Wiring Basics

Even a small model railway layout requires a considerable length of wiring. It pays off to do the electrical installation meticulously even when the wiring is one of the invisible "behind-the-scenes" components of a layout. Saving in wiring is a wrong place to cut the corners. In "spaghetti" style wiring, troubleshooting later can be a real nightmare and it is way more problem prone than a carefully planned and installed wiring system in a layout.

Colour Coding

In a typical layout there are many categories of wires like power feed of the track, wiring for turnouts and signals, illumination, wiring for digital operations to mention a few. A clever electrical installation involves colour coding for the wires. Of course, the electrons inside don't care what is the colour of the vinyl cover of the copper, but adopting a colour coding makes the initial installation of cables easier and helps in later troubleshooting too.

Märklin has traditionally used a colour coding system in their model railways and 1972 introduced z-scale was no exception in this respect. However, while the colours used are the same as in Märklin H0 Scale, they are used in a slightly different manner in Z scale. Märklin Z colour coding is simple and logical and the colours are already there in place in Märklin accessories like turnouts, signals and transformers. Therefore it makes sense to standardize the layout to use just Märklin colour system like was done in Zweibären layout. There are, however, a few exceptions and additions to standard colours in Zweibären due to the digital control system and LED light signals.

Märklin Z Scale Wire Colours

Colour		Plugs	Usage
	Red	Red	Track power feeder wire. The red wire is connected to the right rail in the direction of travel. This assures that the locomotive travels to the right when the controller knob is turned to the right from its center position. Track power with switches in signals or relays that switch the power on and off.
	Brown	Brown	Track feeder wire, ground. The brown wire connects to the left rail and, in case of live catenary, to the catenary feeder mast. This assures that there is no short circuit when an electric locomotive is placed on track with the arrow on the bottom pointing to the right.
	Yellow	Yellow	Accessories power feed with alternating current (AC). Connects to illumination accessories, turnouts, signals and other solenoid powered accessories.
	Grey	Grey	Accessories feeder wire, ground. Illumination accessories. Feeds control boxes that control turnouts, and signals. Grey wire is also used to feed contact tracks for the automatic operation of  solenoid accessories like turnouts.
	Blue	Red Green Orange	Solenoid accessory impulse current from manual control boxes or contact tracks which are connected to the power pack by grey wire. Plug and socket colours to match signal and turnout positions (red: stop(signal)/straight(turnout)).

Track Power Feed

An example showing how the colour coded track power feed terminals are connected.	Track power feed to distribute power to several spots  on the track using power distribution strips.

Digital Operation

The colour system is virtually identical. However, instead of the analog system's direct current (DC) to the track, in a digital control system, the red and brown wires carry the digital track signal. Instead of manual control boxes, the blue wires are connected to digital decoders that control turnouts and signals.

Wire Gauges

The voltage in an electrical wire drops as a function of the diameter and the length of the wire and also the current. NEM-standard 604 gives 10% as a guideline for maximum allowable voltage drop in model railway wiring. With a return wire of equal length, this means actually 5% maximum voltage drop per wire and is small enough to ensure trouble free operation.

Commonly used copper wire gauges for z-scale model railways are 0.14 mm²,  0.19 mm², 0.25 mm² and 0.50 mm² measured by the wire cross-sectional area. Märklin standard wires which come with their accessories are 0.19mm². In z-scale, the physical diameter of the wire can be an issue and therefore avoiding too thick wires makes sense. However, a too thin wire is not good either and the minimum allowed wire diameter is determined by the electrical current flowing in the wire.

The rule of thumb for determining the minimum safe cross-sectional area of a wire is:

• Continuous current of 5 Amperes per one square millimeter
• A peak current of 10 amperes per one square millimeter

The selected wire should be strong enough to tolerate the peak current of the application. For model railway wire gauges this gives the maximum currents indicated in the table below:

In z-scale the nominal voltage for accessories is 10 Volts AC and the maximum voltage for the track (locomotives) is 10 Volts DC. Typical power requirements of various z-scale model railway elements are as follows:

• Locomotives: up to 3.5 VA (corresponds to 350 mA)
• Light bulb: 0.3 VA (30 mA)
• Turnout mechanism: 3.5 VA (350 mA)
• Universal relay: 5.0 VA (500 mA)
• Grade crossing: 4.5 VA (450 mA)
• Transfer table or turntable: 3.0 VA (300 mA)

The following table indicates the maximum allowable lengths in meters for various copper wire gauges as a function of the electrical current and with the voltage drop of 5% in z-scale.

In Zweibären layout 0.14 mm² wire is used for most purposes like the wiring of turnout solenoids, illumination and LED-signals. Track feeder wires are 0.25 mm² gauge which is quite sufficient for individual track segments even with up to three locomotives running simultaneously on the same segment. To feed the various distribution strips and digital solenoid accessories decoders and similar, 0.50 mm² gauge wire is used.

Plugs and Sockets

Märklin has traditionally used 2.6 mm industry standard plugs and sockets. Their z-scale equipment makes no exception in this respect and control boxes, distribution strips, switches etc. are shipped with 2.6 mm sockets and plugs. Using plugs and sockets even with extension wires is one way to avoid soldering and get started with layout building easily. Märklin has recently replaced the standard 2.6mm plugs with a proprietary design which, however, is mostly compatible with standard plugs and sockets.

Zweibären is mostly wired without plugs or other connectors. The wires are soldered to distribution strips or attached on decoders with their supplied connectors. The plugs from standard wires in switches and other equipment are removed and an extension wire long enough to reach the corresponding distribution strip is soldered to the end of the original wire and the soldering points are protected with shrink tubing.

Wiring Accessories