Something isn't right. Can someone advise me on this.
Ed = (12*I*L*2)/CMA

Ed is allowable voltage drop
I is amps
L is length in feet
and CMA is Circular Mils

In this case:
I = 13.3 (8 20 watt bulbs 12Volt)
L = 40'
CMA is 6530 for #12

This means that Ed is about 16%(1.955 volts).

I know from experience we have put many out door lights on a transformer and have had no problems (watts ? can't remember). What gives?