According to :
I have a 200 Watt transformer running 35' of 16 gauge cable to 7
lights of 101 Watts(total), then it is split off to another 25' (16
gauge) for 4 more lights of 40 Watts. The first 7 lights always work
fine, after the split my 4 other lights work for about a minute then
turn off. I checked the split with a DVM and I'm getting only 10
Volts at the split. Why the voltage drop?? I've change the split
twice (butt splice) and it doesn't fix it... I've also try another
transformer with the same results... Seems to me the cable is causing
the voltage drop.... Any ideas? Thanks..

Copper wire has a resistance to current flow.

If you placed a 12A load (140W) on the end of 60' of 16ga wire,
the voltage will drop 5.8V. Given that the lights are distributed
across the wire, it won't be that much. But it'll still be
significant.

16ga is too light for this load and distance. One way to fix this
would be to run a new wire from the transformer to the second batch
of 4 lights. Since you've split the load, you could use 16ga
for the new stretch.

Most of the time I use 14ga for this reason.

The "going off" of the second string very much points to a defective
wire or connector at the joint, nothing to do with the transformer.
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.