3 Power feed ============ 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 ??? 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 will confess that I have my power switching relays next to the throttles and run wiring out to each track, but if I were starting again I would put a relay card under each baseboard area. Cable types 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 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*2 * 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.