Wiring Standards

Model railroads run on electricity, but it’s not just the trains that need to be powered. Accessory systems (such as turnout switch motors and signaling), building lighting, and control systems (block sensors, automated control systems, and computers) all require various forms of electricity. Different systems have differing requirements and need to be kept clearly distinct. Also, there are wiring systems for other purposes (e.g., signaling, building lighting, etc). All of these other than the LocoNet Control Bus are described here. For LocoNet, see the Overview and Addressing pages.

For additional information on wire and its application to model railroads, see the Wire page in the Electricity for Modelers section.

Note: some of this material is repeated on the Block Wiring page for ease of reference.

DCC & DC/DCC Track Bus Wire Sizes and Types

Standard: 14 gauge, stranded, normally color-coded Red/Black
In some places other colors (Yellow/Purple) were used to differentiate separate sets of wires.
Red is DC “+” or DCC “Rail B”.
Terminal Strips used are typically #8 post. Crimp and Suitcase connectors are “Blue” (16-18 ga).
Wires will be lightly (3.5 turns/ft) twisted.

All bus wires are accompanied by a single Green “common ground” wire of the same or larger gauge. This is both a reference ground between systems and a safety ground tied to earth. There is one ground system regardless of the number of bus sets, and all ground wires are a single tree rooted at the main power panel (no loops) and connected to earth via a power cord and the building ground system.

DCC Track bus wires are the wires from the control station (or booster) that supply track power to the circuit breakers (or directly to track feeders). DC/DCC Track bus wires are similar, except that they are connected via an A/B switch and may alternately connect to either the DCC source or to a DC Power Pack. Although DCC wiring has more stringent requirements than DC, the DC/DCC wires will be similar to permit design flexibility.

Track Bus Background

For an N-scale layout the size of mine (total track circumference, and thus bus length, of about 30 feet), using a 5-Amp command station and boosters the bus could use 16-gauge wire and still have an acceptable voltage drop. Use of 14-gauge wire would provide a slight improvement (less voltage drop) and allow for future expansion to a longer bus without rewiring. Unless you’re using 10 Amp boosters and really long runs, nobody needs 12-gauge or heavier wire.

Wiring will thus be 14 gauge for track power bus wires, to minimize voltage drops (and because 14 ga wire is available at relatively low cost from home supply stores). I originally planned to use solid-core wire for this; its rigidity is of benefit in bus wiring, which tends to be straight, or follow a simple curve. But problems using crimp-on lugs and insulation-displacement connectors led me to replace it with stranded wire instead.

For this kind of wiring I’m using something sold as “Primary Wire”, rather than in-wall electrical wire as I’d originally planned. The reason is that primary wire has a more flexible insulation, and I don’t need the more robust characteristics of in-wall wire for low-voltage open-air wiring.

DCC wires do not need to be twisted for noise reasons, although this may have some benefit when high-frequency signaling systems (i.e., transponding) are in use. There is also some anecdotal information that this has a beneficial effect on signal quality in long runs (an impedance issue of some kind). This also has the benefit of clearly associating two wires (handy in places that have two sets of bus wires with the same color code) and reducing under-table tangles. Some of my track bus wiring is not twisted, but I’ll eventually fix that. Twisted wire should have about 3.5 twists per foot, but this is only approximate.

DC & DCC Distribution Wiring

Standard: 16 or 18 gauge, stranded, (16 preferred) outside of power-control system panels, normally color-coded Gray/Blue
Gray is DCC “Rail B”
Terminal Strips used are typically #8 post. Crimp and Suitcase connectors are “Blue” (16-18 ga).
Wires will be lightly twisted.

Distribution wiring will connect the bus to circuit breakers, the circuit breakers to the occupancy detectors, and the occupancy detectors to the track feeders. It is typically limited to short runs within the bounds of a single “table” or at most a two-table “scene” (i.e., under ten feet).

Distribution Wiring Background

Distribution wiring on each table can be smaller 16 ga (stranded) as connectors for this are fairly common, and the short runs don’t create a serious voltage drop. I could have used 18 ga, but 16 was easier to find in a variety of colors.

For this I’m mostly using “hook up wire” which is even thinner and more flexible than 16-ga Primary Wire, although Primary Wire is used in places that may be subject to more stress (like the power control panel).

