baseboard heater trips breaker - Bad stat?

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For the same reason there is no pump on most sprinkler systems. For fire protection in buildings. If there is a need, the fire department comes, and brings a pump. In your case, you're the electrician, and you bring the ammeter.
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wrote:

What you are forgetting, however, is the cold resistance will be the lowest resistance and therefore the highest current. The Ohmeter will give you "worst case".
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Bypass the stat with jumper wires. If the breaker trips, it is not the stat. If the heater heats properly, it may be the stat. A line voltage thermostat is nothing buy a heat operated switch. While it may be bad, why not do the simple diagnostic with a couple of short wires and wire nuts?
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*I would go to the heater and disconnect the feed wires and cap them off. Then go turn on the thermostat and circuit breaker and see what happens. If the breaker doesn't trip replace the heater. If it still trips then go remove the thermostat, connect the wires together and reconnect the heater and see what happens. If the breaker still trips then you've got a problem somewhere else on the circuit.
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Just went to a known good thermostat/heater combination in another room, and took the same meter readings.
And got the same results. 240 across line, 0 across load, 120 from each line to load terminal. Had the stat make, then 240 across line, 240 across load, 0 from line to load terminals.
What that tells me is the meter readings in the bad room are not diagnostic of a grounded element because a good one reads the same, but logically it seems a likely place to start.
Except for the fact that it worked fine before the panel was changed, and doesn't work now. And going around the house i found two more heaters that don't work, though they don't trip anything. Seems more and more likely the installer did something wrong, but I'm not sure what it would be. you don't touch the stat to change out the main panel.
It's time to turn this over to a pro and see what he comes up with.
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TimR wrote:

Is there a pilot light either on the t-stat or wall heater? If there is a 120V light, (or any other 120V device), on the load side to neutral then that would make the meter read 120V between a hot terminal and a load terminal when the t-stat switch is off. I would still check for short to ground/neutral in the failing unit. Ever find a label to tell brand model# and ratings?
Kevin
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Are the 20A breakers you are talking about 2 pole 20's? Assuming 2 wire with ground was the white wire accidently hooked up to be a neutral in stead of the second hot? Just asking because we just wired up some 240v heaters with 12/2. Used 2 pole 20A breakers and marked the white as hot. I would think it would have to be wires crossed some how in the new panel.
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wrote:

Was the panel installed by a qualified electrician?
If not, he likely mixed up some wiring, puttingboth sides of the heater on the same side of the panelor something stupid like that. Replacing a panel is a job for an expert, or at least someone who understands how it is SUPPOSED to be done and is smart enough to figure out how to make it work the way it is supposed to.
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I don't know how expert the guy was. But I got him back out to look at his work.
He pulled the power wire off the heater, the breaker still tripped. So not the heater.
Then he pulled the load wire off the thermostat. It no longer tripped. So not the thermostat. So I have a short between the thermostat and the heater, and it's going to be really difficult to get to. But at least I know the next step.
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Uh-huh. Did you ever check the rating plate on the heater?
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On Tue, 25 Nov 2008 23:05:10 GMT, snipped-for-privacy@milmac.com (Doug Miller) wrote:

Not necessary. With the heater disconnected it still trips. Sounds like a mail through the Romex somewhere.
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Never let your mail man install heaters.
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wrote:

He doesn't have the heater disconnected yet. Not all the way. He says the "electrician" pulled "the power wire" off the 'stat. But it's a 240V circuit. There are *two* power wires. Only one of them is disconnected.

Or a 120V heater connected to a 240V circuit -- which a quick look at the rating plate would confirm or disprove. Think about it: we already know it's a 240V circuit. If the heater is 120V, and the neutral side of it is bonded to the chassis -- entirely possible for original equipment in a home built in 1960 -- then when *either* leg of the circuit is connected to the heater, the breaker trips.
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On Nov 25, 6:05pm, snipped-for-privacy@milmac.com (Doug Miller) wrote:

Well, no, I didn't. There's not one in an obvious location.
But do you realistically think this could be the problem?
These aren't plug in portable heaters. They are installed baseboard heat, have been in place since the house was built c. 1960. They all appear to be the identical make and model. I've put a meter on several thermostats, ones that work and ones that don't. All have 240 volt power. All the thermostats are rated for 240.
Do they even make a 110 V model? I guess it's possible but I've certainly never seen one.
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Keep checking until you find one.

Yes, I do, as I've explained several times. If you have a 120V heater, original to a home that's nearly 50 years old, it's entirely possible, maybe even likely, that the neutral (cold) side of that heater is bonded to the chassis -- a home that old is likely to have been wired with 2-conductor Romex *without* a separate grounding conductor.
What do you suppose happens when a grounded 240V circuit is connected to something like that?
In case it's not instantly clear, I'll explain.
One hot leg of the 240 is connected to the hot side of the heater. The other hot leg is connected to the cold side -- where there's *supposed* to be a neutral conductor, not another hot. The grounding conductor of the 240V circuit is connected to the chassis of the heater. And that makes a dead short to ground from the second hot leg.

But, without finding the rating plate, you don't know, do you?

That would argue against this particular one being 120V, I admit, but it's still possible. Maybe even possible that the same unit can be used as either 120 or 240, by changing the posistion of an internal jumper wire.

There are *many* 120V baseboard heaters.
Bottom line is that you need a competent, qualified electrician to look at this and find out what's wrong -- and that description clearly does not include either you or the guy you've had working on it. You need to get somebody out there who knows what he's doing.
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