltage of 16 V).

Check the power specifications for the sounder bases. The current goes up as the voltage goes down. At the end of a 24-hour electrical blackout, the voltage at the end of the circuit still has to be has to be at least 16 volts. When using the high-volume setting on the sounder, the sounder will need 0.14 Amps (140 milli-amps).

Make It Work - Conventional Fire Alarms

Perform NAC Voltage Loss Calculations on Each Power Circuit

The power circuit is a Notification Appliance Circuit (NAC). The low-frequency horns are "Notification Appliances", and the appliances are attached to a "Circuit". Because it's a NAC, calculations need to be performed on each NAC to make sure the circuit will provide enough voltage at the end of the circuit.

As a rule of thumb calculation, don't even bother with using 18 AWG or 16 AWG wire. Start your calculations with 14 AWG wire.

You may be able to use the tables in the fire panel's installation manual to make the calculations, or even software that may be provided.

As an alternative, use the resistance calculations found in the back of the NEC Code Book, and MULTIPLY THE LENGTH OF THE WIRE BY TWO. That will give you your resistance of the wire to the end of the circuit, plus the resistance back from the end of the circuit.

To figure out how much current will be used when there's an alarm, start with 10 low-frequency sounder bases and add their current (at 16 volts) to find a total current.

Look in the installation manual for the fire alarm panel. The voltage to use for the NAC voltage loss calculations will be in there, somewhere. As an easy alternative, contact the technical support team for the sounder bases you're using. They'll be glad to give you this information.

You will probably have to use more circuits than you planned on using.

Signaling Line Circuit

Check the installation manual for the requirements for the Signaling Line Circuit (SLC). If the requirements don't specify to use shielded cable, don't use shielded cable. Shielding just increases the capacitance of the cable.

In general, there's not very much current running through the SLC wires, so larger wires don't really affect voltage loss on the cable.

Make It Work - Conventional Fire Alarms

On the other hand, larger wire, like 14 AWG, or especially 12 AWG wire, has much more capacitance in the cable than, say 18 AWG. Because of capacitance (and therefore capacitive lag), larger wire (16 AWG, 14 AWG, 12 AWG) means the cable can't be run as far as 18 AWG wire.

Keep in mind that the specifications for the SLC wire is not for distance from the panel. It's for the total amount of wire that is used on the SLC, including the wire used for any t-taps. Add all the distances up, that's what the panel is dealing with.

Because it's the SLC, if you can use the smaller wire rather than the larger wire, go with the smaller wire. Smaller wire, in the case of the SLC, is better than larger wire.