Microwave Issue

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carleaux

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Naperville, IL
Hello all, first let me start by saying that I have already done a search and found no useful answers.

I recently replaced and relocated (along with 100A to 200A upgrade) an electrical panel, keeping the existing conduits mostly intact throughout the house. The old panel was a Stab-Lok and the overhead service was non-compliant and had to be moved to the side of the house to get in compliance. Everything has gone smoothly except now the microwave keeps tripping its 20A dedicated breaker. The original was a 16-year-old 1000W model and the owner replaced it with a new 1000W model from Costco - still tripping. She also moved it to a 20A countertop outlet which also happens to be dedicated and it trips there too. The wiring is 12AWG and the microwave is on a MWBC with the refrigerator since the 3/4" pipes going to that area were near fill capacity and I couldn't run a dedicated neutral for both or I'd exceed the current-carrying conductor limit. Most of the other house wiring is MWBC as existing. The scary part is that the original configuration had both this microwave, the refrigerator and the outlets in that vicinity on one 15A breaker. No surprise to anyone I'm sure that the Stab-Lok wouldn't trip.

Does anyone have any ideas or has anyone run into this before? I'm working out a plan to try reconfiguring the wiring to get a dedicated neutral to the microwave but I don't feel 100% sure that this is going to solve anything. I've dealt with MWBC for years and I know it's done properly and I've never had issues with them. I've checked all the connections and sizes twice so I know that's okay. The customer wants me to upsize the breaker and/or wiring but she doesn't understand that for a 120V cord-connected appliance we are at the end of the line already.
 
carleaux said:
It pulls around 15A while it's running. I haven't been fortunate enought to see what happens when the breaker trips.
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Seems high for a 1000 watt unit--- What is the voltage with a load?

Anyway is it possible the microwave is faulty. I would try and get another countertop mw and see if it has the same problem.
 
This is the second microwave, a brand new one. The power consumption is close to the same as the old one. I am going to monitor the amp draw with it running again to see if anything comes up.
 
carleaux said:
I'm working out a plan to try reconfiguring the wiring to get a dedicated neutral to the microwave but I don't feel 100% sure that this is going to solve anything.
Click to expand...
It may solve your problem, but only by by-passing whatever is wrong with the circuit you are having trouble with.

I would try my own load on the circuit, two heat guns for instance and check voltage and amps.
 
Something seriously is wrong if it can trip a FPE! :lol: Check the voltage while it is running, extreme voltage drop will cause microwaves to blow the internal 20 amp fuse, which if I remember correctly, is a fast acting, not slo-blow type, so if it is the microwave, you should be blowing it first. So I'm leaning towards a problem being somewhere else.
 
You are saying this was wired in conduit, no separate EGC? I'm thinking a short to ground somewhere, but a high resistance ground path. Don't know why it would follow the microwave around to different circuits though.
 
My money is on a faulty microwave. You said it was a new replacement, right?

Is there a known good receptacle close to the panel? I'd take the microwave down there, put a large cup of water in it, and run it to see if it trips a breaker.

(Don't run it empty - its not good for the microwave.)
 
I am wondering if it is a multiwire branch circuit with a loose neutral and that leg is getting the high voltage. Not sure how a microwave works but if it is burning out the MW I would check that
 
It didn't trip the FPE, and it was on a 15A circuit with the fridge!

I checked, the microwaves both draw almost exactly 15A. The voltage drop was 3, from 122 to 119. It tripped twice when we didn't want it to and wouldn't wih the meter attached. I did see a 30A spike I think at one point but I'm not sure the source.

The only separate green wires are when flex is used. Here is IL most jurisdictions prohibit NM and using the EMT for grounding is allowed.
 
carleaux said:
It didn't trip the FPE, and it was on a 15A circuit with the fridge!

I checked, the microwaves both draw almost exactly 15A. The voltage drop was 3, from 122 to 119. It tripped twice when we didn't want it to and wouldn't wih the meter attached. I did see a 30A spike I think at one point but I'm not sure the source.

The only separate green wires are when flex is used. Here is IL most jurisdictions prohibit NM and using the EMT for grounding is allowed.
Click to expand...

Oh, it's no longer on a FPE breaker? That explains why the breaker trips now!
 
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