Powering Thermostats Without a C-Wire (ADC-T3000 / ADC-T2000)

Most thermostats can run on batteries, but hardwiring is preferred for reliability. If your system lacks a C-wire, here are safe options and requirements for Alarm.com Smart Thermostats.

Caution: Thermostat wire labels can vary by brand/model. Always verify with the HVAC and thermostat manuals before wiring.

Power Requirements

Thermostat	Voltage	Max Current
Alarm.com Smart Thermostat (ADC-T3000)	24 VAC	Up to 1 A
Alarm.com Smart Thermostat (ADC-T2000)	24 VAC	Up to 1 A

Note: For other thermostat models, confirm voltage and current limits in the manufacturer’s manual.

Typical C-Wire Connections

On 24 VAC systems, one transformer lead lands on R/Rh/Rc and the other on C. Verify the wiring diagram for your specific thermostat and air handler/furnace.

Option: Standalone 24 VAC Transformer (No C-Wire)

The ADC-T3000/T2000 can be powered by an external 24 VAC transformer separate from the HVAC, with limitations:

Wire transformer leads to R (Rh or Rc, whichever is unused) and C on the thermostat.
Remove any Rh–Rc jumper if present and follow the thermostat’s configuration prompts.
Limitation: This setup supports heat-only or cool-only control (not both) when using a standalone transformer for power.

Best Practices

Confirm transformer output is 24 VAC and adequately rated (≥ the thermostat’s current requirement).
Secure low-voltage cable runs and strain-relief at the thermostat and transformer.
If adding a C-wire kit or fishing new cable, power down the HVAC at the service switch/breaker first.
After wiring, restore power and complete thermostat setup; verify mode and fan operation.

When to call a pro: If your system has multi-stage heat/cool, heat pump with aux/emer heat, or shared transformers, professional wiring is strongly recommended to avoid equipment damage.

Need Help?

True Protection can assess your wiring, provide options (C-wire add-a-wire kit, transformer, or new cable), and complete a safe install.