wire size for powering a 16 by 20 layout.

[lionelsoni]

Suppose that one control of one of your Zs is set to the lowest voltage, say 6 volts, and another control on the same transformer is set to 24 volts, and these outputs are connected to the center rails of two adjacent blocks.  When a train crosses the gap between blocks, the pickups connect these two transformer taps together, literally shorting out an 18-volt secondary winding.  Not only does that draw a very heavy current from the transformer winding, but none of that current passes through the transformer's circuit breaker.  There will be arcing and, if the train should stall, things will burn up.

Now of course you wouldn't deliberately run between blocks set at those extreme voltages.  But you could easily have several volts difference even if you intended them to be exactly the same.  And if you move one knob but forget to move the other, you could accidentally get much more voltage between the blocks.  I have a couple of transformers that I took from Zs that have burned sections on their secondaries; and I think I know how it happened.

The right way to handle this situation, I believe, is to make the blocks switchable among the transformer outputs.  The simplest way to do this is a single-pole-double-throw-center-off toggle switch for each block, to connect the block to either of two transformer outputs, or neither.  Then use these switches to assign any blocks that the train goes through to the same transformer output.  Note that the number of blocks is unlimited in this scheme.  For more than two transformer controls, you can use rotary switches or simple arrangements of pairs or even triplets of toggle switches.

[71835]

Thanks for the information.  In reading your post, I discovered that I don't know as much about electricity as I thought.  Could you expand on your concern about crossing between separatly powered blocks?

I use the z's to power the whole layout.  The 8 power taps provided by the 2 transformers allow me to use 2 to power two blocks for multiple train operation.  The other 6 taps are used to power switches and accessories, which are a mix of post war and modern lionel. Accessories with the same voltage requirements are powered from the same tap.

The transformers are in phase so I use a common ground linked to both transformer's ground taps.

What unwise thing have I done????

[lionelsoni]

For track power, your wire should be big enough to handle the greatest current that the transformer can put out, regardless of the layout size or the current that the trains normally draw.  The type Z has a 15-ampere circuit breaker, which means that you should use no smaller than 14 AWG wire.  It is big enough that, if a condition occurs that draws just less than 15 amperes, your wiring will not get dangerously hot.

This wire is also probably plenty big enough (2.5 milliohms per foot) for any feeders that you might need to run and is easy to find at home-improvement stores at reasonable prices, solid or stranded.

If you solder your track joints, you will probably find that you don't need very many feeder connections.  In fact, a complete loop with all joints soldered may need none at all.  The track itself is roughly the same resistance as 16 AWG, so the resistance to the farthest point of a 50 foot loop (about the longest you could put on a 12x16 table) is 50 milliohms, which will result in only 250 millivolts drop under the load of a heavy 5-ampere train.

Multiple-output Lionel transformers like the type Z have only a single circuit breaker.  This provides no protection at all for the fault current that can flow if you (deliberately or accidentally) run between separately powered blocks.  I recommend adding individual circuit breakers to the outputs to cure this shortcoming.  You can get suitable automatically resetting breakers at automotive parts stores for a couple of bucks.

It worries me that you may indeed intend to run between separately-powered blocks, since the 8 outputs of two type Zs are a bit of an overkill for the size of your layout otherwise.  Say it isn't so!

[71835]

I am expanding my 12 by 16 foot O layout by adding an additional 8 by 8 segment. It adds a second return loop, made room for a transfer table, and a few more siddings. Its all one level with track screwed to the plywood.  I also replaced most of my O22 post war switch tracks with K-line O42, most of the latest design, with only 4 of the old Marx based switches.  About half the sidings remain O22 post war. 

I would like advice on standard wire sizes for the main applications of powering track and the switch tracks (as I will power them independantly of track power).  I use 2  Lionel Z type transformers in phase, for all power,  track,  switches, and a bunch of accessories.  With the size of the layout, voltage drop can be a problem so advice on spacing of feeders would be helpful.

Finally, I use Lionel TMCC for remote control of motive power which is lionel post war or k-line, I do not yet own any TMCC locos but want to add some soon, so that is a consideration.

Any suggestions would be most welcome including pointing me to existing posts which address my situation.

Thanks

Greg