TimR wrote:
The baseboard heat in one of the bedrooms trips the breaker as soon as
I turn the thermostat on. We didn't know this until now because it
didn't get cold enough to need it until now.

But wait, there's more. The house (we just moved) had fuses until the
panel was replaced with a modern looking circuit breaker panel. We
tested that heater on inspection but that was while it was still on
fuses. I never thought to try it with the new panel.

The electrician says most likely bad thermostat, he'll come back and
look when he can schedule it, may be a while.

If so, I'd like to know what in a thermostat goes bad like that. I'm
not sure how they're wired inside.

I put a meter on the thermostat. The stat has four terminals, two
marked line and two marked load. With breaker off and stat off, I
read 28 ohms across load. That sounds reasonable, it should pull 8.6
amps if my math is correct, and the 20 A breaker should be fine.
Breaker off and stat on, 26.7 ohms appears across both load terminals
and line terminals, that sounds okay to me too, .8 and .7 ohms from
line to load across the contacts.

With the power on, I read 242 VAC across line, 0 across load, 120 from
each line terminal to load terminal, 120 from each line terminal to
ground. I didn't expect that. I thought a thermostat this old would
be single pole, and so one load terminal would read zero. That's with
the thermostat turned off, if I turn it on it trips the breaker
immediately. Can this be a double pole stat? or am I just
misunderstanding the readings?

Curious symptom: with the breaker off, I still read 1.12 VAC across
line, 2.66 from line to one load terminal, 1.48 to another. It's a
radio shack DMM, and I've always assumed low voltage readings like
that were due to some kind of capacitative coupling, not real. Maybe
not?

Anyway, heater sounds okay at 28 ohms, so it should be the thermostat
or the breaker. But there would have to be a dead short in it to trip
the breaker immediately, shouldn't that show up on the resistance
readings?

Any more diagnostic suggestions? What else to try? I'm still
suspicious of the breaker, that's the only thing known to have
changed.
Hi,
Be logical. If it worked with fuse OK and now breaker trips wouldn't it
mean the breaker is under rated? What is in there now?