Bjarney's chart is close. While I agree the
voltage drop is load dependent you have to
consider the fact that if you install a 20 amp circuit someone is going to try and plug in a 20 amp load. VD=2k*L*I/CM
where: VD= voltage drop
L= length of conductor
I= current in circuit
CM= area of conductor in circular mill
K= resistivity of the conductor- 11
for copper
Thus in this case:
VD=2(11)*150*20/16510=
a voltage drop of 3.99 volts for no.8 wire
A little over 3% for 120 volts or 3.19 volts
for an 80% load.
