I am not aware of a specific reference source, but I have dealt with several transformer and generator pads. I pay particular attention to the critical section of the slab near the line(s) of equipment bearing on the pad. This is where I have seen most pads fail (not enough reinforcement to resist shear or cantilever action of the slab projection). Most equipment, such as tranformers and generators, do not produce even loading on the pad and consideration must be given to the underlying soils (elastic foundation). In other words, watch out for diffential 'settlement' and subsequent failure conditions due to the specific loading (crictical with gen-sets). Check with the equipment supplier on the equipment framing; pre-packaged booster pumps, generators, pump stations, etc, come on skids that better distibute their loading to the slab. Also, I believe Cat provides some basic slab designs for their generators.
For reasons beyond my understanding, most contractors do not exercise the care on equipment pad construction that they do on 'occupiable' slabs. Probably, because the labor constructing the slabs are mechancial equipment contractors and not flat work contractors. I always make sure to annotate everything (especially compaction and bearing capacity) to an excruciating level of detail.
If you have some more specific information on the loading, it may prove helpful in soliciting the input of others.