Trains4Africa
Because all Boys (and some girls) love Trains

TRACK WIRING BASICS

Basic track wiring (applies equally to DC and DCC layouts).

How should I connect power to the tracks?

  • Special plugs. Toy train sets and some makes of track offer track pieces which provide wiring access. If such a connector is visible it might be considered unsuitable for a sceniced layout.

  • Soldered. Most railway modellers prefer to solder wires to the track to supply power. Options include:
  • Wires soldered to the sides of track. This is quite common and has the advantage that it can be done after the track is layed, however it does produce rather unsightly blobs of solder. If you want to go this route, remember to solder the wires on the outside surfaces, otherwise the solder will obstruct the wheel flanges.
  • Wires soldered to the underside of the fish plates. This can be practically invisible and is easiest if using flexitrack so that the fishplates are not welded to the tracks. When one piece of track is in position and the next is ready to be placed, drill small holes in the baseboard where the fishplates will be, then gripping a metal fishplate in pliers, solder a wire to it. Feed the wire through the hole and push onto the track.
  • Wires soldered to the underside of the track. Potentially more reliable than soldering to fishplates (see below), but has the hazard that the heat will damage the sleepers and webbing.
  • Wires spot welded to the track. I have yet to experiment with this strategy, but in theory discharging sufficient energy from capacitors should be similar to electric arc welding, producing a solid and reliable joint, with minimal danger of a dry joint and very little chance of producing enough heat to damage the plastic sleepers and webbing.
  • How many feed wires should I use?
  • One only, or one per loop. This is the toy train set approach. It relies upon the fishplates for section to section connection, and upon the turnouts to feed power where required. Turnouts are notoriously unreliable for routing electrical power – just a tiny dust particle can destroy the connection, especially in the smaller scales (N & Z), but is still not totally reliable in 00 and larger. Fishplates are a serious problem if you ballast your track by the classic method of flooding with PVA adhesive as the PVA is drawn into the fishplate to track junction by capillary action. Even without such ballasting the connections become unreliable over time, especially is temperature is not constant (the metals are always expanding and contracting).
  • One per route. Each stretch of track between turnouts is fed with its own power, which could be switched by an auxiliary contact on the turnout. Insulating fishplates are used to separate the tracks from the diverging end of the turnout. The unreliability of the contact between switch-blades and stock rail (the moving and fixed rails that make contact in the turnout) is mitigated, but power feed to the switch blade and frog (where the rails cross) upto the insulating fishplate still relies on the switch blade contact.
  • One per piece of track. If maximum reliability is required then a wire attached to each piece of track, and then commoned under the baseboard must be the best solution, at the cost of rather more effort during track laying. It could be argued that this is a waste of time unless turnout unreliability is addressed first.

How to route power to your tracks

There are many ways to feed power to your tracks. With classic switched control panels the natural method is to run wires from your throttle(s) to the switches on the control panel and then a wire from each power switch to its associated track section. This naturally uses a lot of wire, and makes lots of thick cable bundles to route around.

An alternative is to run a ‘ring main’ from each throttle around your layout and then route power as required to each track section with relays. If the wire from each throttle runs around the whole layout and then back to the throttle (if it makes sense in your installation) then you can use thinner wire because current can flow both ways around the loop. The relays are of course controlled from the switches on the control panel, but to save wiring you can use RPC modules, with either a PC or a Merg RPC PTP module to drive the relays from the switch settings. This way you have far less wiring, and if you want to add an extra track you only have to connect an extra relay to the appropriate ring main(s).

I do something different again. I use one remote controlled throttle per track section. It sounds expensive but my QTU (Quad Throttle Unit) module works out quite cheaply. So I put one or more QTUs under each baseboard to feed all the track on that board. I then run a ring main of low voltage AC around the layout to feed all the QTUs. The power output from each QTU feeds everything else on its baseboard such as infra-red sensors, solenoid and slow action turnout motors, lamps. The only long-distance wiring I have is the AC power feed and an RS485 data cable. The only inter-baseboard wiring is for tracks that cross baseboard boundaries.

What wire should I use?

  • Wherever a wire might be flexed, even slightly, use multi-strand. So feed wires that connect to the tracks are ideally multi-strand as the tracks might move during and work on the layout, and also as temperatures change – railway tracks expand and contract considerably with temperatures and this can be a serious problem in attics where nights are cold and summer days very hot. All connectors not solidly fixed to baseboards should use multi-strand.
  • You can use single strand wire on fixed layouts – where the layout is not is modules or sections. Ensure that the wire is securely tied down to avoid stress when work is carried out on the layout. It is quite possible that a small modification can move wires and if they were partially fractured, or joints imperfect (such as dry joints) then faults in other wiring can occur.
  • Personally I like to use grey ribbon cable for my N scale layout, and would also use it for 00 provided the wiring runs are not too long.

