Wiring standards part 2: Terminals and Feeders

The first wiring post covered power cabinets, power districts, and power buses. Now that we've got power to the underside of the layout in the form of the bus, we now need to get it from the bus to the track. The wires that connect the track to the bus are called feeder wires or just feeders.

Later in this post I'll discuss the standards I use for attaching feeder wires to the track, but the first question I want to address is how to attach the feeders to the power bus. There are two basic approaches:

1. Directly connect each feeder wire to the bus wire of the same color (i.e. same polarity). This makes sense for layout areas where track is sparce, such as a single track main line on a shelf. In such cases there aren't a lot of feeder wires so a short, direct feeder connection is best.



2. Connect all local feeder wires to a terminal that is attached to the power bus. This makes sense where track density is high. The staging area on this layout is an example of just about as much track as you can squeeze into a given area. In this case you simplify the wiring by connecting feeders to several central terminals.

There are other published wiring methods, but these appear to be the most commonly used. On this layout I use the terminal method predominantly, and the direct connect method in places where track is sparce.

An example of a terminal used on this layout is shown in the photo below. This picture illustrates the various wiring standards in practice:



There are many things to note in this photo. First, the terminal strip itself is an 8-slot barrier strip that I buy at Radio Shack for about $3 each -- cheaper than I've seen elsewhere. I don't say this is the best choice for this application, but it's the best I've found available and the price is good.

Second, note the power bus. This is the pair of 14 gauge wires near the top of the photo, one red and one black, that each weave through 4 of the slots on the terminal strip. It's a bit of work to strip that much 14 gauge wire and snake it through 4 terminals, which is the one drawback of this method. If you use this method be sure to use solid, not stranded wire for the power bus as it's much harder to do the same thing with stranded wire.

Note also that on the left of the terminal strip you'll see a label on the power bus wires -- "PD-1". This means "power district 1", per the standards discussed in the first wiring post.

Third, under the terminal you can see a label "1-J". This means power district 1, terminal J. By convention, terminals on a power district are named A, B, C, etc., with A being closest to the power source, B next closest, and so on. This convention is not a guarantee, as future layout revisions may result in, for example, a new terminal "P" inserted between terminals "C" and "D".
Fourth, you'll see smaller red and black wires connected to the wiring screws on the bottom side of the terminal. These are feeder wires. In general the feeder wires are on the side of the terminal that is most accessible given the location of the terminal.

Feeder wire conventions are as follows:

1. Wire is 22 gauge, red for attaching to the red power bus, black for black. 22 gauge is large enough to carry the power the short distance to the track, and small enough to easily work with when attaching to a rail. I prefer solid wire but I use stranded when solid is not available. I avoid buying "hobby" wire because it is incredibly expensive, instead buying the large spools from Radio Shack. Unfortunately you can't specify color when you order, and it is important to have an equal amount of each color, so instead of ordering in advance I just pick up a few extra spools when they have some in stock.


2. Feeder wire pairs are twisted in order to keep them together for easy tracking (feeder wire distances are so short that you don't have to worry about impedance).


3. The twisted pairs are stapled to the layout wood to hold them in place with a T20 (narrow) or T50 (wide) stapler, depending on the specific situation. I am careful to use long staples (1/2" or more) to avoid the staple damaging the wire itself.
   
4. Each wire pair has a name. For example, 1-J-3 means power district 1, terminal J, wire pair #3. The track location to which a feeder wire pair is attached is noted on a Visio diagram along with the name of the wire pair. (I'll post a sample of the Visio diagrams sometime in the future.)


5. Wire pairs are labeled with their name, usually dropping the power district as that is obvious by the location of the wire. So a typical label might be "J3". There should be at least two labels per wire -- one near the terminal and one near the track.


6. By convention, feeder wires are attached to one of the 4 terminal screws for their wire color according to the following pattern: From left to right, wire 1 goes to the leftmost screw, wire 2 to the next, 3, 4, then wire 5 back to the left most and so on.


7. I try to keep to a limit of 12 feeder pairs per terminal, which equals 3 feeder wires per terminal screw. If more are needed then another terminal should be added. This means that the screws, from left to right, would have the following wire numbers: 1-5-9; 2-6-10; 3-7-11; 4-8-12.

The next wiring post will discuss attaching feeders to track.