The "miniature" zip ties, the ones with a cross section of about
1/16" x 3/32", have a "loop tensile strength" of 18 pounds.
The next size up, "intermediate" size, is 40 pounds. The
"standard" size is 50 lb. See here for actual tie dimensions:
3M Zip Tie Data
I have used these for many years for securing plywood parts in
"stitch and glue" small boat building process. If the little ones
break, just drill more holes, use more ties and close them up a
little at a time so that no single tie sees the whole load. The
alternate of course is to enlarge the hole and go to the next
larger size tie. All three sizes are generally available at Home
Despot, Lowes, etc.
This is when securing a roughly circular cross section wire bundle.
There is a significantly lowered value when the tie goes around hard
edged corners, essentially an arc of zero radius.
So, give your corners the largest radius that you can.
NEMA