How thick must my wires be?

  • For individual wires (those not in an insulated multi core cable) you can put several times their rated current through them safely.
  • From a safety perspective the key issue is heat, not current. It is heat which can start a fire. Wires not closely enclosed dissipate heat extremely efficiently and so do not heat up anywhere near to ignition point. If you use multi-core cables, or like to tie large bundles of wires into neat wiring looms then use thicker wire.
  • Track power requires cables that will not drop too many volts. A drop of 1 volt is the maximum you should be prepared to accept, but there is little reason to demand as low as 0.25V. Aim at an average section dropping half a volt, but tolerate 1V on the longest runs with double headed trains.

The volt drop depends upon three parameters:

  1. The current being drawn
  2. The length of the wire
  3. The thickness of the wire

The current varies a little between locos, and a little with speed, but mainly it is the scale you are using that defines current. Some starting point assumptions might look like:

Scale Slow speed Full speed Stalled

Z  80mA   250mA  400mA
N  100mA  300mA  600mA
OO 150mA  400mA  800mA
G  400mA  800mA  1200mA

but these are all approximate and will vary between locos, track and throttles.

You do NOT need to design for stall current, except to ensure that your wires will not melt – do not worry about this even ribbon cable will not melt on a stalled G scale loco.

The length of each run is obviously a function of your particular installation and I cannot give you any guidance at all on this one.

The other variable is obviously the thickness of the wire. Thicker wire costs more, is harder to install and make much bigger bundles. Think carefully before jumping in with wire that is thicker than you really need.

To calculate what thickness of wire you need:

  • V is the acceptable volt drop you are prepared to tolerate.
    Start with half a volt. If you don’t know then try an experiment with a throttle, some thin wire (rated at less than your loco consumes), a loco and a voltmeter.
    See how much volt drop you can insert (by adding more and more thin wire) before loco behaviour degrades. Pick a volt drop perhaps half of the drop where you can actually notice a performance change.
  • I is the current you need to handle, in amps.
    Take the figure for full speed running (not the stall figure). Double it if you plan on double heading (but on the other hand does it matter if you get more volt drop with a double-headed train).
    You could try measuring the running current taken by your lococs. Do NOT measure the motor resistance to calculate current – this does not work.
  • L is the length (in metres) of the average wire from throttle to track. Allow exceptionally long runs to have a little more volt drop than is ideal.
  • R is the maximum resistance of the cable run, calculated from: R = V/I
    eg. 0.5V with 400mA (OO scale loco) gives R = 0.5/0.4 = 1.25 ohms

r is the resistivity (in ohms per meter) of the cable you need: r = R/L
eg. 10m cable gives r = 1.25/10 = 0.125 ohms per meter.

Now pick a cable or wire with that resistivity or less.

Solid core

diameter AWG ohms per meter

1/0.52    24    0.094
1/0.55          0.070

Multicore

wire type sq.mm ohms per meter
7/0.1    0.055  0.384
7/0.13   0.08   0.227 (ribbon cable)
7/0.2    0.22   0.092 (ribbon cable)
19/0.127 0.25   0.0836
19/0.15  0.35   0.0561
19/0.19  0.5    0.0401
19/0.25  0.933  0.0212

I use ribbon cable on N scale and so I need 300mA and my runs are typically 6m.

Working the other way around my volt drop would be:

V = I*R = I * r * L = 0.3 * 0.227 * 6 = 0.4V

Which I consider perfectly acceptable.

Of course if you were to use a “ring-main” of ribbon cable, then you could clamp on a connector at any point to tap off power to a relay board. The resistance would then be halved (worst case). For example a layout in a room 3m by 4m might use a ring-main 14m long (the circumference of the room). The worst-case run from throttle to track is two 7m lengths in parallel. If you were using some double headed trains in OO scale then the volt-drop would be:

V = I*I * r * L/2 = 0.4*2 * 0.227 * 7/2 = 0.635V

which might be considered acceptable for the double headed trains.

Of course garden railways need much thicker cable. The currents are larger and the cable runs much longer. Suppose a typical length was 20m and we were double-heading and so wanted to supply 1.6A, but accepted a full 1V drop (G scale throttles have more volts to play with than OO throttles) then:

r = V / I / L = 1 / 1.6 / 20 = 0.031 ohms per meter and so you might need a 19/0.25 or 32/0.2 wire.