We have about the same size layout mine is10x12 around the wall
thats 44 ft of main line I run 6 locos in 3 consists of 2 each
one sound in each consist Using a Zephyr and two seperate UT-4s with no problems
The difference is i use 14 gauge buss and 18 gauge drop wires every 4 ft
when i did the wireing i checked the voltage and found only one siding
that had a drop due to a loose joiner
I’m with the others I really don’t think your Zephyr is the problem