Low Voltage In Walls with 22ga Wire - Charging

Question

I've tried to figure this out on my own, but it exceeds my ability.

I'm trying to replace my old DSC Security Screens with Android Tablets. My house was originally wired with 22ga solid core copper wire for the DSC security screens. I've also got a bunch of old spare 12V 1.5A power supplies and a couple of transformers to step the voltage down from 12V to 5V for USB. I'm unsure of the total length of the wire, but its probably approximately 20 meters. My plan is to put the 12V->5V transformer in the wall behind the tablet and mount the tablet. I'd use the 12V power supplies in the room where the 22ga wire terminates. The original DSC panels would use a max of 230mA through a single conductor.

The system looks like 120V->12V 1.5A (PSU)->Appox 20 meters of 22ga wire->12V to 5V 3A step down transformer ->USB->Phone/Tablet

I did try things out a little bit and measured 12.3V at the power supply and 11.2V at the 12V->5V transformer. This seems like a large amount of Vdrop, but the 12V->5V transformer is good down to 8V. I measured the amps going into the 12V->5V transformer and they were about 0.86 amps. The phone said it was getting 1600mA into the USB port.

I let an old device charge for about 30 minutes and the step down transformer only got a little warm. The wires were not warm as far as I could tell.

Is this safe/how much headroom do I have? I've tried finding out the max amount of amps I can push though 22ga wire, but every website says something different... If its not, safe, could I use a 24V PSU instead?

Answer 1

Based on everything you have described, this situation sounds like a reasonable approach. Once the phones are charged, they should draw less power too.

I would consider using a fuse on the output of each power supply, since 18W is enough to cause a problem if there's a dead short somewhere in the wall. A 1A fuse will probably work well.