The wire ‘inside’ locomotives is indeed short, and indeed fairly fine-gauge. It is also extremely short by ‘feeder’ standards, and is right at the ‘last mile’ to the devices that are to be powered. The purpose of adequate feeders is to limit the voltage drop up to the wheel contact, wipers, and small wire that necessarily (in most actual locomotives) constitute sources of loss. Think of it the same as good steam jacketing and internal streamlining reducing temperature and throttling loss to steam-locomotive cylinders.
High resistance at wheel contacts almost immediately invokes a risk of microarcing. That is always and everywhere a bad thing. Technically that may be worse with higher available voltage, which may need to be borne in mind when assessing really good high-amperage feeder systems…
The two most sensible approaches (and I suspect they can be combined on a particular layout) are feeders to every rail joiner with that joiner then soldered to both adjacent rail ends with a lower-melting solder, and feeders to the midpoint of each rail segment, whether or not that segment has rail joiners or just gaps at its ends. As I understand the latter, even if you solder the rail joints you still want a feeder to the midpoint of each stick of rail, going to an adequate bus through a good soldered areal contact. gregc and others are indicating that the nominal voltage drop through rails with good soldered joiner connection is relatively low, so there certainly will be cases where just having some long strings of soldered rail without feeders will be adequate, but it still seems rational to me to take the time to drop a feeder from every rail, as a one-time action that guarantees the ‘best’ reasonable electrical supply right from the time track is laid. (See our good threads on track fixation and benchwork for advice on holding either soldered or unsoldered track effectively once i
Some of my feeder runs were a little on the long side (over 8 feet) with 20 awg so I’ve replaced them with 18 awg. The rest are about 6’ or less 20 awg but will be tied in to the main bus with 18 awg via on/off switches.
Last two layouts I built, the feeders were soldered to rail joiners - do not EVER buy the Atlas terminal joiners, for the price they charge for those, you can make a couple dozen pair, and since you can solder them at the workbench, there’s no danger of melting ties. ANyway, I used terminal feeders at EVERY track joint. I only soldered every other one though, so it was flex track - soldered joiners - flex track - joiners - flex track - soldered joiners - flex track, etc. ANd All 3 legs of every turnout (except where I needed insulated joines to divide power districts) got wired joiners - none soldered.
This was highly reliable, even after painting the rails. And I wasn’t shy with the paint around joiners - I wanted to hide all the shiny.
The traick is - I have a handful of joiners I use to test fit things - they are well used and pretty loose fit. However, all ther terminal joienrs I made up were fresh out of the pack joiners. Only installed after the track was fitted and cut to length. So they were very tight. Slid on once, and done. The soldered sections, I soldered at the bench.
If any of the non-soldered ones failed, I never knew it, because I never had any power issues. There was effectively a soldered joint with power feeds every 3 feet, with an extra ‘helper’ one in between that wasn’t soldered to the track. And the turnouts (Atlas) were extremely reliable with feeds at all 3 legs. Even without powering the frogs, nothing ever stalled.
I assembly lined it. Cut lengths of wire from my roll of feeder wire. Strip the end to solder to the joiner of all pieces, and bent the stripped end over at 90 degrees. Then soldered all the prepped wire to new joiners. With a built up supply of a dozen or more pairs, I could just keep on laying track without having to go back and solder things. Can make a couple dozen sets in an hour of bench time.
Practically speaking if on your home layout you only run an engine or two at a time, and they aren’t drawing much amperage at all, the voltage drop is much less, even in the provided calculator (I got it down to about 3%). I have a decent mainline run at about 81 feet, and the maximum wire length is about 30’ to any of the three total drops. I use Kato track and none of the joints are soldered anywhere. It’s just a simple folded dogbone, and most of the time for plain dc operation it functions just fine. Do I wish I’d put in another feeder or two? Perhaps, but it works. It even functions ok in dcc operation too. I think it’s because the NCE system tells me that even maxed out some of the (Genesis) engines are only actually drawing 0.2 amps. At that kind of amperage, the voltage drop is a lesser concern. I do have to keep my track clean. The issues occur if/when there are any dark, grimy deposits on a track section or particularly on the rail surface at a joint. As long as my track is clean, things run very well, whether dc or dcc.
