On January 20, 2019 at 00:14, tca said...
We constantly run into situations where we only have 1 cat5 cable at a gate and need to provide power for a door strike, keypad, and request to exit button. Most situations we need to use 12v dc to accommodate all 3 things. Runs can be anywhere for 100 feet to 300 feet. If we can run new wires, we do, but I want to understand voltage drop.
12vdc 1 amp power supply should be enough to power a keypad (typically draws not more than 200ma), a strike (typically draws no more than 300ma), and a request to exit (typically draws no more than 100ma for led light).
I've seen all the voltage drop calculators, but I don't understand fully. I know the voltage drops, but what about the amps? Can you install a 13 or 14 VDC power supply to make up for the drop? Does that also help with the amps?
Typically we would quadruple up the pairs of a cat5 and connect to a 12vdc 1 amp power supply. As an example, if we do this for a run of around 200 feet, and connect it to the strike, keypad and request to exit button using the above current draws, it may work, but is it mathematically appropriate? If not, what would we need to do to make it correct when we can't run new wires and only have 1 cat5 cable, and no local power?
Thanks for helping me to understand.
Here's a chart for wire gauge, with current limits. It shows the resistance for 24ga wire at about 25 Ohms/1000 feet, so you would cut that to ~25% by combining the wires and further to 20% of that number if it's 200', so it would be around 1.2 Ohms. You would be better off using a regulated power supply rated for more current than trying to send higher voltage.
[Link: powerstream.com]The thing that's creating/being a load will cause heat- if the wire has no resistance, it will cause no heat and therefore, no voltage drop. Also, if the temperature of a conductor increases, the resistance increases, too. It's counter-intuitive, but that also increases the current if the power is to remain constant. It's what causes a weak car battery to kill the starter.
