Is the Generator Outside (earth)? Inside (on concrete?)
Any Local requirements regarding Seismic and the attatchment of the pad to an
existing/new concrete slab?
Do you need the pad to be re-inforced (ie: Re-Bar)?
But given all that, you would be better off getting a Local Structural
Engineer to do the design of the pad as there is the weight of the generator
to also consider (12-inches is a thick pad by any means - but if you are
talking 1500 Kw generator... may not be enough).
I'd call for chamfered edges (personal tast).
And if you have Seismic considerations, then there is a minimum amount of area
around the physical dimension of the genrator base to allow proper seismic
bolting down of the generator.
Other than that, I'm also a bit unclear to exactly what you are asking. As
far as a plan view is concerned, a rectangle with a note to it informing the
contractor that a concrete pad is required is usually what we do here
(Vancouver, Canada). We have in our Spec that the Contractor is responsible
to have a local Seismic Engineer design the pad. We just call for that one is
required under the contract.