The following chart is a guideline of copper wire current carrying capacity following the Handbook of Electronic Tables and Formulas for American Wire Gauge. The rated capacities are a rule of thumb. If one were to push the capacity to the limit, the insulation temperature limit, thickness, thermal conductivity, and air convection and temperature should all be taken into account. The Maximum Amps for Power Transmission uses the 700 circular mils per amp rule, which is very conservative. The Maximum Amps for Chassis Wiring is also a conservative rating, but is meant for wiring in air, and not in a bundle.

In a modular set up, it is very possible to have a length of 50 feet from the nearest booster, so the wire gauge is very important to avoid loss of performance at the end points. Three locomotives and a couple of switch machines operating near the end point would realize a 12.5% voltage reduction using 16 ga. Wire and assuming a 3 amp load. If you add ohmic contact resistance for each of the connections, you could realize performance losses using wire smaller than 16 ga. For this reason, 16 gauge is set as the standard wire size for power transmission in a module.

* voltage drop for a 50 foot length of wire of the given gauge, at the specified voltage AC or DC, with a load of 3 and 6 amps. Per the Voltage Drop Calculator by electrician.com