The requirement is from your local power company.
This part of CA, PG&E requires a minimum of 1.25" IMC or RMC for overhead service if the service will be attached to the mast. The size will also depends on conduit fill.
I learned the hard way that SMUD's (Sacramento Muni. Utility District) has a 1 1/2" min size, found out before it was installed but was a pain....
