Wiring: colours, feeders and wire size

Benjamin | TrackDrawer.com · published 13 November 2026

Good wiring rests on three rules: a fixed colour for each kind of wire, a heavy bus that runs under the layout past every part of it, and short, thinner feeders (droppers) from that bus up to the rails, spread over the layout. Build like that and you get less voltage drop, find faults faster and can extend later without pulling everything out again. How thick a wire needs to be depends on how long it is and how much current flows through it.

Colours: choose a code and stick to it

A wire under the baseboard is usually only seen up close, and often years later. By then its colour has to tell you what it does.

Märklin three-rail has a common code you will find in the manuals: red for the track power to the centre conductor (B) and brown for the return to the outer rails (0). For point motors and other solenoid accessories, yellow is the common supply and blue the control wire. TrackDrawer uses the same colours in its wiring diagram: red and brown for every terminal track, yellow, blue, blue for every turnout.

Two-rail has no code that every brand follows. Choose one yourself and write it down. A practical rule is a fixed colour for each rail, so the left and right rails never get swapped. Many modellers use red and black, the colours the NMRA code for loco decoders uses for the right and the left rail.

A colour per function helps as well:

The exact colours matter less than the discipline. A wire in the wrong colour is a trap you set for yourself later.

Bus and droppers

Run a bus under the layout: two heavy wires that pass every part of the layout, starting at the command station, booster or controller. Short droppers run up from that bus to the rails. That way the current travels most of the distance through a thick wire and only a short stretch through a thin one.

• Keep the pair together. Run the two wires of the bus side by side, or twist them loosely. On digital that keeps the signal cleaner.
• A bus of its own for each power district. If your layout has a command station and one or more boosters, each district gets its own bus. The track power of two districts must not touch. What a district and an isolating gap are is covered in track sections and isolation.
• Turnouts and signals separately. Do not run the supply for turnouts, signals and lighting over the track power wires. A separate wire keeps interference apart and makes fault-finding easier.

Feeders: how many and where

Every joint between two pieces of track adds a small contact resistance. The further a loco runs from the feeder, the more joints lie in between. That is why you feed a layout in several places.

• A common guideline is one feeder every one to two metres of track. Treat that as a starting point, not a law: old track with tired joiners needs more, a short layout with new track fewer.
• Every section its own feeder. A piece that is electrically isolated gets no power otherwise.
• After every gap, feed again. A holding siding, a reversing loop or a block with a detector needs its own feeder.
• Flexible track. With flexible track, long lengths and soldered or clamped joints, many modellers feed every length separately.

For a feeder you use a terminal track from your system or solder a wire to the rail. Which terminal tracks our catalogue has for each system is in the table in train runs jerkily.

Wire size: how thick does a wire need to be?

A wire has resistance, which grows with length and falls with thickness. Current through that resistance costs voltage: the voltage drop is current times resistance. Copper has a resistivity of about 0.0175 ohm per metre at a cross-section of 1 mm². A worked example for a 5 m bus (10 m of wire, out and back):

What that tells you:

• The bus can be heavy. Over several metres the cross-section makes a big difference. On a larger layout many modellers choose something between 0.75 and 1.5 mm² for the bus.
• Droppers can be thin, as long as they are short. A 0.25 mm² dropper 20 cm long (40 cm out and back) has only 0.028 ohm. At 2 A you lose less than 0.06 V in it.
• Thin and long is the trap. Running a thin wire round the whole layout, such as the one that comes with a starter set, works on a small layout and fails on a large one.

Treat the table as a worked example, not a rule: the current on your layout depends on how many trains run at once. What to do when the voltage on an existing layout still sags too far is covered in voltage drop on a large layout.

Connections that last

• Solder or clamp. A soldered joint or a good clamp connection lasts for years. A wire just twisted round a screw works loose over time.
• Strain relief. Fix wires under the baseboard so that a tug on a wire does not pull the connection at the rail loose.
• Within reach. Put junctions and terminal strips where you can get at them, preferably at an open part of the baseboard frame.
• Label everything. A number on both ends of each wire, and a drawing with the same numbers.

Wiring in TrackDrawer

With the digital layer (Pro) for Märklin C track, TrackDrawer builds a wiring diagram from your plan and your collection. It numbers turnouts W1, W2, detection tracks C1, C2, terminal tracks V1, V2 and uncoupling tracks O1, O2. For each power district it lists which terminal tracks are fed, each with red (B) and brown (0). For each accessory decoder it lists the outputs with address, turnout and the wire colours yellow, blue, blue, and for each feedback module which input belongs to which detection track. At the bottom the diagram counts the wires by type: supply pairs, turnout cables, feedback wires and booster cables.

Lay out your track in the app, add your digital components to your collection and take the wiring diagram with you under the baseboard.