My occupancy detectors (BDL168), circuits breakers (PM42) and transponding sensors (RX1) were originally planned to mount to the underside of the table. This proved to be difficult to fit in the available space, so after doing the Riverside Station scene tables this way, later ones were built on separate boards and attached to the layout. See the DCC Occupancy page for details about this.

Bus wires will be mildly twisted on long runs (about 7 twists per 4-foot section) to reduce impedance effects on DCC signals. Radiated electromagnetic interference is unlikely to be a significant concern at the frequencies in use here, so that is not the reason to twist wires (although it is important with Transponding sensors and the distribution wiring connected to them).

Power Control Panel Wiring

Standard: 20 gauge input (20 ga acceptable), 22 gauge output, stranded, normally color-coded Gray/Blue
Gray is DCC “Rail B”
Terminal Strips used are #8 post on the panel edge, and #6 post for internal wires. Crimp connectors are “Red” (20-22 ga).
The Blue (“Rail A”) wire is looped through the occupancy detectors and transponding sensors.
Wires are not typically twisted (in most places they don’t run together) except near the transponding sensors where a wire twists back on itself.

Power Control Panel Wiring is wire within the bounds of the panel containing the circuits breakers and occupancy detectors. This connects to the track bus on the input side, and to Track Feeders on the output side, but Distribution Wiring will be between the panel and both of those.

Power Control Panel Background

The edge connectors used by the Digitrax Electronics can accept 18 gauge with effort, but are really designed for 20 gauge or smaller. Either 20 or 22 gauge can be used for the 5 Amp input side (20 is preferred, but can be hard to find at my local supplier). Output wiring will typically be handing less than 2 Amps (often a lot less) and can be smaller, so 22 gauge will be normal there.

Twisted wire here serves a different purpose from twisted bus wiring: it is used within two inches of the transponding sensor to reduce noise from the wire that could induce a signal on the transponding output and affect that function.

Track and Track Feeder Wiring

Standard: 22 or 24 gauge, stranded (solid may be used as below), normally color-coded White/Blue.
Note: any soldered feeders MUST use copper wire, not aluminum.
White is DCC “Rail B” and is also “Rail furthest from table front” (Blue is “Rail A” is “front”).
Terminal Strips used are typically #8 post. Crimp connectors are “Red” (20-22 ga).
Wires are typically pre-made feeders, as short as possible, and not twisted.

Track feeders are the links from the track to the under-table wiring.

Track Feeder Background

Feeders to the track will be relatively thin (22 or 24 ga) “hook up” wire. In general this will be stranded wire, but I’d likely use solid if I’m soldering it to the rails. Kato feeders use stranded wire, and I’ll be using those to start. This will be as short as possible (but will run from the track to underneath the table), and connect to thicker feeder wires via a terminal strip. Since a feeder powers at most one train if the track sections are short, current across these will be low enough (typically 0.5 to one Amp) that small wire is okay. I used similar wiring on my old HO layout and it worked well.

Feeder wires are not twisted. This is both for simplicity, and because they are relatively short and thus less of a concern as a source of inductance.

DC Power Supply Wiring

Standard: 16 or 18 gauge (20 ga acceptable), stranded, Yellow/Orange (accessory and signaling) or Brown/White (lighting)
Terminal strips used may be either #8 or #6 post. Crimp connectors may be “Blue” (16-20 ga) or “Red” (20-22 ga).
The lighter color (e.g., White, Yellow) is always positive (“+”).
Accessory supplies may be >12V (up to 16VDC). Lighting buses are always 12V DC regulated supplies at less than 1 Amp (fused at 1.25A).

Several sets of 12V DC power supply wiring exist to provide power to accessories. These are similar, but for equipment connections they are divided into three sets: Accessory Power Bus, Signaling Power Bus and Lighting Bus. Each table with have one of each, but individual tables may be powered independently. Note that this is not the high-current power supply to the DCC Command Station or Booster(s), which is kept short and uses heavy-gauge Red/Black wires.

