20 amp breaker trips

[kentannenbaum1]

Occasionally the breaker trips as soon as the 1100w microwave is turned on. More often however, it doesn't trip. In fact, most of the time, when it's running ALONG WITH a 1300w toaster, it does not trip.
Any clue why? Thank you.

 

[Jadnashua]

Circuit breakers, unlike many fuses, have a time delay curve on them. A significant overload will trip them fairly fast, a load slightly over their limit can take some time before it trips. Starting both units at the same time could cause enough surge to trip it. Your supply voltage is unlikely to be always the same. 120vac nominal at 20A= 2400W, but if it was say 110vac*20A = 2200W...so in one situation, it wouldn't be overloaded, but in the other, it would.

Without knowing the supply voltage, in both cases, you're close to the maximum and may exceed it sometimes. If there's anything else on the circuit, that can easily push it over the edge. That type of appliance typically isn't used continuously, but if they were, there's an 80% rule, and in all cases, you'd be overloaded. That generally only applies to something expected to be on continuously for a long time.

 

[Sponsor]

 

[kentannenbaum1]

Circuit breakers, unlike many fuses, have a time delay curve on them. A significant overload will trip them fairly fast, a load slightly over their limit can take some time before it trips. Starting both units at the same time could cause enough surge to trip it. Your supply voltage is unlikely to be always the same. 120vac nominal at 20A= 2400W, but if it was say 110vac*20A = 2200W...so in one situation, it wouldn't be overloaded, but in the other, it would.

Without knowing the supply voltage, in both cases, you're close to the maximum and may exceed it sometimes. If there's anything else on the circuit, that can easily push it over the edge. That type of appliance typically isn't used continuously, but if they were, there's an 80% rule, and in all cases, you'd be overloaded. That generally only applies to something expected to be on continuously for a long time.

Hey, thanks. Actually, when it happened today, the toaster was not on, just a light + the microwave. Which is why I thought it strange. This has happened previously too, though not often. So was it was at 1100w plus the light. Could it be a defective microwave (it's fairly new) which just when it starts might overload the circuit? Again, thanks.

 

[Reach4]

Does this happen more when the microwave oven is nearly empty? Do you have an AFCI breaker by chance?

 

[Jadnashua]

Are you confusing a GFCI with a circuit breaker? Lots of things can make a GFCI trip (and an AFCI, too).

If it is a CB, and it trips with only the microwave on it, it could be either the CB is defective or you have a wiring problem. Could be the microwave, but that's less likely. There is a surge when you turn on either the toaster or the microwave, but that's why the CB has a time-delay in it...it should not trip as long as the current remains below the max after initial turn-on. It can usually handle a fairly big turn-on surge since that ends very quickly.

I had a GFCI issue that took awhile to isolate...it ended up being a wire between one receptacle and another...rather than tear up the wall, I just ran a new wire...it's been fine now for years. The wire probably had a nail or was pinched somewhere, but not enough to always trip things.

 

[Bluebinky]

What else is on the circuit in question? Refrigerator, perhaps?

 

[kentannenbaum1]

What else is on the circuit in question? Refrigerator, perhaps?

Sorry for the late reply. There's no breaker of any kind except the panel 20amp breaker. It is true that just a cup of java was in there. Could be the wiring I guess, although I ran it several years ago, 12 gauge and don't believe I've damaged it along the way. What's different is the more powerful microwave. I think the old one was 800 or 900 watts, this one's 1100. The additional stuff on the circuit are a light bulb and a 1300w toaster which when all are on, the breaker does NOT trip.

 

[Reach4]

A cup of coffee is a pretty good load. I was wondering if it was some dry thing and there could be an arc. What breaker? Some might be more subject to falsifying. If it is a new AFCI breaker, I would get that out of your kitchen outlet. If it is a GFCI breaker, it is possible there is some leakage at the microwave oven. Unlikely....

I am suspecting the breaker, but the problem could be the microwave oven. I am not a pro.

 

[kentannenbaum1]

A cup of coffee is a pretty good load. I was wondering if it was some dry thing and there could be an arc. What breaker? Some might be more subject to falsifying. If it is a new AFCI breaker, I would get that out of your kitchen outlet. If it is a GFCI breaker, it is possible there is some leakage at the microwave oven. Unlikely....

I am suspecting the breaker, but the problem could be the microwave oven. I am not a pro.

Already said, NO AFCI or CFCI breaker. I may borrow another microwave to check it out. By the way, it's intermittent, it did NOT happen today.
Again, thanks.

 

[Reach4]

Already said, NO AFCI or CFCI breaker.

No,... you didn't previously say that. Perhaps you were thinking that those features are not built into many 20 amp panel breakers. A breaker with such a feature is still a breaker.

 

[JRC3]

I'd start with a new $5 breaker.

 

[Jadnashua]

A breaker can wear out and is more likely to die if one of two things happen:
- long-term high loads can cause the contacts to the bus bar to get warm and thermal cycling can loosen them, making the contact less than ideal (check the clamp where the wire attaches, too)
- corrosion on the bus bar and/or breaker contacts

You might want to pull the breaker and first, see how hard it is to remove (an indication of the spring tension on the contacts) and second, verify that the bus bar is not damaged. Either of those can increase the load, and the combined load could cause the thing to trip.