Originally Posted by
rvtraveller
I've studied the manual that Iride01 linked. It led me to ask if that junction box was necessary.
So the Y-cable you mention was specifically developed for drop bar bikes that use a bar end junction A, as opposed to the original style that is hung from the stem. In theory you could use the Y-cable. But, lets go back and review your set up...
Currently you need to get 3 items linked together. The most expeditious and easiest way is to use either a SM-JC40 (external) or SM-JC412 (internal) as Shimano describes in that manual you referenced above, and use 3 eTube wires EW-SD50 of the appropriate lengths to connect each component to the JC40/41 to build the system.
If you want to use the EW-JC130 Y-cable first you need ensure to get one long enough to at least get two of the components links, and you only need to worry about the end that is quite short at 50mm (about 2 inches). To extend that short cable end, you'd need to get an EW-JC200 junction and an EW-SD50 cable of the proper length to make that short end reach. No idea on the pricing, but in the end, it may not save you much, and it may be simpler to just build you system according to the diagrams in that manual you've already studied. Also, I've never heard of anyone using a Y-cable to do anything except connect an EW-RS910 junction box at a bar end.
Good luck!