I have a 20 amp variac. Yesterday I tried plugging it into a 20A circuit with
its output voltage set to 30% of input voltage, and it (consistently) popped
the breaker. When I set it to 100% of output voltage, however, it didn't pop
the breaker. Obviously the magnetizing current is higher with the variac turned
down. The question is why? What's going on here?

I believe the variac and my electrical circuits are both in good working
condition, in other words I don't believe this is faulty behavior.

Grant Erwin
Kirkland, Washington