Understanding LSP's FlexCalculator Suite - Part 2: Wire Size

In the first part of our series, we learned about the Voltage Drop calculator in LifeSafety Power's FlexCalculator Suite which can be downloaded from the Calculator page on our website.  If you missed Part 1 of this series, you can find it: 

Part 1 - Voltage

In Part 2 we will cover the Wire Size calculator, which tells you the minimum wire size required based on starting voltage, wire length, and load current.  Again, 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 "Wire Size" button.  The Wire Size Calculator page will open.


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

Start Voltage - As in the Voltage Drop calculator, 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 12.5, which is the measured voltage of an FPO power supply set for 12V.

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.  As in the Voltage Drop calculator, 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 12V lock which will operate down to 10.1V:

(Start Voltage-End Voltage)*100/Start Voltage=% Drop
(12.5-10.1)*100/12.5=% 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 12V battery set will be 10.2V.

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 300 feet.

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 426mA, so 0.426A is entered.

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

Max Voltage Drop - This is the maximum allowed voltage drop for the device to operate based on the entered parameters.  Our example gives 2.4V of maximum allowed drop (19.2% of 12.5V).

Max Wire Resistance - This is the maximum allowed wire resistance, calculated using Ohm's Law.  In our example this result is 5.63 ohms (2.4V/0.426A).

Minimum Wire Gauge Req. - This is the minimum wire size in AWG necessary to meet the requirements.  Our example gives a result of 19 AWG.  Since this is a non-standard wire size, round this to 18 AWG.

Note that a negative result in this field indicates a wire size larger than 0 AWG.  A minimum wire size of -1 converts to 00 AWG, a result of -2 converts to 000 AWG, etc.

Actual Wire Resistance - This is the actual wire resistance of the run using the calculated minimum wire gauge.  Our example scenario shows that a 300 foot, 19 AWG solid wire run is about 4.90 ohms.

End Voltage - This is the voltage at the end of the wire run under normal conditions.  In our example, 10.41V.

End Voltage at Low Battery - this is the actual voltage at low battery.  In our example, this value is 8.1V - which is below the 10.1V 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 shown 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 an 11V battery voltage because of voltage drop in the wire run.

If battery standby is required, use the Minimum Required AWG result as a starting point in the Voltage Drop calculator to determine the wire size required for operation at low battery.

The next part in this series will cover the Battery Size calculator, which calculates the minimum required battery size in AH with given standby and alarm times and currents.  As always, if you need assistance our Technical Support  department is always here to help.