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Old 02-09-2013, 01:38 PM   #1
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does a household circuit breaker have thermal memory?


A circuit breaker in our home is tripping in a way that I do not understand.

We purchased a toaster oven for Christmas. It is plugged into the same circuit as the coffee maker, fridge, and a few other outlet.

The breaker will trip as expected if everything is on so we do not use them all at once. The breaker never tripped before we purchased the toaster oven.

Here is what is confusing. On at least two instances the breaker tripped when we turned on the coffee maker on within a minute or two of turning the toaster oven off. I do not understand why that would occur.

The breaker is a square D 15A.

Would the breaker have some sort of thermal memory? I thought the breaker was electromagnetic? Any explanation would be appreciated

Thanks

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Old 02-09-2013, 02:05 PM   #2
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does a household circuit breaker have thermal memory?


What are the wattages of the appliances ? that circuit is too small for those loads.

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Old 02-09-2013, 02:24 PM   #3
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does a household circuit breaker have thermal memory?


Thank you for the reply. Those appliances do not trip the breaker as long as the toaster oven has been off for a long time. They may trip the breaker if the toaster oven has only been off for one minute.
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Old 02-09-2013, 02:27 PM   #4
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does a household circuit breaker have thermal memory?


Household circuit breakers are either the electromagnetic type, which senses an overcurrent condition by direct "measurement" of current, or the thermal type, which incorporates a bimetallic strip that gets warm due to flow of current through it, and bends. When the bimetallic strip gets sufficiently warm, it bends far enough to contact a switch, and shuts off the power. Many home breakers incorporate both the electromagnetic mechanism, which operates very quickly in large overcurrent conditions, and the thermal type, which is more effective in tripping under slight overcurrent conditions that last a long time.

My guess is you have a dual type of breaker, and the bimetallic strip gets warm from the initial use of the coffee maker. The strip is still warm when you turn on the toaster oven, which then trips the breaker due to a relatively small overcurrent condition, not sufficient to trip the electromagnetic trigger.

As previously noted, you are grossly overloaded on that circuit, so you may want to consider installing an additional circuit.
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Old 02-09-2013, 02:30 PM   #5
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does a household circuit breaker have thermal memory?


Do you have any 20 amp appliance circuits in your kitchen?
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Old 02-09-2013, 03:43 PM   #6
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does a household circuit breaker have thermal memory?


Thank you Daniel Holzman!

Your response referring to the "bi-metallic strip" answered my question. I was not aware of household breakers incorporating both techniques.
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Old 02-09-2013, 03:47 PM   #7
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does a household circuit breaker have thermal memory?


That's one reason why newer codes would call for all 20 amp. GFI protected outlets in a kitchen.
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Old 02-09-2013, 03:58 PM   #8
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does a household circuit breaker have thermal memory?


Quote:
Originally Posted by joecaption View Post
That's one reason why newer codes would call for all 20 amp. GFI protected outlets in a kitchen.
And a coffee maker and toaster over would probably still trip a 20 amp circuit...
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Old 02-09-2013, 05:06 PM   #9
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does a household circuit breaker have thermal memory?


Quote:
Originally Posted by joecaption View Post
That's one reason why newer codes would call for all 20 amp. GFI protected outlets in a kitchen.
GFI is another matter, but...
The code has called for no less than 2 20Amp small appliance circuits
since the days that small appliances have commonly existed.

Their pattern of use then (toasters and percolators and warmers right on the table) is why those SA circuits were made to extend into dining areas.

Last edited by TarheelTerp; 02-09-2013 at 05:13 PM.
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Old 02-09-2013, 05:17 PM   #10
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does a household circuit breaker have thermal memory?


Just the refrigerator on a 15A circuit is iffy at best. No wonder the breaker trips. How old is your house? It doesn't seem to be anywhere near compliant with current (pardon the pun) code.
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Old 02-09-2013, 06:45 PM   #11
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does a household circuit breaker have thermal memory?


Thanks everyone! Here are the details of the situation.

The kitchen circuits never triggered any breakers before Christmas. There are at least 2 20A circuits in the kitchen. (I'll get to the 15A breaker in a minute). We had the normal appliances: fridge, coffee maker, toaster. The home is 17 years old.

As TarHeelTerp mentioned the kitchen circuits do extend to the dining area which we use for our computer area. We never had the breaker trip.

We replaced our toaster with a Breville Toaster Oven for Christmas, 1800 Watts!!!. The breaker tripped when the oven, coffee maker, and computers were on. OK. Understood.

I turned everything thing off and went to reset the breaker however the breaker would not reset. Went to home depot to look at breakers and ask a few questions about breaker operation. We assumed that the breaker was most likely bad. The home depot electrical person recommended that I use an electrician since I never worked in a panel. At that same time a person walked by and said "like me". We talked and he came out to the house to install the new breaker. I observed but did not realize he installed a 15A breaker to replace the 20A. It was a dual breaker but he used only one side. My guess is that is what he had in his pickup truck.

I have since moved the computers to a different circuit. We avoid using the coffee maker and toaster oven at the same time. I did not realize that the toaster oven would warm the bimetallic strip of the breaker and then when we use the coffee maker right after that it would cause the breaker to trip.

I understand the ideal solution is a dedicated circuit for the toaster oven. I plan to install a 20A breaker to replace the incorrect 15A for now and continue to avoid using other appliances with the toaster oven.

Thank you

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Old 02-09-2013, 07:03 PM   #12
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does a household circuit breaker have thermal memory?


Quote:
Originally Posted by djmieczk View Post
The kitchen circuits never triggered any breakers before Christmas.
There are at least 2 20A circuits in the kitchen.
We never had the breaker trip.

Breville Toaster Oven for Christmas, 1800 Watts!!!.
The breaker tripped when the oven, coffee maker, and computers were on. OK. Understood.
Quote:
I observed but did not realize he installed a 15A breaker to replace the 20A. It was a dual breaker but he used only one side.
My guess is that is what he had in his pickup truck.
No excuse. Everything he did is suspect.
You're now starting from scratch. Do it right.

Quote:
I have since moved the computers to a different circuit.
Good.

I understand the ideal solution is a dedicated circuit for the toaster oven. I plan to install a 20A breaker to replace the incorrect 15A for now and continue to avoid using other appliances with the toaster oven.
You're on the right track.
Keep at it.
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Old 02-09-2013, 07:13 PM   #13
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does a household circuit breaker have thermal memory?


Quote:
Originally Posted by TarheelTerp View Post
No excuse. Everything he did is suspect.
You're now starting from scratch. Do it right.

You're on the right track.
Keep at it.
Maybe he prevented a future fire by changing back to the proper sized breaker if that 20A was feeding #14.
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Old 02-09-2013, 07:17 PM   #14
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does a household circuit breaker have thermal memory?


Quote:
Originally Posted by k_buz View Post
Maybe he prevented a future fire by changing back to the proper sized breaker if that 20A was feeding #14.
That's a big maybe. I find it VERY unlikely that any house has 14ga wire feeding a small appliance circuit. Anywhere. In any house.

otoh... I have a LOT of suspicion of HD aisle trolling handyman types.

Last edited by TarheelTerp; 02-09-2013 at 07:20 PM.
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Old 02-09-2013, 07:21 PM   #15
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does a household circuit breaker have thermal memory?


I see it all the time. 15A breaker is too small and trips all the time, just replace the breaker to a 20A. Just about every fuse panel I replace has fuses to large for their conductors. 30A screw in fuses are very common.

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