You may be thinking of this:
760.121(B) Branch Circuit. The branch circuit supplying the fire
alarm equipment(s) shall supply no other loads. The location
of the branch-circuit overcurrent protective device
shall be permanently identified at the fire alarm control
unit. The circuit disconnecting means shall have red identification,
shall be accessible only to qualified personnel,
and shall be identified as ?FIRE ALARM CIRCUIT.? The
red identification shall not damage the overcurrent protective
devices or obscure the manufacturer?s markings. This
branch circuit shall not be supplied through ground-fault
circuit interrupters or arc-fault circuit interrupters.