Whole house generator for 400A Service

[robbione]

As a resi contractor for mostly large custom homes, I am frequently asked to provide whole house generator protection for a 400 amp or larger service.
08 code requires the generator to be sized for the full load unless there is a load management system employed.
I know generac makes a 200Amp transfer switch with built-in load shed capability, but Generac tells me that you can only use 1 of these in a system, not 2 or more. If a load calc on the house is 350Amps, I am looking at a 100KW generator. The GC and the homeowner think I'm crazy - the price is more than the wiring for the house, and of course their old house was run off a 45KW for years with no problems.
What other design options can be implemented to acheive the load management? I was thinking about some kind of relay between the 2 transfer switches so that one kicks on automatically but the owner has to flip a switch to allow the other one to come on? Are there other generator systems that have solved this load shed problem?

 

[Cavie]

You can transfer the load the generator will carry automaticly and the home owner will have to do the rest. If he overloads it it's his problem.

 

[tkb]

Put only the loads you want to back up into their own panel and use the ATS for that only.

 

[Robert Hutcherson]

I would perform a service demand calc instead of sizing to the existing service main. You could use a manual transfer switch and provide the owner a procedure for load management.

 

[iwire]

You can transfer the load the generator will carry automaticly and the home owner will have to do the rest. If he overloads it it's his problem.

There is a code change that does not allow that if it is a automatic system.

 

[iwire]

Put only the loads you want to back up into their own panel and use the ATS for that only.

Now that sounds OK as long as the owners can deal with it.

 

[charlie b]

. . . but Generac tells me that you can only use 1 of these in a system, not 2 or more.

Why? And how did they tell you that (i.e., is it part of their listing instructions, or did you get that from some sales agent)? Put a pair of 200 amp breakers at the generator output, and use each one to serve a 200 amp transfer switch, and put half the house loads on each transfer switch. I do this all the time with much larger generator systems. A building needs to have separate transfer switches for the emergency, legally required standby, and optional standby loads. But in addition, nothing stops us from having two or more transfer switches for any one of those branches.

I would push harder to learn the basis for the "only one" restriction. My suspicion is that you will find that there is no basis.