You should always use shielded cable of the type appropriate for fire detection installations. Please note that cable types are also regulated by local authorities/ standards.
As a rule of thumb you should use:

a) 0,8 mm² for loop lengths up to 500 m
b) 1.0 mm² for loop lengths up to 800 m
c) 1.5 mm² for loop lengths up to 1200m
d) 2.5 mm² for loop lengths up to 1500 m

The lengths specified include return to the main panel.