Most 12-speed Di2 installations use a simple three-wire setup: one SD300 wire runs from each shifter lever down to a junction inside the frame, then a single wire continues from that junction to the seatpost battery. The EW-JC304 is the junction box that makes this configuration work. It provides four SD300 ports arranged in a compact inline housing designed to sit inside your frame's downtube or bottom bracket area where multiple wire paths converge.

The four ports give you flexibility in how you route your system. In the common three-wire setup, both shifter wires plug into one end while the battery wire plugs into the other end, leaving one port empty. That empty port needs a dummy plug, which Shimano includes with the junction box. All unused SD300 ports in the E-Tube system require dummy plugs to maintain proper sealing and electrical continuity, so don't skip this step during installation.

You can also use the EW-JC304 to extend wire lengths by connecting two pairs of SD300 wires through the junction, essentially splicing four wires together while maintaining full power and data transfer. This comes in handy on larger frames or unusual routing situations where standard wire lengths fall short. The junction box itself is small enough to fit through most internal routing ports and tuck away cleanly inside the frame.... Read More