Furnace keeps tripping breaker

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Raindem

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Hi. I have an Arcoaire electric furnace, model no. MF20N2600A. It runs for about a minute, maybe less, and then trips the breaker. The unit itself has a 50a breaker. Then it runs to a box on the wall with a 40a breaker. At the main fuse panel it is controlled by a 100a breaker. It is the 100a at the main panel that is tripping.

I thought it was odd that the 100a trips but the others don't. So maybe the breaker was bad. I replaced it with a spare 50a that I had lying around and it still trips at the main fuse panel.

Are there any things a DIYer with a multimeter can troubleshoot here, or is it time to call an HVAC repairman?

Thanks

Curt

Edit: Forgot to mention that I've already changed the filter.
 
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nealtw

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So the 40 and 50 amp breakers are not tripping. there must be other breakers. so total draw of all of them is more than 100 but non are overloaded. Try turning other breakers off.
 

Wuzzat?

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Go with this /\
 
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Raindem

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It is the breaker at the fuse panel that is tripping. When I reset it the furnace will briefly run and trip it again after a minute.
 

Wuzzat?

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I thought it was odd that the 100a trips but the others don't. So maybe the breaker was bad. I replaced it with a spare 50a that I had lying around and it still trips at the main fuse panel.
At this point no one knows if the 100a is bad or the 50A is bad or good or it is something else.
I'd replace the 100a with a similar unit (but I don't like guessing) or buy/borrow a clampon ammeter for some logical troubleshooting.
What is the nameplate current draw for your unit?
 
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Sparky617

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Replacing a 100A with a 50A will definitely trip unless your total load is below 50A. I suspect your 100A breaker is bad, it happens, they can become weak over time. If it just started doing this that would be my educated guess. Your electric furnace is probably the largest load in your house.

Are you running an electric range, water heater, and a dryer at the same time? Those would be your other high load appliances. Do they all go through the 100A main breaker? At my house the HVAC units and electric stove outlet have breakers in the main panel below my electric meter outside. The sub-panel for everything else is in the garage and runs off of a 100A breaker under the meter.
 

Wuzzat?

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With the Arcoaire off, along with a clock you can use your electric meter as an ammeter and see what current is being drawn.

Your PoCo can supply you with the method, there's some number stamped on the meter that provides the conversion.

100A @ 240V gives 24 kw for an hour (24 kwh) and 0.4 kwh in one minute.
 

Raindem

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Thanks for the follow up responses. I thought the 50a breaker would be OK temporarily because the other breakers in the circuit were equal or less. I guess I just proved I'm not an electrician.

I think what makes sense at this point is to get an exact replacement for the 100a as someone suggested. And then go from there.

I should point out that this is a small furnace in my workshop, not in the main home. There was nothing else running at the time.
 

nealtw

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Thanks for the follow up responses. I thought the 50a breaker would be OK temporarily because the other breakers in the circuit were equal or less. I guess I just proved I'm not an electrician.

I think what makes sense at this point is to get an exact replacement for the 100a as someone suggested. And then go from there.

I should point out that this is a small furnace in my workshop, not in the main home. There was nothing else running at the time.
Is there anything else taking power from that 100 amp breaker?
 

Raindem

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Not that I know of. The fuse panel was mis-labeled and had that breaker as going to a 220V outlet. But I got that sorted out. With the 100a turned off everything else in the shop appears to still be working.
 

Wuzzat?

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A bad connection from panel to CB can heat a breaker and cause it to trip.
Unfortunately, there is no easy way to trace this with a VOM because the points you'd have to probe are inaccessible.

This symptom doesn't make sense, even if you look up the trip curves for the breakers and try to find some fault that snakes past the 50A & the 40A trip curves and opens the 100A, unless there is a steady high-current draw below 100A, as was suggested.
Using this generic curve
http://static.schneider-electric.us/docs/Circuit Protection/Molded Case Circuit Breakers/0100-400 A Frame FA-LA/FA-FC-FH/0600DB0105.pdf
for a 1 minute trip time for a 100A breaker you'd be pulling 150 to 250A. This is like the draw for a whole house.

When you make something out of wood, you measure as closely as possible because the accuracy can only degrade from there.
With electrical projects, the voltage & current are invisible, so it is the certainty as to the root cause that gets worse as you get in deeper, and so measuring the current with a known good meter is a good start.
Otherwise you start off with guesses and more guesses are stacked on top of this initial uncertainty.

Good luck.
 
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Raindem

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Just a quick update. I replaced the 100a breaker and ran it for about 20 minutes with no problem. Will keep my fingers crossed. Thanks for all the help.

Curt
 

Wuzzat?

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Just a quick update. I replaced the 100a breaker and ran it for about 20 minutes with no problem. Will keep my fingers crossed. Thanks for all the help.

Curt
So it seems the old 100A breaker was tripping at less than 40A, either due to an internal problem or a loose connection to the panel.
And the 50A CB that you used temporarily was tripping at less than 40A.
It now seems very likely, but not certain, that both of them are bad.
 

nealtw

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If the breaker is there to protect the wire. Why is it the wire is the last thing that is looked at when a breaker wants to trip all the time.
 

nealtw

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The 100 could be faster acting than the others. Have you checked for a short in the element. Have you turned off all the breakers and checked continuity between power and ground to see if there is a short in the wire.
 

Raindem

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I haven't done any of those other tests yet. I was planning to when I started this thread.

I'm pretty sure the 100a was in fact bad. After a couple days of trying to initially troubleshoot the problem the breaker could not even be flipped back into the "ON" position. It doesn't mean there might not be other problems in the system. But if this fixes it then great.
 

Wuzzat?

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What sometimes happens is that the problem returns after a few months. The new breaker can then still be good or will have mysteriously failed.

Anyway, luck is with you for the present.
 

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