According to Grant Erwin :
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?

It sounds as though you are applying power between one end of
the winding and the wiper controlled by the knob.

The 20A Variac (if a genuine General Radio Variac, and not some
other maker) is likely to have several taps, and may even have windings
good for 240 VAC input.

You connect the input power to a pair of terminals according to
the voltage you have to feed it and the markings on the terminal plate.
(Typically a 0V at one end of the winding, and a 120V somewhere on the
way up from there. There may be a 140V terminal even beyond that, which
means that you can get overvoltage if you connect to the 120V terminal
and run the wiper past that.

The *output* (load) should be connected between the 0V terminal
and the wiper. Never connect the wiper to the input power.

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.

With no load connected to the wiper, it should not pop the
breaker at *any* setting of the wiper. I've used these things for many
years -- Variacs, Powerstats, and other brands mixed in. *None* of them
have ever exhibited the behaviour which you have observed, but then I
never connected input power to the wiper either. I strongly believe
that you have input power connected to the wiper. And if so, with the
wiper set at 30%, and 120V input, you will probably have 360V (or more)
on the high end of the coil -- for a moment before the breaker pops.

Good Luck,
DoN.

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