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Old 20-03-2014, 07:53   #1
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microwave tripping breaker...what to look for?

I'm pretty much clueless when it comes to all things electrical. My new-to-me boat was rewired a few years ago and everything works....except the microwave. It trips the master breaker after about 15 seconds of running. This is with the boat at the dock. I have 30amp shore power so that *should* be plenty. I've tried turning everything else off when running the microwave to no avail. Any thought as to where or what I should I be looking for, solution wise?
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Old 20-03-2014, 07:58   #2
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Re: microwave tripping breaker...what to look for?

You need to check current draw with the appliance running to eliminate a defective breaker or other source of excess draw. Hold the cord to check for heat. A worn outlet can drive the resistance up causing the breaker to trip.
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Old 20-03-2014, 08:23   #3
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Re: microwave tripping breaker...what to look for?

What other AC appliances are being run at the same time? Does your AC system have a couple of AC circuits with individual breakers, plus the master breaker? What time of day does the master breaker shut off?

Often times, folks have three 15 amp circuit breakers and a 30 amp master breaker. One breaker often goes to the battery charger, and the other two power separate circuits serving different parts of the boat. The charger may be working hard trying to charge the battery to keep all the cabin lights in the boat illuminated, while the other AC circuits are powering the electric heater and the microwave. Also, that much current flow heats up the shorepower cord and plugs, which might be a bit corroded or not making a secure connection. Look at your plugs for signs of charring.
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Old 20-03-2014, 09:11   #4
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Re: microwave tripping breaker...what to look for?

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I've tried turning everything else off when running the microwave to no avail. Any thought as to where or what I should I be looking for, solution wise?

The microwave has a short in it! And if it isn't tripping anything with the microwave breaker shut, but not running and only trips when you turn it on, the short is the unit itself. Replace it.
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Old 20-03-2014, 10:15   #5
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Re: microwave tripping breaker...what to look for?

sailorboy1, I picked up on jcolman's hint. My experience has been that folks often overlook how extensive their electrical "vampire's" appetite can be. Many of us are running things that we are not aware of. As far as a "short", he said it took about 15 seconds to appear. A short would be more apparent, but if the heating up of the microwave aggravated an issue that then lead to high current flow, then you got it. Trying out another microwave, or plugging the existing microwave directly into the shorepower cord, via an adaptor, would confirm the source of the problem. Again, my own experience has caused me to believe things get more complicated (hence, Murphy"s Law), but sometimes it's the most obvious cause that needs addressing. Boats are a never ending source of entertainment.
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Old 20-03-2014, 10:34   #6
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Re: microwave tripping breaker...what to look for?

Thanks guys. I'll try another microwave.
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Old 20-03-2014, 10:54   #7
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Re: microwave tripping breaker...what to look for?

What size breaker did they install upon rewiring? Maybe it's too small? Otherwise as mentioned maybe that MW is pulling too much. They seem to go bad regularly now days.
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Old 20-03-2014, 11:19   #8
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Re: microwave tripping breaker...what to look for?

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What size breaker did they install upon rewiring? Maybe it's too small? Otherwise as mentioned maybe that MW is pulling too much. They seem to go bad regularly now days.

I don't know. I'll have to see if I can find out. The refrigerator runs fine as does my cabin heater. I've unplugged both when turning on the microwave to eliminate too much current draw but the microwave still trips the breaker.
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Old 20-03-2014, 11:58   #9
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Re: microwave tripping breaker...what to look for?

What brand of breaker? does it show the amperage right on the lever? Depending on size of microwave it could be close to 10 amps startup.... but the fact that it runs for 20 secs and shuts down indicates it's overheating the breaker. Do you have a space heater you could plug in? a big microwave is maybe 1100-1200 watts. A space heater might be about the same (will say on the back) if the heater works fine then the MV is pulling too much... and defective.
I've had several breakers over the years that went bad and did the same thing.... just trip too easy. Especially those ones that are prevalent in boats with the yellow or black or red or white switch paddles.
If the fridge, heater and MV are on the same breaker you need about a 20 amp breaker (assuming the heater is one that only draws maybe 5 amps)
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Old 20-03-2014, 12:02   #10
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Re: microwave tripping breaker...what to look for?

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... It trips the master breaker after about 15 seconds of running.
Are you tripping the MAIN breaker on the panel????
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Old 20-03-2014, 13:47   #11
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Re: microwave tripping breaker...what to look for?

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Are you tripping the MAIN breaker on the panel????
Yes, it's the main.
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Old 20-03-2014, 13:52   #12
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Re: microwave tripping breaker...what to look for?

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Originally Posted by Cheechako View Post
What brand of breaker? does it show the amperage right on the lever? Depending on size of microwave it could be close to 10 amps startup.... but the fact that it runs for 20 secs and shuts down indicates it's overheating the breaker. Do you have a space heater you could plug in? a big microwave is maybe 1100-1200 watts. A space heater might be about the same (will say on the back) if the heater works fine then the MV is pulling too much... and defective.
I've had several breakers over the years that went bad and did the same thing.... just trip too easy. Especially those ones that are prevalent in boats with the yellow or black or red or white switch paddles.
If the fridge, heater and MV are on the same breaker you need about a 20 amp breaker (assuming the heater is one that only draws maybe 5 amps)
As I mentioned earlier, my space heater and refrigerator both work fine. I unplug them when using the microwave as I thought maybe I was pulling too much amps. But the master breaker still trips about 15 seconds after I turn on the microwave. Here are pics of my electrical panel.



Close up. The master breaker on the bottom left is the one that trips.



I'm going back up to the boat on Sunday. I figure I'll bring the microwave home with me and see if it trips the breaker at my house. If so, then the microwave has a short. If not, then back to square one I suppose.
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Old 20-03-2014, 13:58   #13
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Re: microwave tripping breaker...what to look for?

Run a semi-heavy ext cord from a 110 volt outlet on your servive on the dock . If you blow the dock , its the micro-wave /
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Old 20-03-2014, 14:06   #14
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Re: microwave tripping breaker...what to look for?

I could be wrong (not unknown) and it is a breaker problem.

Is your MW wired into the AC system or is it plugged into an outlet powered from one of those Port/Stbd Outlet breakers in your photo?

If it is plugged into an outlet and the outlet breaker isn't tripping, but the Main Breaker on that panel is, the breaker could be the problem.
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Old 20-03-2014, 14:40   #15
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Re: microwave tripping breaker...what to look for?

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I could be wrong (not unknown) and it is a breaker problem.

Is your MW wired into the AC system or is it plugged into an outlet powered from one of those Port/Stbd Outlet breakers in your photo?

If it is plugged into an outlet and the outlet breaker isn't tripping, but the Main Breaker on that panel is, the breaker could be the problem.
I plug it into one of the starboard outlets, the same ones that I use for the heater and refrigerator (but not all at once)
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