nest Thermostat E Guide D'installation page 7

Incompatible wiring and voltage
Stranded wires
Stranded wires aren't compatible with the
Nest Thermostat.
High-voltage wires
If your thermostat is labeled 120V or 240V,
or has thick wires with wire nuts, or has
L1 and L2 terminal blocks, your system
is high voltage and isn't compatible with
the Nest Thermostat.