<<I have a question about the length limit of 25 feet. The NEC directions say that you should use at least 14AWG wire and limit the wire length to 25 feet. Does the main and each of the sub busses each have to be 25 feet or less? Or is it the length of the main bus up to where the sub bus connects plus that sub bus length? How does this 25 foot limit work? >>
Quoting directly from the NCE Power Pro System Reference Manual:
“For the hobbyist wiring up a new layout our suggested wire sizes based on voltage drop are:
Runs to twenty-five feet #16
Runs to fifty feet #14
Runs beyond fifty feet #12
For best results on long runs (over twenty feet), twist the bus wires about three turns per foot.”
Note that the “limit” for runs of AWG #14 is fifty feet, not the 25 you mention. This alone might resolve your questions.
When we’re talking about buses we need to keep two things in mind:
1) Power loss (voltage) over distance, and
2) Distortion of the DCC signal due to inductance.
Voltage drop is minimized by increasing wire size; signal distortion is minimized by twisting the wires (keeping the wires in close proximity allows the magnetic fields generated by each of the wires to cancel out the magnetic field created by its partner).
The length of wire limits are not “hard” limits – a #16 bus will not fail at 26 feet. The limits are “suggested” because at some distance beyond that recommendation, voltage loss will drop below that needed to provide voltage, current and signal to the track. Wiring should be designed with the suggestions in mind; however, there is _some_ room for exceeding those limits.
I’ll defer to those who know the math of voltage loss over distance to provide the scientific answer to the total voltage loss of a length of size “A” followed by a length of size “B”, but anecdotally there seems to be some room for deviation.
Every layout is very unique, so it is hard to know at what length of bus problems will start impacting operation. Because of this, when wiring new layouts (or gutting and completely rewiring an existing layout to stay as close to the recommendations as possible – its not a binary question of “less than X you are fine, more than X you will definitely have problems” – it is a very analog situation – at some point problems (obvious or not so obvious) will start showing up. The closer you hold to the recommended good practices, the lesser you can expect to have problems down the road.
All of this is a long way of saying:
1) Know _why_ certain recommendations are made
2) Follow these recommendations as much as possible, but don’t anguish over minor deviations resulting in major economic savings.