This is in section 620.51 of the NEC. If you have the handbook it has a good narative. It is required though by ASME A17.1 Rule 2.8.2.3....
Hope this helps...
I work for a small MEP firm that mainly does tenant finish out work and remodels and electrical loads are straight foward to calculate for these. In the future we have several shell building designs that we will be doing, retail, office, and light industrial. Many of the shell MEP plans I see...