I have a number of battery powered, low-power consumption zwave sensors installed around the house, such as contact, PIR or temperature sensors. These are most running on 3v (CR123 or 2 AAA) batteries and takes very little power ( one set of battery lasts for 3month to 1 year.. so maybe < 10ma? )
I am thinking of modifying some of them, to put hole in the battery cover so I can wire it to a 12v wall wart with a buck converter outputting 3V near the sensor. The wire may or may not go through attic/drywall/exterior wall, but the wall wart / buck converter will be accessible, and there will be connectors and fuses to break the circuit.
I read about the regulations and safety concerns, and learned that for low-voltage wiring I would want CL2 for in-wall use, and I should worry about voltage drop for longer runs ( which the buck converter should mitigate). It seems like NEC does not specify too much in the class 2 area?
What am I missing? Is there other related NEC for this kind of setup? What are the regulatory and safety concern should I worry about in this setup? Is this a viable plan?
Best Answer
Low voltage wiring to devices that draw less than 55 watts has greatly relaxed rules, and is covered under a different section of (US) National Electrical Code than mains wiring.
You can see the effect of those rules in how thermostat and doorbell wiring is done -- both of which are subject to Code.
Here, the very fact they can get 3 months runtime on two AAAs implies you have exceptionally low draw devices (in the microamps) that they are not using any more power than you might find on telephone or Ethernet cables (disregarding PoE). So telephone style wiring is appropriate.
Of course, telephones have current limiting resistors at the far end of the circuit, even if you're right next to the central office, the telco won't give you enough current for a short to start a fire.
You should do the same: have current limiting resistors on the "head end" (power supply end) to remove the possibility of significant current flowing, e.g. A resistor calibrated so the devices normally receive sufficient current, but if the wires are dead shorted, only a few milliamps flows - would be about right. So you might consider a 3V supply, skip the buck converter, and put a rightsize resistor on the head-end.
E=IR
E=3V, I=10ma (30mw not likely to ignite drywall paper), R=300 ohm. Assuming your device normally draws <<1ma, it will see <<0.3V drop, you'd have to give it some trial and error to make sure the device isn't browning out at times of key activity such as transmitting, to find the right value of R.