If I were running more units, maybe I’d see more of a difference, but a single unit or steamer will pull most of my trains.
My experience is that you should have no problem using 20 AWG on those staging tracks. I tend to use stranded 22 AWG (or did before Radio Shack closed up locally_ and have had no problems with it. I assume the grade is close to zero in staging, so little added load because of that. So long as you hook up eqch track to feed from a 18 AWG source, it will be fine.
I don’t want to disparage anyone’s formulas but they tend to be overkill for what we do as model railroaders. Such formulas are good for critical circuit design in order to precisely define what is needed so that circuits won’t be degraded by voltage drop nor will excess costs for overly heavy comp[onents be borne, which can confound certain aspects of a design.
In contrast our locomotives operate at a wide range of values and only rarely at full throttle. There’s almost always plenty of reserve power should it be needed. Yeah, some voltage drop may be present, but get out the hair splitter if you think it matters much.
The formula includes the factor of current draw - and where the charts come from and what people often do is look at their system capability, say 5 amps, and plug that in. But that’s the MAXIMUM current - on a given stretch of track, the maximum current would be determined by the current draw of the locomotives and how many can fit in the area. That number is usually significantly less than the max capability of the booster. This means the voltage drop over the same length of the same size wire is less.
Where this matters more is in the case of a short cause by derailment or some other reason. Then, the fully current of the system, or the current setting of any circuit breaker (if your track section has a circuit breaker set to 3.5 amps between it and the booster, you’re not going to get the 5 or 10 amps of the booster, you are going to get a max of 3.5 amps), flows through that wire. If the wiring introduces too much resistance, then the current may never reach the trip level of the breaker. A breaker set at 3.5 amps will allow 3 amps all day long without tripping. 3 amps at 15 volts is 45 watts, a significant amount of heat. This is why the quarter test is important - you set (do not press) a quarter, or similar size coin if outside the US, on the rails all over the layout. In each case the breaker should trip. If it does not, you have inadequate feeders to the area. If it does always trip the breaker, you cna be reasonably confident that any derailment or other issue will also cause the breaker to trip, instead of flowing considerable current through the ‘short’ which isn’t 0 ohms.
And the idea is to have multiple feeders to a given length of rail - so under normal circumstances, you may have 2 sets or more of feeders powering the track a loco is setting one, so your feeders may be 2x #22, instead of just that single length of #22.
All the feeders are approx 11 inches long of 20 AWG from the track down to under the staging yard. Those feeders on each staging track are tied together with longer 20 AWG wires of a distance of less than 8 feet to 18 AWG sub-bus. Any distance longer than that is tied to the short 20 AWG drops with 18 AWG.
Each staging track will be tied-in via connectors to an 18 AWG wire to a on-off switch which will tie them all to a 14 AWG bus. The bus will go from a PSX1 breaker to the booster or command station.
I just wanted to make sure what radiates out from the 18 AWG to the tracks using 20 AWG are not too long so there isn’t a significant drop in voltage at 3 amps or less. From what Overmod commented above, the distance of 20 AWG should be ok if it is less than 8 feet.
Online voltage drop calculator. I set it for copper, pick wire size, 12 volts, DC, single conductor. I put in .25 amps for a modern loco, is that correct? At that current, even 18 gauge can go far without a big drop. There’s lots of technical info lower down. -Rob
I do think that calculator is assuming a single wire - so for your complete circuit, you need to double the length entered to get the real value. But yes, at a mere 1/4 amp, the voltage drop of even #18 is neglible over a very long distance. If you are only ever running 1 loco on your layout, you can make such assumptions. But if you run more - and more importantly, if there is a short, you need to take into account the full current capacity of your DCC system, or the circuit breaker you have feeding that section of track. Too small a wire can add enough resistence that even a pair of pliers across the rails doesn’t draw enough current to shut down the booster.
If you are using a system that puts out say 2.5 amps maximum, it’s pointless to do the calculations with 5 amps as the load - it will never get that high. Or using more than 5 amps if you have a 5 amp booster, etc.