The XPCR Coupler/Repeater is installed at the electrical panel. It contains active circuitry that couples and amplifies X10 signals from one phase of the 220V line to the other.
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Here is a copy of the instructions included with this device:

The X-10 Pro Smart Coupler/Repeater (XPCR) is designed to couple X-10 power line carrier signals between phases in single (split-phase) or three phase installations, while at the same time boosting the original signal and "repeating" it onto all phases.

Note: Installation must be carried out by qualified installers only. The main breaker must be turned off during installation and the coupler must be installed in a suitable workbox or equivalent enclosure. Installation must be carried out in accordance with all applicable codes and requirements, including, but not limited to, the National Electrical Code (NEC).

Installing the Coupler/Repeater
Turn off the power at the main breaker panel.
Install the coupler/repeater into a suitable workbox or equivalent enclosure using the two mounting holes.
Refer to the diagrams above and connect the black/red wires as shown (depending on how many phases you have), using wire nuts. Check that there are no bare wires protruding and cover the wire nuts with insulating tape if necessary.
Connect the white wire to the Neural strap in the breaker panel.
Check all wiring and connections, and turn on the main breaker.
Test for signal strength on all phases using signal transmitter and signal strength meter, XPTT and XPTR