Newbie here.
I am in the planning stages of a multi-room home N scale layout. It will have multiple power districts with circuit breakers and I intend to use 14 AWG stranded wire for my bus with 22 AWG solid feeders every 3-6 feet with soldered joints. My question is about length of the bus (and/or any sub-bus) before adverse effects of voltage drop occur. I have seen “fifty feet” recommended for 14 AWG and am confused if that represents the length of a single wire, or the combined length of both wires of the bus? Hoping it is for a single wire of the pair…
That is correct - 50 feet out and 50 feet back is generally quoted as the maximum bus length, although it really depends on a number of factors (booster design, twisted or not, possible sources of outside interference, etc.).
The best design is actually to place your boosters so as to minimize the length of the bus runs, rather than to see how long you can get away with making them.
Thanks. I appreciate the quick response!
WHartman, it’s good to hear from you. This sounds like an ambitious project. I also have N scale, but a much smaller layout than what you’re planning.
Please keep us up-to-date with your progress, with photos if possible!
not sure where the 50ft comes from.
this tables shows that there will be 0.25V drop when 1A is drawn at a bus distance of 50’
wire run voltage drop: 14 g, 2.5250 Ohm/kFt
ft R V/0.5A V/1.0A V/1.5A V/2.0A V/2.5A V/3.0A
10 0.05 0.03 0.05 0.08 0.10 0.13 0.15
20 0.10 0.05 0.10 0.15 0.20 0.25 0.30
30 0.15 0.08 0.15 0.23 0.30 0.38 0.45
40 0.20 0.10 0.20 0.30 0.40 0.51 0.61
50 0.25 0.13 0.25 0.38 0.51 0.63 0.76
60 0.30 0.15 0.30 0.45 0.61 0.76 0.91
70 0.35 0.18 0.35 0.53 0.71 0.88 1.06
80 0.40 0.20 0.40 0.61 0.81 1.01 1.21
90 0.45 0.23 0.45 0.68 0.91 1.14 1.36
100 0.51 0.25 0.51 0.76 1.01 1.26 1.52
18G wire would have a 0.64V drop
wire run voltage drop: 18 g, 6.3850 Ohm/kFt
ft R V/0.5A V/1.0A V/1.5A V/2.0A V/2.5A V/3.0A
10 0.13 0.06 0.13 0.19 0.26 0.32 0.38
20 0.26 0.13 0.26 0.38 0.51 0.64 0.77
30 0.38 0.19 0.38 0.57 0.77 0.96 1.15
40 0.51 0.26 0.51 0.77 1.02 1.28 1.53
50 0.64 0.32 0.64 0.96 1.28 1.60 1.92
60 0.77 0.38 0.77 1.15 1.53 1.92 2.30
70 0.89 0.45 0.89 1.34 1.79 2.23 2.68
80 1.02 0.51 1.02 1.53 2.04 2.55 3.06
90 1.15 0.57 1.15 1.72 2.30 2.87 3.45
100 1.28 0.64 1.28 1.92 2.55 3.19 3.83
It’s not just about voltage drop. Harmonic ringing can also an issue. If you follow the NCE or Digitrax lists on groups.io, you’ll find 50 feet is about the maximum length you can go before ringing can start to become an issue. (Some good info here.)
Of course it can vary based on the individual situation, but when someone posts about losing control of locos at a given spot on the layout, bus length is usually one of the first questions.
And a quick check of the NCE Information Station even mentions 30 feet: “If your track your track bus wiring has any single section longer than 30 feet your track bus may be conducive to electrical spikes. A decoder can be confused and run away.”
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so 50’ has nothing to do with wire gauge?
Not specifically, no. It has to do with the possibility of ringing at the frequency DCC operates at.
Having said that, of course you still have to select your wire sizes based on the length and expected current flow.
For me at least, it’s not hard to imagine that there may be multiple intertwined factors that need to be considered together when designing something.
i’m an EE and it’s not obvious to me why there’s a problem attributed to ringing on one layout and not another. The club doesn’t seem to have problems with multiple trains running and has bus lengths > 50 using 18g wire
The ‘ringing’ is strictly a DCC concern, associated with the choice to use a square waveform for the pulsed-DC modulation. It is of comparatively little concern in DC power, or even high-frequency PWM smoothed to DC.
Think of it as an artifact of the power needed to drive the bus voltage from reference 0 up to 15 or whatever volts in the shortest possible time. This is very difficult to modulate without transient overshoot and a certain degree of oscillation (the ‘ringing’ in the resonance sense) to settle at nominal voltage. This is a consequence of the Lenz modulation scheme, and has the effect of very short voltage spikes that might be easily attenuated or filtered up to a point, but may cause unanticipated damage to very small or low-power components, or devices susceptible to overvoltage (potentially including some types of supercapacitor).
I suppose that would be true if you were talking about two identical layouts, where ringing on one was an issue but not on the other.
But every layout is different - Different wiring, different physical dimensions, different environment, different brand or model of boosters, etc. So yes, one layout may have a problem, while another layout (possibly similar in some ways) does not.
Search the two lists I mentioned above for ringing. You’ll get lots of hits. Look at the ‘scope captures on Allan Gartner’s site that I linked to. Look at the remnants of Mark Gurries’ web site. Whether it’s obvious to you or not, ringing is a real thing.
So while I’m glad your layout doesn’t have a ringing issue, someone else’s may. Oh, and your mention of multiple trains running - Here’s a thread where another loco on the track actually attenuated the ringing!
i think all you’re saying is square wave with high slew rates overshoot.
yes. so what are the factors that cause ringing, or result in problems “attributed” to ringing
higher load could slow the the transients
Here ya’ go, from Mark Gurries’ site:
There is an equation that describes the physics of what is happening.
V = L di/dt.
Where:
V = Voltage: This voltage spike is on TOP of the track voltage.
L = Henrys: Inductance of the wire. (Function of wire length)
di = Amps: Change in current flow in the wire.
dt = Time: Change in time in which the current changed.
Which is why your layout, “with multiple trains running”, doesn’t experience the issue.
this is just the equation describing the voltage induced in an inductor due a change in current.
the combination of line inductance and capacitance can make the line appear like a longer transmission line by slowing down the propogation of the signal.
when i’ve spoken to the RF engineers at work, they’ve suggested that one cause of ringing is due to the reflection of current/voltage at the end of an unterminated and at very long lengths.
neither these solely explain the problems “attributed” to ringing on one layout and not another.
with a clearer understanding of the cause of the problem, someone should be able to recreate the problem on the bench with either a coil of wire or wire just laided out on the floor, twisted or not twisted, …
then there should be problems when there’s only one loco running. There aren’t problems with just one loco.
“ringing” on twisted wire suggests twisting the bus will reduce inductance and ringing. Twisting also increases capacitance, which should suppress high frequencies (i.e. ringing).
did those layouts that have problems “attributed” to ringing have twisted busses?
Twisting the bus wires together about one turn per foot may help.