Voltage Drop for Fire Alarm Systems

Fire Alarm Voltage Drop Calculations

All electrical conductors include a small amount of resistance.  This resistance increases if the length of the conductor increases or or the conductor size decreases.  Think of blowing air through a hose. If the hose diameter decreases and or the length increases it would be harder to blow through.  You can also think of freeway traffic as resistance.  The freeway is the conductor.  The wider the freeway, the faster and smoother you travel.

As electrical current flows through the conductor it will experience a decrease in voltage between the source (starting point) and at various points along the conductor path.  Another example to look at is the voltage drop in a 1000 foot run of 16 AWG wire would be greater than that of a 1000 foot run of 12 AWG.  This is simply because a 16 AWG conductor is smaller in diameter than a 12 AWG conductor.

Fire alarm equipment LISTED to the standards of the National Fire Protection Association and Underwriters Laboratories (U.L.) is tested to determine if can operate properly at 85% of the rated nameplate voltage.  This limit was set in place to make sure the circuit can deal with a "brownout" condition or a possible voltage drop which might result from excessive resistance in the system wiring.

As required in the CFC (California Fire Code), fire alarm designers are required to prepare voltage drop calculations for the notification appliance circuits (NAC) as part of the design.  These voltage drop calculations must be included in the submittal plans and specifications.  This is to assure that the devices on the system are supplied with electrical power within the operating voltage range.

You as a designer can use several different methods to calculate voltage drop on a fire alarm circuit. One method calculates the actual voltage drop for each length of cable and device within the circuit and the other calculates the overall voltage drop.  Either method will have slightly different results but should be acceptable by your local AHJ (authority having jurisdiction)

The suggested maximum allowable voltage drop on a fire alarm circuit is 10% or the voltage drop included in the fire alarm control panel installation guide, whichever is less.

"Lump Sum Method"

Step #1)  Take the total current of the circuit.  You can achieve this figure by adding up the current draw of each device on the circuit.  This will represent "A"

Step #2)  Measure out the length of the circuit in feet.  Do not double the distance of the circuit for 2 wire loops unless you want to use a multiplying factor of 10.8 versus 21.6 (see step #3).  This will represent "L"

Step #3)  Use a multiplying factor of 21.6.  This number represents the resistivity of copper conductors.  This is a constant used in the formula.

Step #4)  Find the Circular Mils for the particular gauge wire you are using.  This can be found in the National Electrical Code (NEC) chapter 9 table 8.  #14 AWG is 4110 and #12 AWG is 6530.  This will represent "C.M".

A x L x 21.6
-------------  =   VD
C.M.

Example

.356 x 450' x 21.6
-------------------   =   0.530 Volts Dropped
6530

To find the percentage of voltage dropped do the following:
0.530 / 24 = 0.022
0.022 x 100 = 2.2
= 2.2% Voltage Drop

Now remember you can also perform this calculation for each individual length of wire and device on the circuit.  This is known as the "point-to-point" method.  This is a better way to perform the calculation as it gives you a chance to really break down the circuit and pin point exactly where a circuit must end do to voltage drop.  Simply use the above formula for each wire run and add the voltage drop totals for each circuit section together for the total voltage drop.  Then divide by the source voltage (in this example we will use 24VDC) and then multiply by 100 to come to a total voltage drop percentage.

I will be adding info in the near future on the calculations for the "point-to-point" method using Ohm's Law