Grant Erwin wrote:
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

it sounds like you have a short, or have the autoformer wired wrong.

The slider connects to the output side. The input goes across the windings
of the toroid, either at the ends for a 0 to 100% transformer or one end
and a tap for the 0 to 140% ones.

The slider should have no effect on the transformer at no load otherwise.