Section 108.409 - Location and spacing of tubing in pneumatic fire detection system(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) No portion of the overhead is more than 3.6 meters (12 feet) from the nearest point of tubing;(2) Beams or girders extending below the ceiling or other obstructions do not detract from the effectiveness of the tubing; and(3) Damage to the tubing, is unlikely to occur if it is not protected.(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.(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.