DC Power Supply Wiring Background

I had originally planned to use the two spare 5 Amp outputs of the PS2012 that supplies the command station for all my accessory needs. That would have been more than sufficient. This was before I discovered the comments in Digitrax’s documentation about the PM42 and BDL168 power. As a result the accessory bus was split into one bus for the DS64 and other accessories (the Accessory Bus) and a separate bus for the BDL168 and SE8C (Signaling and Occupancy Bus), plus a third bus dedicated to the PM42 circuit breakers (PM42 Bus). And lighting was split off (the Lighting Bus) so that it could be kept to a 12V regulated supply presuming I needed to switch one or all of the others to a higher voltage (some comments online implied 14-16 VDC was preferable as a power source for the Digitrax equipment). In the end, I made the PM42 and BDL busses 15V, the miscellaneous accessory bus remains connected to a PS2012 output.

There’s a bit of ambiguity in Digitrax’s manuals. The PM42 circuit breaker (used to supply track power to the BDL168) isn’t supposed to share a power supply with anything besides other PM42s, and the SE8C and BDL168 may share power supplies, as long as nothing else uses the same supply. My suspicion is that this is really just a caution about overloading the 300 mA PS14 (which could supply two PM42s or three SE8C/BDL16s), rather than a concern over some kind of transient (surge or noise) being injected into the DC supply that could affect a different module. So I’m going to allow the DS64s to share the PM42’s bus. If that causes problems, I’ll revisit this later.

The current requirements for the PM42 (125 mA), BDL168 (100 mA) and SE8C (100 mA) are clearly stated in their respective manuals.The DS64 manual states that it needs 300 mA (likely most of that is needed only when throwing a turnout but I’ll budget all 300 mA to be safe).

The other systems are a bit less clear-cut. The UR92 documentation only says to dedicate a PS14 to it (which means the maximum load is 300 mA).

For the two large busses (Signaling and Accessories) I’m going to use 16ga wiring to distribute the power, which is seriously oversized. The PM42 bus and each lighting bus will use 18ga distribution wiring, but Lighting will step down to smaller wires for the actual runs to the LEDs.

Other Wires

There are some other wires under the table that need to be of specific sizes and color-codes, although this is less critical than for power wiring.

Track feeder: soldered, 22 ga or 24 ga, solid copper (not used at present)
Blue (outside), White (inside)
- This might change depending on availability of the appropriate wire.

Switch throw leads: Kato-supplied (26 ga, stranded)
Red/Black feeders (because that’s how Kato wires them)
Yellow/Orange 22 ga used between switch controllers and the actual feeders.

LED Lighting Bus: 22 ga
White (inside) / Brown (outside) - originally I’d planned orange in place of brown, and 18 ga.
Note: white is positive (+) and brown is negative (-); to avoid blowing LEDs, polarity is critical.

PM42 Power Bus: 16 ga, stranded
Blue/White (due to the wire I had available, but this shouldn’t be easily confused with a track feeder)

Under-Table Bus Wires

These wires are part of the above-described systems, but have a specific layout that needs to be documented.

The “inside”/“outside” labels refer to the connected rail’s position relative to the front edge of the table for each pair of rails (e.g., DCC bus #1 has a black wire that ultimately connects to the rail closest to the table edge on any track wired to that bus). Power supply numbers designate the voltage and circuit fuse panel position (e.g., 12H is position H with a 12V wall-wart) or “PS2012” for one of the 5A outputs off the main supply.

Outer Terminal Blocks:
DCC Bus #1 (Commuter): 14 ga
Black (outside wire) / Red (inside wire)
Green (Ground)
Supply: PS2012 (5A)

Accessory (Switch actuators, panel meters, etc) Power Bus: 16 ga
Yellow / Blue
Supply: PS2012 (5A)

DC/DCC Bus #3 (Rapid, Outer loop): 14 ga
Orange (outside) / Purple (inside)

Inner Terminal Blocks:
DCC Bus #2 (Subway): 14 ga
Black (outside wire) / Red (inside wire)

Signal & Occupancy Power Bus (BDL168/SE8C): 16 ga
Yellow / Blue
Supply: 15E (15V, 1.6A)

DC/DCC Bus #4 (Rapid, Inner loop): 14 ga
Orange (outside) / Purple (inside)

PM42 Power Bus: 16 ga
White / Blue
Supply: 15C (15V, 1.6A)

Lighting 1: Riverside Station Scene
Supply: 12F (12V, 1.25A)

Lighting 2: River Crossing Scene
Supply: 12H (12V, 1.25A)

Lighting 3: Urban Station Scene
Supply: 12G (12V, 1.25A)