Douglas Krantz - Technical Writer - Describing How It Works
Book: Make It Work - Conventional Fire Alarms

Do We have to Provide Voltage Drop Calculations?

By Douglas Krantz | Life-Safety | 4 minutes of reading

Do We have to Provide Voltage Drop Calculations?


Do We have to Provide Voltage Drop Calculations?


Greetings Douglas,

In the case of addressable sounders, is it required to provide voltage drop calculation? If yes, why?

Thank You, S E

Voltage drop calculations, showing that enough voltage will be available to all the sounders and strobes, are your assurance that the sounders and strobes will work when there's a fire. Any circuit that the calculations haven't been performed on might not work to the last device.

NAC or Addressable Power Circuit

Fire alarm systems have to work, even during a power outage. All wires have resistance, and all strobes and sounders (horns, chimes, etc.) use electrical current. Also, strobes and sounders only work if there's enough voltage.

Whether it's a conventional Notification Appliance Circuit (NAC), or a circuit that has addressable Notification Appliances (like addressable sounders, horns, chimes, strobes, etc.), the NAC circuit or power circuit is providing power to the devices.

Calculations

Ohm's Law is used for the voltage loss calculations.

E (Voltage in Volts) = I (Current in Amps) x R (Resistance in Ohms)
  • The wire has resistance. The resistance is higher for thinner wire, and the resistance is higher for longer lengths of wire. The total resistance on both wires, the wire carrying current out to the devices and the wire carrying current back from the devices, is the resistance.

  • Each sounder (horn, chime, etc.) and each strobe on the circuit uses current. The worst-case scenario for current used by the sounders or strobes is shown on the installation sheet for the sounders and strobes. The added-up, worst-case scenario current from all the sounders and strobes on the circuit is the current. Of course, another device might be added later, so add at least another 20% to the current, just to make sure.

Because the manufacturer never knows the wire size and the wire length you're going to use, and the manufacturer never knows how many sounders or strobes you are going to use, you are going to have to figure out the voltage drop yourself. Once you know the resistance of the wire (total for both directions) and the current that will go through the wire, you can use Ohm's Law to figure out the amount of voltage that will be dropped, or lost, in the circuit.

This lost voltage is subtracted from the panel's worst-case scenario voltage (check with the technical support team for what voltage to use). Use that subtracted voltage to confirm that resulting voltage will power all of the sounders and strobes.

Notification Appliances

It's important to remember that the Notification Appliances (the sounders like horns, chimes, etc. and the strobes) are what notify the occupants of the building about fire. Your job, when you design a circuit is to make sure that all of the Notification Appliances receive enough voltage to do the notifying.

Listing

As far as the requirements for voltage drop calculations go, the NFPA (National Fire Protection Association) says that all the Notification Appliances have to be listed-for-use in that fire alarm system. As part of their listing, all Notification Appliances like sounders and strobes show the minimum operating voltage. To meet the listing, in your calculations you have to show that you are going to provide at least the listed minimum voltage to each sounder (the horns, chimes, etc.) and each strobe in the building, even during a power outage.

That means, whether it's a NAC for a conventional Notification Appliance, or a power circuit for an addressable Notification Appliance, you have to calculate the voltage drop for each power circuit. For the Authority Having Jurisdiction (AHJ), make sure you provide the required CYA paperwork on each circuit.

Douglas Krantz

facpdoug@douglaskrantz.com
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