I have a 15 amp branch circuit where the first box from the breaker contains a switch for a ceiling light at the door leading to the garage. This box supplies the rest of the branch. On the same branch at the opposite corner of the room I am adding trailer circuits for multipoint switches located at each entry point to the room to control new recessed lighting fixtures. These trailer wires end back at the garage door where I am replacing the first box with a multi gang box. This box will now contain the switch for the existing ceiling fixture, feed the rest of the branch, and also be the end point for the trailer wires where they connect to the load through the final S3 in each chain. This ends up making a big loop with sub branches branching off in the middle. I have included a diagram for clarity. The question is, how do I handle the neutral and ground connections in this box (Multi gang box combining B1 and B5). I am inclined to wire the neutrals and grounds as if they were still in separate boxes but everything I've looked at seems to indicate I should wire all the neutrals together and all the grounds together. Does anybody know the correct approach?
What you will want to do in your case, instead of nutting all the neutrals together (which is technically a 310.10(H) violation!), is keep the neutral from the breaker at B1 and going to B2 and the existing light fixture separate from the neutrals coming in from B4 and going out to the two recessed-light circuits -- in other words, B1 and B5's neutrals are separate. Also, in boxes B4 and B5, keep the neutrals separated from each other as per Tester101's revised illustration -- again, NEC 310.10(H) applies.
One other tip -- the power feed to all the lights should come from the unswitched hot in B3. Otherwise, you wind up having to return the switched hots for the two four-way loops back through B4 and B5 from B3, leaving you without a neutral wire in those cables unless you switched the 14/3 for those runs out for 14/4. Using the netural from the three-way loop isn't an option because it'd create a large, annoying current loop that could interfere with the operation of timer or electronic switches that require the neutral at the switch box.
I'm guessing your wiring looks something like this.
So if you also draw the grounded (neutral) conductors, it looks something like this.
Drawing it like this you can clearly see, if you connect the grounded (neutral) conductors in box 5 (B5) to the grounded (neutral) in box 1 (B1), you'll have a giant loop.
It shouldn't cause any problems, electrically speaking. Your electrons will just be able to take different routes back home. However, as ThreePhaseEel points out, paralleling conductors in this way would be a code violation. To avoid paralleled conductors, you should wire the circuit as shown in the second image above. Notice that the grounded (neutral) conductors are kept separate from box B3 on.
All fed from one breaker... all neutrals together, all grounds together. That's how I was taught.