Microwave blows different circuit

I just bought a new microwave. It is rated at 1300 watts. Whenever I turn it on it trips a breaker but not the breaker controlling the circuit it is plugged into. The microwave continues to operate.

Background. I have lived in this house for over 22 years. The house is about 26 years old. The last time anything was done to the wiring was about 10 years ago. Our previous microwave rated at around 650 watts (it is around 20 years old) has been plugged into the same outlet since it was first purchased. I replaced the new one with the old one and everything works fine.

I went to Lowe's and a salesperson who was or currently is an electrician (at least he talked about how when he wires houses) said the breaker that trips may be weak and the sudden load on the entire system when the new microwave is turned on causes it to trip. He suggested replacing the breaker that trips and/or putting the microwave on it's own circuit.

Any opinions/suggestions on this situation?

Thanks

Gary

 

Four questions: (1) Is 1300 watts the input power or the output power? (2) What circuit number (on the panel cover) is the microwave on? (3) What is the circuit number of the breaker that trips? (4) What is controlled by the circuit breaker that trips?

 

Putting the microwave on its own circuit is always a good idea if you have one available for it.

As for the weak breaker it is possible although I have never seen it happen that way when a breaker is weak it usually trips under its own load first. If it were sensitive enough to trip from a change in the household then it would trip all the time.

Is the microwave and the other circuit completley separtate or are they sharing a neutral? If this is the case then the breaker may be tripping do to something wrong in either circuit, a loose connection or a burnt wire. The curcuits work fine as long as the load is low but as soon as a strain is put on them the electricity that is backfeed from one circuit trips the other. this is not really a very common thing but it can happen.

What is on the second circuit that trips is there anything on it that you might use only when the microwave is on so it appears the microwave is causing the problem but in reality there is another device turned on when this happens? If you disconnect everything on the second circuit and run the microwave does it trip then? This could be an indictation of something else running at the same time.

 

According to the microwave manual the unit draws 11.5 amps and 1320 watts with an ouput of 1300 watts. It is on a 20 amp breaker and the only other thing I can find on that circuit is a 300 watt torch lamp that is usually on at the time the microwave is used though not always. Whether or not the lamp is on does not make a difference.

The circuit that trips is the first one under the main breaker. Then there are six 220 breakers, then a 20 amp breaker then the microwave breaker.

The breaker that trips is a 15 amp GFI breaker. On that circuit are the widescreen HDTV, Satellite receiver, VCR, and DVD. Also are the outlets in the two bathrooms which each have only an electric toothbrush charger plugged into them. That's it. If all those are unplugged the breaker does not trip. When the TV system is plugged in it trips as soon as the microwave is turned on. As you might expect the TV and components are on regularly during the evening. The breaker only trips when the NEW microwave is turned on. The TV and components are plugged into a Monster Powerbar which filters and protects against surges.

If I put the old microwave back it works without tripping the breaker.

The circuit only trips when I don't know if the circuits share a common neutral.

To experiment I moved the microwave to a different circuit in the kitchen and it tripped the TV breaker but continued to operate. I moved it to the living room and it tripped the TV breaker but continued to operate. I moved it to a bedroom and it did not trip any breakers.

Gary

 

The problem is the surge protector. You should never use a surge protector on a GFCI. When the microwave comes on, it causes a voltage surge. When the surge protector sees the surge, it shunts some of the current to ground. The GFCI sees the current imbalance and trips.

Your TV should not be on the same circuit as your GFCI-protected bathroom circuits. I'm not sure what the code was when your house was built, but it would not pass current code.

Solutions? (1) Take the surge protector off, or (2) Replace the GFCI breaker with a regular breaker and put GFCI receptacles in your bathrooms, or (3) Move your electronic equipment to another circuit that is not GFCI protected, or (4) Use a smaller microwave that doesn't cause voltage surges, or (5) Upgrade your service so that the microwave doesn't cause a voltage surge.

In any event, quit getting advice from people in the electrical aisle at Lowes.

 

John, your post makes sense. I was just finishing a long post on RF on the line maybe coupling into the GFI breaker causing it to trip when I saw your post and hit the delete key.

Anyway, the conculsion to my post was your solution #2.

 

Thanks for the advice. Like Mike I like solution 2. As part of a remodeling project I will be replacing every outlet and switch in the house. They are offwhite and the wife wants white ones to go with the new paint and decor. So I'll replace the bathroom outlets with GFCI outlets and replace the breaker with a none GRCI type.

One other question. There is an outside outlet on the patio which is also on the GFCI circuit. I assume that outlet would need to be replaced with a GFCI outlet. Is this correct?

Gary

 

The simplest answer is yes. If you are willing to do sufficient investigation of exactly what is downstream from what, you might be able to save a few bucks by buying the minimum required number of GFCI receptacles. But you can avoid that analysis work by buying as many GFCI receptacles as you have receptacles on this circuit in each bathroom, garage, unfinished basement, kitchen, and outside. You would then not use the "load" side of the GFCI, only the "line" side.

P.S. When replacing receptacles, be sure to look at each side of the receptacles you are removing to see if the removable tab is removed. If so, then remove the tabs to match on the new receptacle, and wire it up exactly the same. So, so many people screw this up.

 

John,
Great thinking !!!! This one had me scratching my head.

 

Gary,

I know you have been soliciting and getting second opinions elsewhere. So what is your conclusion now?

John

 

Hi John

All my investigation seems to support your theory that the surge suppressor on the same circuit with the GFCI breaker is causing the problem. When I remove the surge suppressor the circuit does not trip.

I don't know how many of the people who have responded to my post, including you, are experienced electricians. As you have probably noticed I have received a variety of answers and it makes me wonder how many electricians I would have to hire and how much money I would have to spend before I found someone who could accurately diagnose the problem. So unless I get convincing information that points to another cause I'm going with your theory and advice.

Soon I will be running a new circuit to my HT. I will seek the assistance of my father in law who is a retired phone company electrical engineer. I did mention this problem to him and he could not come up with anything. However, since talking to him I have done more investigation and have gotten several possible explanations here and elsewhere so I now have more information to provide him. When we run the new circuit he will examine all the wiring in the panel to see if there are any obvious problems. When he built his house many years ago he did all his own electrical. His knowledge is not current but neither is my electrical system so he is familiar with it.

Thanks for your advice.

Gary

 

That is a good thought John,, it sometimes amazes me how you can trouble shoot from there. I would have missed the connection between the gfci and the supressor too. I just get back from one where the guy put all the bathroom stuff on a gfci brkr. Got one of those fans with the heating element in the ceiling. He thought it was tripping breaker at first but it tripping gfci. Going to give it its own circuit this winter during serv upgrade. Kind of similar though and at first this post didnt sound too interesting till I read it a time or 2,, was waiting for a conclusion. Definatly going to file that thought for future use,, its gotta come up again with all this electronic stuff in use. I like the gfci at point of use too. In my own I rarely use feed thru. Just saves a lot of greif.

 

That was a great catch! The poster deserves an attaboy, too. He provided complete information, otherwise John would not have been able to catch it so quickly (without resorting to ESP)!