Understanding LSP's FlexCalculator Suite - Part 1: Voltage Drop

LifeSafety Power's FlexCalculator Suite provides the integrator a bevy of Microsoft Excel-based tools to help plan and install access control, fire, or other life safety systems.  It is available under the Calculators page on our website.  In this post, I will show you how to use the Voltage Drop calculator page of the suite.  The Voltage Drop calculator will tell you the voltage at the end of a wire run, based on the starting voltage, load current, and wire gauge.  It was designed for solid wire, but will give reasonably accurate results for stranded wire as well.

Once the suite is downloaded, open the file.  A main menu will appear with buttons for each of the calculator pages.  For this exercise, click the "Voltage Drop" button.  The Voltage Drop Calculator page should open.


General Information

To begin, enter the information required in the General Information section.  

Start Voltage - This is the voltage of your power supply.  Enter either the nominal (12V or 24V) rating, or the actual measured voltage.  For this example, we will use 25.0, which is the measured voltage of an FPO power supply set for 24V.

Temperature - This is the average temperature of the wire run in degrees Fahrenheit.  Typically, this can be left at 75 degrees, but if the wire run is in extreme conditions, this value can be changed for more accurate results.  For this example, we will leave this at 75 Degrees.

Allowable Drop - Enter the percentage of voltage drop allowed from the Start Voltage entered above.  This value is determined by the minimum operating voltage of the device being powered at the end of the wire run.  For this example, we will use a 24V lock which will operate down to 19.2V:

(Start Voltage-End Voltage)*100/Start Voltage=% Drop
(25-19.2)*100/25=% Drop

Low Battery - This is a calculated field giving the low battery voltage based on the Start Voltage.   This value cannot be changed.  In this example, a discharged 24V battery set will be 20.4V.

Wire Run Information

Next, the Wire Run Information must be entered.  

Wire Length (One Way) - This is the distance between the power supply and the device being powered in Feet.  Only enter the one way distance - do not double the distance to account for the return wiring.  For this example, we will use 100 feet.

Wire Gauge - This is the AWG of the wire.  For this example, the wire run is 22AWG.

Current Through Wire Run - This is the total current draw of the load at the end of the wire run in Amps.  For this example, our lock is drawing 300mA, so 0.3A is entered.


The results appear in the Results section.  To get the results, click outside of the last field you entered information into, or click the "Calculate" button.  

Wire Resistance - This is the calculated wire resistance in ohms, based on the information given.  Our example information gives approximately 3.3 ohms.

Voltage Drop - This is the calculated voltage drop under normal conditions.  Our example gives 0.98V of drop.

End Voltage - This is the voltage at the end of the wire run under normal conditions.  In our example, 24.02V.

% Drop - this is the actual voltage drop over the wire run.  In our example, the drop is 3.9%.  This field will be green if the value is below the Allowable Drop value.  If the % Drop field is greater than the Allowable Drop field, this field will turn red.

End Voltage at Low Battery - this is the actual voltage at low battery.  In our example, this value is 19.42V - just above the 19.2V minimum our lock will operate at.  If you are not using batteries or do not care about the operation of the device while on battery backup, this field can be ignored.

This field accounts for a loss of AC, where the battery has fully discharged to the "Low Battery" voltage in the General Information section.  This is important when using batteries in a system - your 24 hour standby time will be greatly reduced if your devices stop working at a 23V battery voltage because of voltage drop.

To illustrate this, we will increase our wire run to 500 feet.

When we do this, the End Voltage under normal conditions is still 20.08V, which is still enough to power the lock.  However, our End Voltage at Low Battery is now 15.48V - well under the minimum voltage of the lock.  In fact, in this situation the voltage at the lock would be below the minimum at a battery voltage of 24.1V - Your 24 hour standby has been reduced to just a couple of hours.

We hope this has given you a better insight into the usage of the Voltage Drop Calculator.  The next part of this series will cover the Wire Size calculator, which calculates the required wire size given a wire length and current.  And as always, if you need any assistance our Technical Support  department is always here to help.