My Voltage Drop Problem
Written by Shawn Lee
Occasionally I suffer from a mild form of Obsessive Compulsive Behavior (OCB). It generally has something to do with the way I use or process information relating to fire protection– or when I am attempting to repair an electrical system. Both situations usually produce the same reaction when they don’t go as planned. In the case of repairing an electrical system, everything is “supposed” to work the first time and everything is “supposed” to fit exactly as it should; however, many times it doesn’t. When it comes to fire protection subjects, everything is “supposed” to be standardized and make sense, but again, that is not always the case. Let me tell you about my latest issue.
__
A few days ago, I was updating one of my fire alarm system presentations. When I got to the section that explains how to perform voltage drop calculations, I ran into a little difficulty. It wasn’t because I didn’t know how to do the calculations or that the calculations were overly difficult. I just wanted to make sure I provided solid information that would help future students. However, as I finished my slides, with the two calculations I use when I do voltage drop calculations (point-to-point and a version of lump sum), I decided that it would be helpful to everyone (including myself) if I added one more formula to give everyone as much information as possible.
I did a little research to make sure I had the correct formula. I inserted a few more slides into my presentation for this third formula. I then took some time to do a few calculations using all the same wire size, amperages, and wire distances. The example circuits were run with #14 AWG solid copper conductors with three horn/strobe appliances– each with a current draw of 0.75 amps, 1.05 amps, and 0.75 amps respectively. The wire distances starting at the control panel to each appliance were 25 feet, 50 feet, and another 50 feet. I ran the numbers for each formula and got three different results!
Now, I’ve been using the point-to-point method and “my” lump sum method for well over a decade. I am used to having two different results from the two formulas I use. I’ve never liked it, but I understand why it is that way, and I’ve learned to ignore my OCB on that subject. Therefore, I was not surprised that I had different results between “my” two formulas. On the other hand, I didn’t realize there would be almost a half-volt variance between the two different versions of the lump sum voltage drop formula. Now, to be fair, all three formulas (there are more out there…I found ‘em) are valid formulas for figuring out voltage drop on a fire alarm circuit, and other circuits for that matter. I am not advocating for one formula or the other, as I do have a favorite, but I am advocating for something….
This might be opening a can of worms, but I believe that we should all work from one voltage drop calculation for fire alarm systems. I have three reasons why I think this is important.
1) Using a single voltage drop calculation means we are all on the same page. If we must review the system documentation years after installation, we would all have the same understanding of circuit operation. If we look at an as-built of a fire alarm system and it shows a specific number of notification appliances, let’s say 20, we are all on the same page. Twenty appliances are 20 appliances…period. However, with the numerous ways we can do voltage drop calculations, and the different ending voltages we can come up with, may cause us to not be on the exact same page. I can’t speak for others, but that leads me to question what we can expect from the circuits during the worst-case scenario.
2) The whole idea of the fire alarm and signaling code, or any code for that matter, is to standardize the requirements so that it doesn’t matter who is designing, installing, or maintaining the systems. The performance-based design calculations located in NFPA 72 2022 (Annex B) will provide the same answers to anyone– so long as the designers are using the same information. The formulas do not change based upon the folks using them. Voltage drop calculations do not necessarily follow the same logic. As I stated earlier, even with the same information, I still got three different voltage drops for the same circuit.
3) Past editions of the fire alarm code, state we must perform voltage drop calculations; however, it doesn’t state any specific formula that must be used. Why would someone do that?
Now, I can imagine there may be some folks that would suggest that I just pick one and move on with life. The truth is, I am doing just that (more or less). I can continue to do it and adjust to my AHJs as needed. I freely admit that I have never run into an AHJ that mandated one voltage drop formula over another. Therefore, I might be creating a problem that generally doesn’t even exist for anyone; however, I still think my point is valid. Using one universal formula could fit the purpose of the fire alarm code and standardize everything we do so that we all work from the same page. This is especially important when we review a system that may be decades old and the original installer is either no longer in business or we are unable to contact anyone who either designed or worked on the system.
Having a single standardized voltage drop formula is probably not in cards for the time being, so I will continue to teach the three calculations that I have in my presentation. Why wouldn’t I just pick my favorite and eliminate the others you ask? I considered doing just that, but then decided against it because I strive to provide as much information and training to my students as I possibly can, there may be students who are in a jurisdiction where one calculation IS the standard. Or perhaps their supervisor or employer dictates a specific voltage drop formula. I would rather give them the different options and hopefully those options will help them in their day-to-day jobs. It is important to provide the best fire alarm education and training I possibly can, even if that means teaching three different formulas. So, until the day we have one standardized voltage drop calculation, if ever, I will give as much information as I can to train others. I’d rather do that than take a chance that I am not providing the training they need.
But seriously, we need one formula.
_
