Before embarking on the design of the electrical side of a model railway it is vital to first establish the likely power consumption of the planned rolling stock. From this the specification of the power source, controllers, wiring and accessories can be derived.

A Forum Straw Poll in August 2023 arrived at the following consensus:

• Older mainline steam locomotives and early twin motor diesels can draw anything up to 5 amps, more when heavily loaded.

• Most modern ready-to-run and kit built models draw between 1 and 2 amps under load with most being at the lower end of this range.

However, contributors to the thread also pointed out the need to allow for a safety margin to avoid stressing any part of the electrical system and many controller intended for O Gauge are in the 3 to 4.5 amp range.

Similarly, for safety and operational reasons, all parts of the cable infrastructure should have a current capacity greater than any fuses or cut-outs. Small conductors have high resistance and become hot at low currents, 3.5 amps at 12 volts is 42 watts, enough energy to cause an undersized cable to overheat, not good if it is hidden under a baseboard somewhere!

All cables resist the flow of electricity to some extent and the resistance causes the cable to heat up and the voltage to drop. If the cable is appropriately sized in relation to the current being carried the resistance and hence the voltage drop should be negligible. However, as noted above, undersized cables can overheat and the excessive voltage drop interfere with the proper running of the model railway. Most on-line sources advise that voltage drop should be limited to a maximum of 3% of the initial voltage.

• Conductor data based on Rapid Electronics information
• Maximum length of cables derived fromOnline Connections - DC Voltage Drop Calculator

Other on-line Voltage drop calculators are available at:-

• 12 Volt Planet
• Fab Habs

Note that these calculators all give slightly different results but not enough to influence the choice of cable size for a particular application.

The table above and the various on-line voltage drop calculators mentioned are based on DC (Direct Currant) circuits. Other on-line resources are available based on AC (Alternating Current) circuits. The DCC signal is neither DC or AC and these calculators will thus give slightly misleading results.

Two further complications are that:-

• Firstly, unlike a DC layout where each controller and it’s power source only controls one locomotive at a time, in a DCC layout the power source (Booster) powers the entire section to which it is connected and all locomotives running on it. Consequently the cables (and the power supply) have to have sufficient capacity for as many locomotives that are likely to be run simultaneously.
• Secondly, the DCC decoders fitted inside locomotives are voltage sensitive electronic devices that can malfunction if the track voltage falls below a certain threshold.

However, there is a limited range of cable sizes available suitable for DCC power buses (1mm², 1.5mm², 2.5mm² etc.) so, using the above table as a guide, use the next largest available cable for the power bus and divide the layout into separate Power Districts if necessary.

Voltage drop can also be caused by poor connections between the track and the power bus and it generally recommended to securely solder droppers to every piece of track. Track joiners cannot be relied on to conduct electricity between track sections, they are prone to corrosion particularly if PVA glue is used to fix ballast in place. PVA is a very poor conductor and also very corrosive of certain metals.