I think the answer depends on how you are starting the 15HP idler. The
issue isn't the fact that the motor says "15HP" on it, the issue is how
much current does it draw when starting. If you're using capacitor
start, you get a pretty large surge current, related to capacitor size,
not HP itself. I know the caps are sized based on HP, but I'm trying to
explain what the real issue is.

In your situation I'd have the 10HP contactor between the 15HP idler and
the compressor. This way you start the idler and let it run. The
compressor would start via its contactor exactly as if it were on power
company 3 phase (assuming your third leg doesn't sag...).

Steve

gtslabs wrote:

I finally got a 10 hp air compressor running. I have a 10 hp rated
contactor for it. But my Idler motor for my phase converter is 15 hp.
I have used a momentary switch in the past to start it but I was
reading that a contactor is preferred. Since I am starting the 15 hp
converter first and it is only idling do I need a 15 hp rated
contactor? Or since it will eventially see the additional 10hp
compressor (once the converter is running) do I need a 25 hp rated
contactor?

I want my converter to cycle on and off outside my compressor range so
I was looking at some type of relay and protecting the motors.