Wire Current Ratings

The question we are atttempting to answer is how big of a wire do I need to reliably carry the DCC current around my layout.

Under DC, wires have resistance.  The value for a given wire gauge is expressed in ohms/foot.   (DCC, which is a form of AC, also brings in other properties but that is not what we are going to discuss here.)   The length of the wire is typically twice as long as than you think it is because to run DCC power to the any point on the layout, you must run TWO wires one for each rail.  (Defintion Bus = 2 or more wires the peform the same function grouped together).

1) What is the absolute Maximum current rating of a wire.

Excluding all other constaints, a wire will only have one physical destructive breaking point when it comes to current.  It is called the fusing current which is where the heat generated in the wire reaches a temperature where the wire melts.  AKA the wire turns into a fuse.   Below that, the wire will support any current.   Clearly the fusing current is not a practical value for building our layouts but it allows you to understand that bare wire can handle a lot more current than you think.

2) How does the National Electric Code (NEC) provide current rating for wires?

The NEC code is a very conservative electrical code who's number one goal is high reliable and safety in the power transmission field.   Power Transmission mean the transportation and distribution of AC power from the moment it enters into a given building all the way to the outlet or light switch.  This includes power cords and any tempoary wiring such as extension cords.  In achieving the goal of high reliability and safety, the NEC must take into account all the properties and limitations of the materials and methods used to move power from any given point to any other given point.    This results in a complex code that involved many variables. The key variables are:

A) Wire Insulation material temperature rating
B) The wire is installed in free air or inside a conduit or cable.
C) If part of a conduit or cable, how many other wires carry the same current.
D) Wire Gauge.

Of these variables the Wire Insulation Temperature Rating is key variable that first sets the maximum current rating and the current rating drops down as the additional operating conditions are imposed.  Wires with insulation breakdown due to excessive operating temperature is not a good wire. 

Therefore, If you set a maximum temperature rise based on the wire insulation material, you then can go back and take any given wire gauge and rate it for a maximum current. 

The NEC codes do not apply to DCC since the voltages we are working with are not hazardous.

2) What
ooster provide both the track voltage and current.   They all have a maximum current rating.  When there is a short circuit, they shutdown.  The question is, what is a short circuit?   A short circuit is simply a resistive load that will require more current that the booster can provide.

You can calculate the maximum load resistance from the booster ratings using Ohms Law

V = IR —>   R = V/I

Maximum Wire Resistance allowed = Booster_Track_Voltage / Booster_Current_Rating

So a 14V 5A booster = 14V/5A = 2.8 Ohms.   So:

If the load is less than 2.8 Ohm, the booster will shutdown. 
If the load is more than 2.8 ohms, the booster will not shutdown.

What is the load when there is a derailment resulting in a short circuit?

It the total resistance of the conductive metal that must carry the current starting from the booster out to the short circuit and then back to the booster.

There are 4 conductive metals that we are dealing with.

A) Wiring resistance.   Found both under the layout and inside the locomotive.
b) Rail resistance.  What rail code size you are using.
c) Wheel resistance.  Can include axel bearing surfaces that are lubricated but must also carry current.
d) Rail to Wheel contact resistance.

We only have 100% control over the wiring under the layout.  It is assumed no one want to rewire a locomotive with heavier gauge wire.
We have some limited control over the rail resistance.  The rail code is chosen by scale and cosmetically goals.
We have almost no control of the Wheel resistance
We have zero control over Rail to Wheel contact resistance.  The track and wheels get dirty.