Planning for Every Voltage-Drop Scenario: Calculation crunching, part 2

Shutterstock / Alona_S / Oleksandr Khoma
Shutterstock / Alona_S / Oleksandr Khoma
Published On
Sep 15, 2021

In my July column, I reviewed the reason and importance for performing battery calculations. In this article, I will continue the conversation and review voltage-drop calculations. There has been some confusion about what must be included in these calculations. NFPA 72 only requires performing voltage-drop calculations for notification appliance circuits, not for every circuit. However, it is a good idea to perform them for any other circuits that draw a lot of current when in alarm.

Performing accurate voltage-drop calculations can save a lot of aggravation later in the job. It will improve your confidence that the system will work when needed.

Common misinterpretation

A common misinterpretation is that you can only have a voltage drop of more than a few percentage points. That is not the case. Neither NFPA 72 nor the National Electrical Code has requirements for a voltage-drop percentage. NFPA 72 10.3.5 states that equipment must be designed to operate at 85% and 110% of nameplate primary and secondary voltages. The 85% could be considered a 15% voltage drop. The bottom line is that no matter the percentage, the system still has to work when needed.

The important thing is to ensure that under worst-case conditions, all the horns and strobes will still operate. All you have to verify is that the voltage at the end of a notification appliance circuit is within its listed operating range.

Three calculation methods

There are three methods for performing voltage-drop calculations. One is load averaging, which assumes the entire load is in the middle of the circuit for calculation purposes. I feel this is the least accurate method and wouldn’t personally use it.

The second is the point-to-point method. In this method, you will know what the voltage is at each device. Unless you need this information for a particular reason, it is a lot of unnecessary work for little benefit. If you suspect your circuit may be overloaded and need to see how many horns or strobes you need to remove, then this is a good method for you. Otherwise, who cares what the voltage is at each device? You only need to know what it is at the end of the circuit. If it is good there, everything in the circuit will work.

The preferred method would be end-of-line. It is the most conservative, and, for calculation purposes, you assume the entire load is at the end of the circuit. To perform this calculation, you need to know the length of the circuit, the size of the conductors to be used, the number of appliances to be connected to the circuit and the amount of current each appliance draws. You will use Ohm’s Law to perform the calculation. Voltage drop (Vd) = conductor resistance (R) times current draw (I). You determine the conductor resistance from Table 8 in Chapter 9 of the NEC . Remember that if you have a 500-foot circuit length, you have 1,000 feet of wire, so always be sure to double the distance to determine wire length. So if you have 1,000 feet of 14-gauge wire, the circuit resistance will be 3.07 ohms. If you have 10 horn and strobes, and each draws 100 milliamps each, you have a total load of 1A. Remember that you always use amps, not milliamps (explained in July’s column, “Calculation Crunching”). If you multiply 3.07 times 1A, your voltage drop will be 3.07V.

Next you will subtract the 3.07V from the source voltage. Not everyone agrees on what that level is, but I am a big believer in calculating for the worst-case scenario (and hoping for the best). So if you have a 24V circuit, base the source voltage on 85% of that number (remember 10.3.5?), which will be 20.4V. Subtract 3.07V from 20.4V, giving you 17.3V. If the listed operating voltage for your appliances is less than 17.3V, this circuit will work when needed.

I have found that these calculations are quite accurate. A simple way to verify that in the field is to turn the primary power off so the system is only operating on battery and use a voltmeter to read the voltage at the end-of-line device and subtract that voltage from 20.4V. That will tell you if it will work under worst-case conditions.

That is all there is to it. It is much better to perform these calculations before installing the conductors and appliances, because if you are wrong, it can be a lot of work to fix it then.

About the Author
Tom Hammerberg

Thomas P. Hammerberg

Life Safety Columnist

Thomas P. Hammerberg, SET, CFPS is an independent fire alarm presenter and consultant in The Villages, Fla. He can be reached at TomHammerberg@gmail.com.

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