Lighting contactors are available up to 400a and diffinite purpose contactors shoulde be also. I'm assuming you're talking about 480vac. Usually when one inquires about a product one includes the voltage rating of the device in question, the numer of poles, the type of load, latching of magnetically held, coil voltage as will a the current requirement. Otherwise, should an answer be provided based upon limited information the answer is likely to be incorrect for the actual application.
Whoa...you're trying to build your own transfer switch??!! Whatever you build is not going to be listed for the application, and won't be as relaible as a purpose-built transfer switch. This is not something you want to mess around with! Besides, you will have a hell of a time finding a double throw, 250A relay!
FYI, relay contacts are usually limited to 10A and sometimes as high as 20A. Above that current rating the devices are usually called contactors. Normally closed contacts are not usually available above 100A.
For 300A non-life-safety semi-automatic transfer switches some fire truck manufactures use mechanically interlocked 400A contactors, one coil wired to the on-board generator and the other coil wired to shore power.
For home-built applications stick with a listed commercial transfer switch especially if you need engine start and exercise functions.
The service is 225 amps for the house. The generator/transfer switch are rated at 100 amps. Generator controlled circuit breakers will drop enough circuits in the house to keep the generator happy.
It sounds like a contactor rated for 225 amps would work to disconnect the mains service before the generator fires up, then let the transfer switch relay dump the generator to the house feeders once the warmup period has elapsed. I just thought an external relay (or contactor) rated for the full normal service current would negate the need for two separate electrical items.
The transfer switch has all the bell and whistles, it just isn't built into the generator as some are. I'm well aware of the safety issues! Thanks for the response!
Sounds like what you are trying to accomplish with individual components is classic 'over-engineering'.
A manufactured transfer switch, automatic, or perhaps manual would be the 'preferred method'.
Your statement "The service is 225 amps for the house. The generator/transfer switch are rated at 100 amps. Generator controlled circuit breakers will drop enough circuits in the house to keep the generator happy."
Please explain what a 'generator controlled circuit breaker' is??
I am afraid to think of what could go wrong in the possible reconnection of the POCO and your generator due to relay contacting at the wrong time. Unless you have a fool proof make before break circuit, I would not want the liability. Also what will you be doing with the neutrals?
Mark Heller "Well - I oughta....." -Jackie Gleason