The existing two-wire unit can be replaced by a Class 300 EOS: - building with one or two apartments: replacement without modification of the video intercom system wiring - multi-apartment building: depending on the characteristics of your system, replacing the indoor unit could be sufficient or it would be necessary to add the dedicated power supply ref. BT-350020. For the sizing of multi-apartment systems, see the General Installation Rules in the Technical Manual of the two-wire system