Interesting enough, many fire departments won't allow roof mounted
generators anymore because storing fuel along with it is considered
hazardous. We had an awful time getting our 250 kw generator installed
and ended up with it on the ground level next to the parking garage
with an integral fuel storage tank; it was the easiest to get permitted.
At PAIX they put the generator on the roof and the fuel tank in the basement.
The fuel pump has its own UPS and it's also on the building UPS. It takes
a minute or two to pressurize the fuel line, but the UPSs could run the
whole kaboodle for a lot longer than the generator's startup time.
The permits would have been a huge problem had this building not once housed
the Pac Bell CO for Palo Alto. But all we had to do was more or less exactly
what they had done and the city agreed that if it had been OK for Pac Bell to
do it, it was probably OK for Digital to do the same things.