ZW-L MultiLoop Power

You need to insulate the center rail between the 'D' powered track and the 'A' powered track.  If you don't do this both loops will run the same voltage and putting power from one output into the other is a bad thing.  (Think 'Ghostbusters, "Don't cross the streams!").  16 Gauge is a good minimum size for track power, 18 for accessories. 

The multi loop setup needs insulated pins IF the loops are connected by a crossover track AND you need to run conventional full time or part time. It sounds like you conventional loop isn't connected to the other loops so no insulated pins are necessary there. If your other loops are going to be command only no insulated pins needed. If you want to run convention from time to time you will need an insulated pin in the crossover tracks between each loop.

As far as the wire goes you can use 18ga for the feeder to the track but keep it as short as possible. Make the main buss 16ga or even heavier.

Ron

First the gauge of wire to use:  I depends on, as always, depends on your current draw.  If you are running multiple engines with possibly lighted cars you could draw enough current to melt the insulation on 18 AWG wire. So you can do two remedies.  Use a lower gauge (14 or 16 AWG) wire capable of handling the high current draw with multiple engines and lighted cars.  Or, you can use "Blocking" to put an engine in each Block so the current draw is less (one engine per block).  Each Block is differentiated by using an insulator pin on the hot rail.

Multi-loop:  Is in essence the same as using separate blocks by introducing insulating pins between the loops and using separate ZW-L channels to power the Blocks/Loops.

Maybe I forgot to say, loop D and the loops on A are not physically connected.  They are physically two separate loops.  Loop powered form D is standalone loop.  Loops powered from A are interconnected with track.  So, since these are different terminal posts, am I ok?  I am trying to understand.  Thanks.

Ron, You hit it on the head.  So I am OK.  Yes, the separate loop is conventional, and the connected loops are command.  And, the conventional and command loops are not physically connected.

thanks for the help on the wire sizes.  Do I need to worry about that for accessories?  A lot of those come with 22 or 24 ga wire...  

ATSF

I assumed the loops were connected by switches, so the insulating pins would go between each pair of switches connecting the loops.