Maybe we're beating a dead horse here, but keep in mind that an under-the-table bus with #12 solid wire won't cost much more than a #18 stranded and is much easier to strip and solder taps. A drop of .3 volts doesn't sound like much from a 17 volt supply, but you have to consider real operating conditions. A DC driven motor probably won't start rolling with 3 volts and will probably run faster than you'd like with 4 volts and it's tough to make a realistic start. When you're trying to operate at low speeds that .3 volt variation is a very big deal. The why is a little complex, but it has to do with the back emf (electro-motive force), where a spinning motor tends to act like a generator.
At rest there is no back emf generated and the current is the result of the applied voltage divided by the total resistance. Once the motor starts spinning the current is the result of the applied voltage minus the back emf divided by the resistance. With a fictional frictionless motor and no other loads the current would drop to zero because the back emf would be equal to the applied voltage. In the ideal world and with the motor spinning, the speed will be directly proportional to the applied voltage and the current will be directly proportional to the load.
In the real world the resistance of the motor windings, internal wiring and external wiring all gets into the act and the "effective" applied voltage is the source voltage minus the product of the current and the resistance. In this real world, the current will drop once the motor starts and the effective applied pops up, it’s difficult to achieve a smooth start and the more resistance in the circuit the worse the starting lurch will be.
When you’re operating at low speeds and even with a constant load, as you slow down the back emf decreases, current goes up and the “effective” applied voltage decreases. The end result is that you’d slow down disproportionally to the applied voltage and you can’t achieve the slow speeds you’d like.
An extra .3 ohms resistance from the bus wire will affect performance and you may not like it. The penalty of starting with a #18 bus wire and having to re-wire with a heavier wire is severe, let caution rule and start with the heaviest wire you can afford.
Rich
www.railroadmicro.com