The 2nd generation Di2 e-Tube wiring system from Shimano has the SD300 designation. These wires and connectors are thinner than the SD50 generation. if you need to join two wires to one (3 total wires), or two pairs of SD300 wires to create a longer length for both pairs (4 total wires), you will need this EW-JC304 Di2 SD300 4-Port Junction Box. This is actually a simple looking dual tube connector with internal ports on both ends of each side that accept the SD300 connectors and provide the proper, seamless bridge for the power and data transfer rates of the SD300 wires. Use of TL-EW300 Cable Plug Tool is required to preserve connector integrity as the thinner wires and connectors are more difficult to manage by hand.

This is the connector you will use if you choose to connect your Dual Control levers directly to the main battery. This is the most common 3-wire set-up: one SD300 from each shifter out to this EW-JC304 junction box, then a single wire runs directly to the DN300 seatpost battery. You MUST use the dummy plug included with the EW-JC304 in the empty exit port to run this properly. In fact, all SD300 ports in this e-Tube system must have a dummy plug installed if not in use.