I power my entire layout with one z4000 and one TIU. Its about 400 ft of total track, all tubular. The two handles are set at 17-18 volts and there are small voltage changes happening continuously as trains transition from section to section. The ammeters also show changes from output to output as the trains move around the layout. All completely normal.
The amperage loads used to vary in a big way running passenger trains with all incandescent lighting around the layout. When two pax trains happened to be on track sections powered by the same handle, you could easily see loads of 4-6 amps, and higher. Lashups also cause big amp loadings. This causes big sags in the output voltage. I have now converted several pax trains over to led lighting which has made a huge reduction in loads. Piece of cake now. I rarely see a load exceeding 3-4 amps total.
All sidings and storage tracks are switched, so there are few if any engines powered up at any time, except the 2 or 3 that are operating. Continuously powered sidings are no big deal for PS2/3 and Legacy engines, as they will stay dark and silent until started up. But tmcc engines will have most lights powered up anytime there is track voltage, which I personally don't care for. Just me.
I am a big fan of the z4000, but I wish the outputs had been designed to have regulated voltage. Then once set the output would stay steady, regardless of load. But that would have been more expensive of course, so here we are! 
Rod
