About a home-made solution, there is a current sensing relay that has an adjustable range. It's a small current transformer that slips over one conductor and has contacts built in; and, I'm pretty sure that you can find a normally closed contactor with a high enough rating for the non-preferred load. I've used these relays in condos to control exhaust fans when the dryer starts and to monitor exhaust fans in commercial kitchens. The set points aren't real accurate, but they have a range between 5 and 40 amps. I checked availability a couple weeks ago and, even here, they are only a day away. So, I assume they are everywhere.
If your power supply has wide fluctuations, you might consider adding in a timer so, when the relay cuts in, it holds the non-preferred load off for several seconds to prevent contact chatter.
