I thought it might be fun to pose a motor branch-circuit calculation here, just to see how many different answers we come up with, and to see the different methods people use. Give it a whack if you have the time.
Lets say we have a rigid (RMC) conduit containing two 3PH motor circuits.
Motor #1:
25HP
3PH Code letter F
Squirrel-cage induction
Service factor 1.15
continuous duty
480 volt
Motor #2:
40HP
3PH code letter F
Squirrel-cage induction
Service factor 1.15
continuous duty
480 volt
The conduit is short, so we won't worry about voltage drop, but it does run through a boiler room where the ambient temperature is 110 degrees F. We are using THWN-2, 90 degree rated wire. Equipment temperature ratings are undetermined. The rigid conduit will be used as the equipment ground.
Calculate the minimum conductor size for each motor.
Calculate the maximum size dual element time delay fuse size for the short-circuit/ground fault protection.
Calculate the minimum size rigid conduit needed for these two circuits.
Extra points for showing your work.
