The GTD20A works in conjunction with an auxiliary generator or central inverter system to power designated lighting loads up to 20 amps regardless of local switch position. The device consists of relay switching circuitry in a single, wall-mountable enclosure (T-bar mounting kit optional). The GTD20A senses the loss of normal power and, in response, switches the lighting load to a designated alternate power source. The device can be used to: (1) transfer a lighting load from normal power to generator or central inverter system power when normal power is lost; (2) bypass a wall switch to allow generator- or central inverter system-supplied lighting loads to energize when normal power is lost; (3) bypass a dimming panel and prevent back feed to allow lighting to energize at full brightness when normal power is lost; and (4) bypass most dimming controls using an auxiliary relay contact. The GTD20A includes three dry form C contacts, which allows the user a wide variety of wiring options. The GTD20A is suitable for use in indoor and damp locations.