Greetings Douglas,

I have a question regarding the necessity of voltage-drop calculation for NAC in class A circuit. My questions are as follows:

Thank You,**RA**

I have a question regarding the necessity of voltage-drop calculation for NAC in class A circuit. My questions are as follows:

- Is it allowed to connect conventional sounders in class A?

- If yes, is voltage-drop calculation is required? Why?

- In case of addressable sounder, is voltage-drop calculation is required?

Thank You,

In order to properly design a fire alarm system, at least from a technical point of view, we have to figure out how to accomplish the requirements in the rules, codes, and laws.

In other words, instead of going at this from an attitude of "The rules, codes, and laws say I have to follow their guidelines, so I'll do as they say." Designers and technicians should have the attitude of "Whatever I do, the bottom line is that the fire alarm system has to detect fires and warn the occupants of the building."

If you don't have access to the worksheets, you have to do your own voltage-drop calculations. Even if the worksheet is available, I suggest you perform the calculations once or twice yourself, just to see the big voltage-drop issues.

When designing a Notification Appliance Circuit (NAC), we have to look at whether or not the horns or strobes receive enough voltage to even work. This is true of the NAC circuit we normally think of, and this is also true of a power circuit to operate the horns and strobes.

Even though voltage-drop calculations aren't normally applied to the door holder circuits, the door holder circuits require a lot of power to operate. Performing the calculations on these circuits is something that should be done.

The calculations take into account the voltage available on the power supply, the wire, and the total amount of current that is used to power the horns or strobes. The voltage-drop calculations use Ohm's Law, and after that, some simple math.

You can find this voltage in the panel's installation manual, or you can check with the technical support team for the manufacturer to find out what this lower voltage is going to be.

The length of the wire will be the total length of the wire, including length as it takes longer routes through the building, and the length as it drops down and returns for each horn or strobe.

Add up the total length of the wire in the circuit (remember, there are two wires in the cable, and the wires are in series). This gives you the total length you have to work with.

The resistance of the wire for that length can be found on one of the tables in the NEC (National Electrical Code). Each size of wire will have a different resistance. Use the length of the wire and the size of the wire to find the resistance of the wire.

You can find the current amounts for the horns or strobes in the installation sheets that come inside the box for each device. The installation sheets show one or several current values for each candela setting for the strobe, or each sound level setting for the horn.

Use the highest current value for the candela level on the strobe, or sound level on the horn.

Once you've added up the currents being used for all the horns or strobes on the circuit, that is the current that is going to pass through the wires in the circuit.

This is the voltage-drop from the wires in the NAC circuit.

If the voltage going to the horns or strobes is less than the minimum voltage shown in the installation sheet for the device, some of the devices may not work if the power to the building is blacked out for 24 hours.

Performed to give an idea of whether the circuit will provide power to the devices, these voltage-drop calculations are really guesses. If the calculations show that there isn't enough voltage to power all the horns or strobes - change something. Do one or more of the following:

- Reduce the number of horns or strobes on the circuit in order to reduce the total current needed, or

- Reduce the length of the wires in order to reduce the wire's resistance, or

- Increase the size of the wire in order to reduce the wire's resistance

After reducing the current or the resistance, perform the calculations again.

The whole idea behind the voltage-drop calculations is to show you, the technician, the installer, the designer of a fire alarm system that all of the occupants of the building will be warned if there's a fire.

If it's not compatible, it won't work. That's why the NFPA Code requires that the devices be compatible.

Look up in the installation manual for the fire alarm control panel to find out what devices are compatible with the control panel. You can download an installation manual from the web, or contact the technical support team for the fire alarm system manufacturer to get a manual.

The circuit can be an addressable circuit or a conventional circuit. The circuit can be made up of wires, made up of fiber optics, or other signal carrying methods.

See more information on circuit classification at:

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