main breaker trips after additional line install


Old 01-10-11, 04:30 PM
Thread Starter
Join Date: Jan 2011
Location: japan
Posts: 3
main breaker trips after additional line install

Hi guys. I live in Japan and am having an issue with my main breaker tripping when I use my 1000w microwave. Let me explain what was and is happening now.

Before I install a new line for the microwave, the microwave was sharing power with a fridge, in wall AC/heater unit, and couple other outlets that serves my pc. The breaker for this line would trip when I have the microwave and the ac unit on at the same time. I figure I had two many loads on that line.

I have decided to separate the microwave along with the fridge by running a new power line separating the ac/heater unit. The problem I am having now is that whenever the ac unit and the microwave is on, the main breaker would trip, whereas before, the breaker would trip.

To recap, I have added a new dedicated line serving only the microwave and fridge. The new addition to my existing power is cable and a 20amp breaker. What is causing the main breaker to trip now?
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Old 01-10-11, 05:58 PM
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Join Date: Jan 2010
Location: USA
Posts: 9,213
I know nothing of the codes or wiring methods and materials in Japan, but it doesn't appear you have added any load, just split it up. How many amps is your main breaker rated for? What is the voltage of your service? Is the service single phase or 3 phase? There is a possibility your main breaker is bad and just needs to be replaced.
Old 01-10-11, 06:56 PM
Thread Starter
Join Date: Jan 2011
Location: japan
Posts: 3
Thanks for the reply. Here is further testing results. With just the ac, microwave and fridge on and everything else off, it doesn't trip. However, if I turn on all the down stairs lights and my led tv, the main breaker trips.

This makes me wonder if I need to now upgrade my 30amp service to 50amp as the next step up. Of course, I don't see how I added extra load as casualjoe had mentioned.

Here is a photo of my main breaker info.

Old 01-10-11, 08:16 PM
ray2047's Avatar
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Location: USA
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Isn't your voltage 100-110? 30 amps would be half the amount of power common in houses 60 or 70 years ago in the US. I can't imagine even a small apartment with less then 60 amps but I don't know what is common in Japan.
Old 01-13-11, 09:53 AM
Join Date: Jul 2005
Location: Central Indiana
Posts: 190
Just a guess, but if you only have only one "hot" leg, I bet your load was originally enough to trip the main breaker, but the branch breaker maybe tripped faster than the main breaker. In this case, the solution is to install a larger main breaker, and I would hire an electrician for this.

Another possibility is if you have two "hot" legs, you may have had them on different legs before (like a multi-wire circuit), and inadvertantly put the loads on the same "hot" leg. In this case, you could move either the microwave or A/C to the other "hot" leg and see if that works.

Whatever the cause, I agree that a 30 amp main breaker is too small if that is the case.
Old 01-13-11, 05:38 PM
Thread Starter
Join Date: Jan 2011
Location: japan
Posts: 3
Thank you for the feedbacks. I have briefly calculated my load on an average day. It appears I am well over the main breaker allowance(30amp).

Ray2047 is correct. The average amp for a household is 30 amp. Of course, this would be sufficient many years ago. Microwaves, ac and pc were not a common household goods let along being a luxury items. My guess is that the wiring were not thought out properly.

I will call the power company as this is the appropriate procedure. The next amp up is 50 amp. The installation is free but I will incur an extra $6 a month on top of the electrical bill.

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