Replace the relay on your LiftMaster T model trolley commercial garage door operator with this LiftMaster 29-31229 24VDC SPDT C-relay from Garage Door Supply Company. SPDT (single pole, double throw) relays can be used to open or close a circuit. This No. 29-31229 relay for your standard-lift overhead door LiftMaster T series operator is 2 in. long, 1 1/16 in. wide and 1/16 in. high. Confirm compatibility by checking your owner's manual, or contact our in-house pros for help. We know when you need a part, you need it fast, so we specialize in quick turnarounds on all parts orders for commercial garage door openers. We're an authorized online LiftMaster retailer with lowest negotiated prices on all openers and authentic LiftMaster replacement parts.