• I use heavy 12 gauge solid wire between my Z4000 Transformer and TIU and between the TIU and terminal block.  The is a bit of an overkill, but it is better to go smaller gauge (thicker) wire than the other way. 
  

• I use this mainly because I was able to buy this leftover wire for only $5 per roll!  You can get away using 14gauge and maybe 16gauge wire depending on the distance.
  

• The thicker the wire, the longer distance you can run the wire without lowering the voltage.  Thinner wire has more resistance causing the voltage to drop over the length and wire to absorb the energy which is dissipated as heat, causing your wire to get hot.  If you wires get warm or hot, you need to use smaller gauge or thicker wire.
  

• Also stranded is better for conducting or has lower resistance for the same gauge. 
  

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• The heavy or thick 12 gauge wire is used to transfer the voltage a longer distance without lowering the voltage. 
  

• At the end of the 12 gauge wire, smaller, more flexible wire is connected to ends to connect to track.
  

• The smaller wire can be 18 gauge.  Because it is used for a short distance, there is no worry about losing voltage.  It can't be too small however or it may heat up.  Use the thickest wire possible (recommend 18 gauge) that is still easy to solder to the track or connect to lockons.
  

• I Solder all my wires directly to the track.
  

• Also I solder ground to both sides wherever possible as a backup for broken solder joints.
  

Wiring power to track.

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