Sections and bus wires for larger layouts - how many and how to decide

[PDT]

I am thinking of building a fairly big layout over the next year with areas that could be easily split into 3 or 4 sections that might have say 20 points in each section.

Would it be best to have three / four bus wires to each section from one control module and if so how do you manage the train moving across to each section and is there a risk of short circuit?

[Chrissaf]

I suggest that you may be approaching this from the wrong direction. In my opinion you should start with researching what DCC control products [brand and type] you want to use, then build the power distribution & wiring design based upon the requirements of your chosen products.

Let's just say for example, you chose DC Concepts ADS-8Sx Accessory Decoders for your point operation. These decoders have powerful integrated CDUs that draw high inrush charging currents. With a typical 4 amp power supply, the power supplies start to trip when more than four or five ADS-8Sx decoders are attached. Each decoder is 8 ports, and you are indicating 4 sections with 20 points in each, which is 80 points. Thus 10 x ADS-8Sx decoders would be needed. Therefore in that scenario you would need 2 x 4 amp Boosters with no more than 5 decoders powered from each booster.

Other brands and models of accessory decoders each have their own power design requirements. DCC Concepts iP Digital point motors have an integral DCC decoder that doesn't use a CDU, so with that product you would not need hardly any Boosters. So trying to design the wiring before choosing the products is basically putting the cart before the horse. Which is why I am suggesting that you are approaching your layout design from the wrong direction.

Just one other point of clarification, you use the term 'control module'. It is not clear from your post what exactly you mean by that term. If you mean a 'control module' to operate points then that is an 'Accessory Decoder' as discussed above. If you mean the main DCC Controller to operate the layout trains and provide the DCC commands to the Accessory Decoders to operate the points. Then there MUST be only one of these on the layout. So your reference to 'Short Circuits' when traversing from one section to another section is not really relevant, as in DCC you do not have multiple controllers with each controller operating a different section of the layout. That is an Analogue DC control concept and not applicable to DCC power distribution design.

Another criteria that needs to be factored into the layout power distribution design is the number of locos that will be running on the layout. It is the number of locos that define the power distribution and not the size of the layout. As a typical 'rule of thumb' and considering modern rolling stock. It can be assumed that 10 x locos can be run from a 4 amp power supply. So if you are unlikely to operate more than 10 x locos at a time, then a single 4 amp power supply to the track will suffice. If the intention is to run more than 10 x locos, then the design of the layout wiring will be proportionally more onerous to design because you then have to start looking at the layout from a train movement point of view. Say for example you have two powered sections, with each section powered by a 4 amp supply. Let's also assume that you have 20 x trains that are running simultaneously, then if each section only has the power to run 10 x trains, an issue will arise if train movements result with 15 of the 20 trains ending up in one of the two sections. In that scenario, the supply powering that section could become over-loaded and trip.

So once again, designing the wiring part of the layout is much further down the 'things to do' list than your question is actually suggesting. The wiring design is in reality one of the last things to implement on a large scale layout.

The schematic below gives a flavour of the type of power distribution that you might need to consider. It is only a flavour of a design just to get your thought processes stimulated.

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[PDT]

Dear Chrissaf

This is really helpful and exactly what I need to know. Now I can work on this better.

Many thanks for taking time to explain.

[Fishmanoz]

Fist a comment on Chris’s circuit, then another consideration for your track bus.

As shown in his diagram, Chris’s track has only a single section. Consequently, it can be powered directly from the controller and doesn’t need a booster. The boosters are only required to handle the startup inrush current from the accessory decoders, assuming you are using a type that has the initial inrush feature.

Returning to the track, if you are unsure whether your overall design will exceed the 4 Amp limitation, you can initially design for sections but not fit a booster until you see how it goes. By this I mean you separate your track into sections by using IRJs between them, then also have separate track buses to each section. However, initially, don’t fit a booster, just join both track buses together and connect to the controller. Only fit the booster if you find controller is tripping due to overload.

And in case you were wondering about shorts as a loco crosses the IRJs from one section to the other, the booster handles this by instantaneously reversing the phase in the section it is powering if the phases in each section are opposite at the time. In this regard, it operates just like an RLM (Reverse Loop Module).

[Chrissaf]

Just for clarity. I was showing the track with the Booster as an example of a multi-sectioned track [but only showing the Booster powered single section to keep the drawing less cluttered]. And as I said in the text, it was only meant as an indicative [flavour] which represented 'one of many' possible solutions that could be adopted. It was not meant to be taken literally as drawn.

[96RAF]

One important thing to be noted, which was highlighted by Chris earlier is that there is only one DCC controller putting a signal onto that vertical bus. Each booster takes that signal and relays it into its own isolated area. You will also note as previously explained that each booster has its own power supply and it is that which is applied to each boosted section. The controller power supply is only for its own signal bus.

[PDT]

I think I was concerned about a drop in the current as it went further from the source and with the number of droppers I would be attaching.

[Chrissaf]

The more droppers the better in terms of current management. Some consider this 'over-kill' but I have a dropper on each and every single track piece. Thus my metal track joiners are relegated to just performing physical track joints and joint alignment. It is loose and/or dirty corroded track joiners that restrict current more so than the size of the layout. In my case, only my points and cross-overs are 'ready to run' track pieces. All my straights and curves are created with flexi-track [36" lengths] so this does actually reduce the number of droppers over-all across the layout as a whole.

In principle, the DCC BUS and droppers will be all soldered and use 'copper wire'. Thus their current handling capability will be much better than the rails that they power. The silver in 'Nickle Silver' rails is a relatively good conductor and far superior to the much older 'all steel' rails, which in my opinion should be avoided on a Digital DCC controlled layout.

[PDT]

Really good tips here guys thank you so much