For round ducts only! The SID (Slip-In-Damper) is a user-friendly retrofit for round dampers. With a single cut the SID can be installed in seconds. Due to it's 1.5VA actuator up to 20 SIDs can be controlled from a single EWC Controls zone panel with a 40VA transformer. The optional S1A end switch allows for increased flexibility. Available sizes are 4" - 5"-6" - 7" - 8" - 9" - 10". The SID (Slip-In-Damper) is a user-friendly retrofit damper. With a single cut the SID can be installed in seconds. Due to it's 1.5VA actuator up to 20 SIDs can be controlled from a single EWC Controls zone panel with a 40VA transformer. The optional S1A end switch allows for increased flexibility. Available sizes are 4" - 5"-6" - 7" - 8" - 9" - 10 For more information go to: http://www.ewccontrols.com/acrobat/090377a0118.pdf