In recent columns the past few months we’ve discussed the issue of critical calculations often required by fire authorities. NFPA 72 in particular specifies that calculations be made prior to the installation of a new fire alarm system. In a word, these calculations are a requirement, not an option.
For example, in January we covered battery life and how battery size is calculated in order to assure 24 hours of standby operation and a subsequent ring time of either five (evacuation-type) or 15 minutes (voice evacuation alarm communications [EVAC] system).
This month, we’ll discuss voltage drop calculations, especially where it involves notification appliance circuits (NACs). In each instance, we’ll show you how to do the math.
Calculations and Submittal Process
In May’s “Fire Side Chat,” we discussed the submittal procedures used by fire alarm companies when planning a new system in a commercial building where the Use Group calls for a fire alarm of some kind. Part of that procedure, as you will recall, requires battery and voltage drop calculations.
Voltage drop is extremely critical to the proper operation of all electronic and electrical devices attached to a fire alarm system. Nowhere is this as true as with NACs that connect to a fire alarm control panel (FACP).
This is especially important when working in exceptionally large buildings that can require thousands of feet of wire. Knowing what the voltage drop is at the end of a NAC is essential.
The code reference with regard to voltage drop calculations can be found in Section 10.18.1.2, NFPA 72, 2010 Edition.
Assemble Necessary Information
Before you can calculate a single voltage drop anywhere in a field NAC, you must gather all the right information. In the next section, we’ll discuss the steps you need to take in order to make the right calculations.
Begin by measuring the amount of wire you need on each NAC. In practice you’ll have to know the maximum allowable current per in-panel NAC, as well as the current drawn by each NAC device in order to perform the necessary voltage drop calculations.
Most modern-day alarm control panels provide 3 amps of operating current per NAC on alarm. This will vary with the make and model of the panel, so be sure to check the specifications of the equipment you’re using.
Visual NAC devices usually draw in the area of 0.058A and combination audio/visual models typically draw 0.085A. Specification sheets will also provide information on the minimum voltage necessary for proper operation, which is usually 16VDC when working with a 24VDC system.
Remember, just as the NACs above, you need to double check the specifications of your NAC devices so you have the right numbers to plug in to the calculations.
The last element you’re going to need is a reliable conductor properties chart, like the one that can be found in Ugly’s Electrical References book.
7 Steps to Calculate Voltage Drop
In this article we’re going to do the math by hand using a step-by-step process. For example, let’s say we have a 300-foot NAC with 10 Model GE3-24 visible-audible signaling appliances made by Gentex.
Step 1: Find the total per unit current drawn by each device on the circuit. According to the specification sheet, each strobe draws 60mA (0.060A) and each horn (Temporal 3, high setting) draws 28mA (0.028A). Adding the two together we have a total of 88mA (0.088A) per unit.
Step 2: Total the current drawn by each device on the NAC to find total current. In this case, 10 units on a single NAC gives us a total current draw of 10 X 0.088A = 0.88A. Ideally we want 10 percent above this amount for headroom, or 0.88A X 0.10 = 0.088A + 0.88A = 0.968A total current.
Step 3: Determine the to-and-from distance of the circuit. In this case, we have a 300-foot run, or 300 feet X 2 (conductors) = 600 feet total.
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