Microwave Trips Wrong Circuit Breaker

I know very little about electrical wiring, so please bear with me ...

We have a 2 year old townhome. Intermittently (about 25 to 50% of the time) running the microwave will trip the circuit breaker for the kitchen lights. According to the circuit box labeling, the microwave is on a separate breaker from these lights. The breaker for the microwave never trips.

The microwave is a Jenn-air, and has a motor that opens and closes a stainless steel vent cover. The tripping is definitely related to this motor; the lights usually trip after the microwave shuts down and the motor then engages to close the vent cover. There was about a two week period when this motor didn't seem to work and the cover was stuck open, and the tripping stopped. When I then manually moved the mechanism to close the vent, the motor seemed to engage again - and the lights immediately tripped.

We've had two electricians in, and the only thing they could think of was to tighten and and then replace the circuit breakers, which was done. (Apparently the newer type breakers can be especially sensitive - although the two breakers involved are not next to each other.) It seemed to work for awhile, but since this is an intermittent problem it can be hard to diagnose, and of course it started up again. I'm wondering if replacing the vent cover motor would help. Any ideas?

 

Is this a multiwire circuit? Two circuits sharing the same breaker. You will usually see a red and black wire on the two breakers.

Is the breaker for the lights GFCI or AFCI or neither?

 

Here's the info you requested:

The lights and the microwave do not share the same breaker. The breaker for the microwave is an Eaton Type BR220 C220 20-amp double-pole. One side is labeled for the microwave and the other for the hood, but it is really just one (combination microwave-hood) appliance. The lights are on their own Type BR 15-amp combination AFCI. Hope this helps ...

 

If it is a 20 amp double pole breaker then something doesn't make sense. It would normally be a combined feed of a single 120 volt breaker for the vent/microwave. So that would make the vent/microwave on half the breaker The part number indicates it is a double pole not a tandem.breaker. That then would be a multi wire circuit and you need to find out what is on the other half of the multiwire circuit.

 

Can you post the model number of the microwave. I'm wondering if the microwave and retractable vent are separate units. Usually a microwave/hood combo has only one power source, but Jenn-Air is often unique.

 

The Jenn-air brochure calls this a "microwave hood combination" and it is model number JMV8208WS.

Keep in mind that the microwave breaker doesn't trip - but the microwave is causing a completely separate breaker for the kitchen lights to trip - could the circuits have been accidentally crossed somewhere? This house is only two years old and all the appliances were installed new ....

 

Just wanted to note for the Techs:
This is indeed one unit with one power supply, same as any other microwave hood combo.

I have no idea how one breaker that doesn't trip could cause another to trip.

 

That is the reason behind my questions. I have seen that before causing you to need to trip two breakers to kill a 120 circuit. Not your problem but similar. My guess is a circuit box with two circuits and the neutrals accidentally all connected together . Haven't seen your specific problem though and only a guess.

Brian wrote: The number you gave is for a two pole breaker so the question is why. Multi wire circuit is the most likely but what else is on it? What color wires on the hood breaker?
.

 

How do I determine the color of the wires to the hood breaker (without electrocuting myself!

 

Turn off the main breaker then open the panel. Don't touch anything. Area around main breaker will still be hot.

Off topic but sometimes I am surprised by a reply. I understand your caution and it is good but the first time this happened I would have had the cover off the panel. Didn't occur to me you hadn't been in the panel looking. There may not be a second wire connected to the breaker if so that may not be related to the problem. Post a picture of the breaker and one of the whole panel with the cover removed. http://www.doityourself.com/forum/li...rt-images.html

 

A very simple test might be to turn off the breaker for the microwave and then see if the microwave and/or lights work. Then do the same thing with the 15 amp lighting circuit breaker and see if the microwave and/or lights work.

 

Thanks for the test suggestion:

- Turning off the breaker for the microwave only turns off the microwave, it has no effect on the lights.

- Turning off the breaker for the lights only turns off the lights, it has no effect on the microwave.

 

But does it turn anything else off? We really need to know about the wires connected to the hood breaker.