46 CFR §108.409

1. (a)All tubing in a pneumatic fire detection system must be on the overhead or within 300 millimeters (12 inches) of the overhead on a bulkhead in a location where—
   1. (1)No portion of the overhead is more than 3.6 meters (12 feet) from the nearest point of tubing;
   2. (2)Beams or girders extending below the ceiling or other obstructions do not detract from the effectiveness of the tubing; and
   3. (3)Damage to the tubing, is unlikely to occur if it is not protected.
2. (b)If tubing in a tubing circuit is installed in an enclosed space, at least 5% of the tubing in the circuit must be exposed in the space, except that at least 7.6 meters (25 feet) of tubing must always be exposed in the space.
3. (c)A pneumatic fire detection system must be set to activate after approximately a 22 °C. (40 °F.) per minute increase in temperature at the center of the circuit in the system.