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.

Are you asking about indoor or outdoor equipment?  And can you be more specific on the type of equipment being supported?  Mass seems to be a primary factor for pads supporting vibrating equipment.  Settlement/movement can be a controlling factor for equipment with rigid connection (like gas lines, conduits or rigid electrical bus).  

If you are dealing with electrical transformer pads, most utilities have their own standard pad details.  They are usually very particular regarding their requirements for a transformer pad.  Try them.

For other mechancial equipment, the manufacturers usually have some basic requires for size and anchor bolts.  They then leave the final design to the engineer, so that local soil conditions can be taken into account.  So, the basic answer is, each pad is a separate case.

I usually approach it like I would a spot footing for a building column, much as RG88 mentioned.