Voltage drop issues in security systems can be avoided with some simple calculations leading to correct cable selection.
Theory
We power electric locks and other devices via sheathed copper cable. This cable has a known resistance per meter and the electric device will require a known current.
The resistance causes the voltage to drop as per Ohms law. Naturally the voltage drop will rise as the cable gets longer. The third factor for calculating the voltage drop is the current required. The formula is V=IR (Voltage = Current X Resistance).
Resistance and current values are usually stated in the equipment data sheets and cable data often also comes with a formula that is essentially Ohms law.
Formula and Calculation
Basically V(drop) = I X R
Where
I = Current required by the end device
R = Total resistance for the cable for the whole run
An example is provided below.
Cable distance = 100m X .03 ohms per meter = 3 ohms per 100m
Lock current draw = 1Amp,
V(drop) = 1Amp X 3 Ohms = 3V
A 3V drop from the 13.8V power supply would be unacceptable.
Solution
Usually as the copper conductor size increases the resistance will drop. So select a larger cable that will provide a voltage drop within operating voltage tolerances of the device at the end of the cable. Be warned that the resistances between cable manufacturers and types can vary. So it is best to check the cable data prior to using a new brand or type.
Cable is relatively cheap compared to labour, so it is a good practice to rough in a slightly larger cable than you think you will need. This will save a costly re-run if the voltage drop proves too much or the current requirement is changed. It also means you don’t have to worry about the V(drop) calculation on every run, just the long or high current ones.