A Letter from a local Fire Alarm authority concerning Voltage Drop

Lassen Wir Das Thema Voltage Fallen (let's voltage drop the subject).


As an apprentice, I read the National Fire Alarm Code only when I needed to look up bits and parts, a procedure here, or find a prescription there, like a cook book. I didn’t, and I suspect most beginners don’t browse the National Fire Alarm Code like a fishing catalog. I call this the JAW (Just Add Water) method. Such information is initially incomplete, and assumptions, inferences, and deductions thus drawn are imprecise and usually wrong.

The most common use of this document is to find expert to evidence to bolster a belief if not our ego. When that evidence is not found; it is the deliberate twisting of the intentions by manipulation of the language that causes confusion, not the document itself. On it’s face the document language is deliberately precise, relatively complete, and readily understood. That is to say, when we presume this thing or that as fact, and only then look to the document, we are using the standard the way a drunkard uses a lamppost, not for enlightenment, but for support.

Consider if you will a rejection notice from a city employee (note all references are from the 1996 edition NFC and remain unchanged in later editions)

‘Please revise and resubmit your voltage drop calculations. “Voltage drop calculations exceed 15% allowable line loss in accordance with NFPA 72, (National Fire Alarm Code) 1-5.5.1”.

The following 6 points leap to mind upon reading this statement;

1. Article 1-5.5.1 establishes a upper limit to a minimum operating parameter (voltage), it does not establish a maximum (voltage) loss parameter, and these two parameters are not reciprocal

2. The article 1-5.5.1 DOESN’T EVEN ALLUDE to voltage drop.

3. The number 15% is not enumerated in the article. In fact it article 1-5.5.1 has nothing to do with establishing a fixed value for voltage drop.

4. The NFC doesn’t have a need to (and thus would not) set a fixed value for voltage drop, for any device. That is done by the manufacturer. The NFC is intended to establish the required (minimum) levels of performance, not to establish the methods by which these requirements are to be achieved.

5. The NFC relies on the operating parameters set by the manufacturer and approved by the listing authority; and requires these are to be marked on the device itself..see 6-1.2)

6. Voltage drop specifications for circuits (if any) would be found in the National Electrical Code (NEC), as with all conductors. Not the NFC

So what made this person think that 1-5.5.1 specifies only a maximum 15% “Line Loss”? Well the NEC gives us a clue; it is a classic example of the third stage JAW method.

Voltage drop is quantified in three NFPA standards including the NEC (National Electrical Code), and where found; it is always expressed as a percentage.

None of those standards regulate or reference building fire alarm circuits.

Now here is what I think happens, follow my logic;

Being the AHJ and Upon reading section 907.1.1 (8) in the IBC 2000 edition I am in need of guidance (see below)

[F] SECTION 907

FIRE ALARM AND DETECTION SYSTEMS

907.1 General. This section covers the application, installation, performance and maintenance of fire alarm systems and their components.

907.1.1 Construction documents. Construction documents for fire alarm systems shall be submitted for review and approval prior to system installation. Construction documents shall include, but not be limited to, all of the following:

1. A floor plan.

2. Locations of alarm-initiating and notification

appliances.

3. Alarm control and trouble signaling equipment.

4. Annunciation.

5. Power connection.

6. Battery calculations.

7. Conductor type and sizes.

8. Voltage drop calculations.

Having heard the NEC limits the amount of voltage lost on branch circuits and feeders to the farthest outlet to 5% so I assume that Voltage drop is always express as a percentage (which is not true) and I look for the allowable percentage voltage drop for fire alarm circuits in the National Electrical Code. And I find nothing there not even in article 760 for fire alarm circuits. In the certain belief that if the building code requires voltage drop calculations there must be a standard that establishes their value I look to the National Fire Code in desperation and..“do de do dum de do… Ah Ha! “

“There it is!

the word voltage! right there in the title..

now what does that say .. yep! There’s the other word percentage that must be it, ..

but wait, ..oh this is tricky, it doesn’t actually say what the percentage of drop is.. it makes me cipher it. I’ll need my calculator, ..lets see 100% minus 85% is 15%, there! That must be it, the maximum allowed voltage drop is 15% of the name plate voltage.”

This is of course wrong

NFC 1-5.5.1 states..

“equipment shall be designed so that it is capable of performing its intended functions.. At 85 percent and at 110 percent of the nameplate primary (main) and secondary (standby) input voltage(s)”

That statement above, requires that any device connected to that circuit reliably operate even when the voltage supplied is a little higher or lower than the manufacturer expected, either on primary power or on (secondary) battery power; it does not say how much lower or higher. What it does say is; if you use a relay rated at 10 volts, it should be able to work even if the applied voltage is as little as 8.5 volts or as high as 11 volts. Now if your particular relay can operate on as little as 5 volts or as much as 20 volts, that’s good too, better even. Any way that’s still not voltage drop.

To understand we need to recognize what voltage drop is and then, why this is important;

Voltage drop is the difference in the voltage applied at one end and the voltage available on the other end. We need to know these values to be sure enough voltage remains on other end so that thing, whatever it is, will work the way it was designed even when operating on backup power. It is also important to note that 24 volt batteries often used as a secondary power source, are rated by their ability (Ah capacity) to support a given amount of current at a voltage of at least 20 volts for 20 hours. This means a tightly estimated system at the end of its designed standby time, could be operating at voltages as low as 20 volts when called upon to operate the appliance circuit. It is also important to note that when a manufacturer claims a minimum operating voltage of 20 volts the UL listing authority will require it to operate properly at voltages as low as 16 volts in order to be labeled with a rating of 20 volts.

The question then becomes a common sense one, and the NFC doesn’t attempt to regulate or standardize common sense (because it is so un-common).

The question should properly seeks to define the lowest voltage that must remain after voltage losses and that is of course simply the lowest (minimum operating) voltage designed by the manufacturer and certified by the listing authority to work!

e.g.

24 volts nominal primary supply – 20 volts minimum required supply = 4 volts maximum losses (16.6%)

And..

20 volts nominal secondary supply – 16 volts minimum operating supply = 4 volts maximum losses (20%)

Then Res Ipsa Loquitur: (The Thing Speaks For Itself)

Maximum Voltage drop = starting voltage - minimum operating voltage

It is only when the MINIMUM OPERATING VOLTAGE designed by the manufacturer (which can be ANY number smaller than .85 times the nameplate) is subtracted from the NAME PLATE VOLTAGE designed by the manufacturer that the maximum voltage drop is defined. And brother there aint no percentage in arguing that.

Can we voltage drop the subject now?

* footnote

When the minimum performance is defined as “capable of performing its intended functions at 85 percent and at 110 percent of the nameplate voltage”;

(NFPA1996 art. 1-5.5.1) it sets a minimum and maximum parameter. 85 percent is the maximum minimum parameter and 110 percent is the minimum maximum parameter