Robert is correct but for reasons I will explain, I think you need more feeds on the mainlines to avoid what will rapidly become a very irritating problem.
You should be aware that when the points are set against a particular direction, that direction has the same feed on both rails - ie there is no potential between them so decoders or coaches with lighting will see "nothing". This is not the same as insulating points which only put a feed in the "continuing" rail. The switched rail is "dead" - there is nothing there.
Numbering your squares A to H along the bottom from left to right and 1 to 4 along the side from bottom to top,
Here's the
two feed scenario:-
Feed the inner loop at square B2 (actually it's on the intersection of AB, 23)
Isolate the inner loop at square H2
Feed the outer loop at square H2
Isolate the outer loop at square A2
Isolate between the crossovers at D1 and E4
Whenever the crossovers are set for the crossing direction, all power on the "straight" direction beyond the crossing point will be lost.
The point to watch on the continuous run is if you have the crossover switched to cross over, any loco entering the straight section from the opposite section will cause a short. Here is an example to illustrate what I mean:
Consider the crossover at D1 set so that a loco on the inner loop is travelling to the outer loop. In this position all the track on the outer loop from D1 back to A2 has the same feed. Now consider another loco travelling on the outer loop starting at D4 into C4, B4, A3 and then A2. As soon as this loco wheels cross the insulating rail joiners at A2 there will be a short and the controller will shut the whole system down.
So long as you only have one feed per loop and only one insulator per loop, there is no avoiding this situation. Now you might think this will not happen on your layout; it will. The control on DCC and the performance of modern motors is such that a loco is perfectly capable of creeping along imperceptibly on speed step 1. You think you have stopped it, but it just keeps going, millimetre by millimetre until that loco you thought had stopped enters the "dead zone" and bingo - you've got a short.
The way to avoid this is to add extra feed sections to the mainline
loops:-
For the outer loop you will have three sections. These are (anti clockwise from F4)
1) From the output of the point at F4 to the output of the point at B4. The insulators go on the curved side of the B4 point and the straight output of the point at F4. You can feed at D4.
2) From output of the B4 point right round to the output of the point at D1. You will note that this section has no points in it.
3) From the D1 point round to the F4 point.
For the inner loop you will have just two sections. These are (anti clockwise from D4)
1) From the point at D4 round to the point at D1. Note that we have included the point at C1. The insulators go on the output side of the points.
2) From the output of the point at D1 round to the output of the point at D4.
This gives you five mainline sections rather than three. It's all based on not allowing a loco to enter a section which has the same feed on both rails from a section where it has both feeds. This possibility is created by the points, so it is the points and their locations which determine the number of sections. The basic rule of thumb is one point = one section. In this case since C1 is so close to D1 I have put them in the same section.
On my DCC layout, I indulge in what most people consider "overkill" and feed the points as individual sections with insulating joiners on all six rail ends. You don't need to go that far. I think usual practice is to power about a foot beyond the point exit if it is on a continuous circuit and the whole siding if it is not.
I hope that's clear. If it isn't I apologise and hopefully someone else can provide a clearer explanation or an alternative one.
David
