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I have a circuit that powers several things in my house:

  1. Kitchen microwave
  2. Basement lights
  3. Basement sockets
  4. Sump pump

Recently, this circuit has been thrown with alarming frequency when I use the microwave. No other combination of uses has thrown it. It seems especially likely to throw if the microwave is used after a big rain (when the sump pump is active).

This is not something I encountered in the first 2 years of owning the house, however it has happened a couple dozen times in the last six months.

Any ideas on what it could be and how to fix it? Will it cause permanent damage to not fix? If it's relevant, I plan to sell the house in about a year.

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what size is the circuit breaker? 20A? – longneck Aug 15 '10 at 15:19
Did you plug something new into one of the basement receptacles in the last 6 months, maybe something that draws constant power? It could be drawing just enough power to put the circuit over the limit when the sump and microwave run. – Tester101 Aug 15 '10 at 15:25
up vote 12 down vote accepted

This sounds like pure luck that you haven't had problems before. Niall has the ratings right: My microwave says 1100W power output, and 1500W max power use (which is 13A), and a 1/3HP sump pump is 9.5A (or 10A for a 1/2 HP pump), with a peak around 15A when it first starts up. If this is a 20A circuit, you're likely over the limit, and if it's a 15, you're definitely over.

After rain is very easy to explain: your sump pump is coming on more often. The starting surge of sump pump is around 5A, and the microwave probably has a surge of at least 2A when it starts up.

Basement lights are easy to figure out, just add up the wattage of the bulbs (and current in Amps = # Watts / 115V). Likewise for anything plugged into the basement outlets.

Since the sump pump turns on and off at random times depending on the water level, it's likely that any time it tries to start while you have the microwave going it's tripping the breaker. When the microwave is on a lower power setting, it may not cause the same problem.

If possible, your best bet is to run a new circuit for the microwave: by the sounds of it, it's been retrofitted, and someone just picked the nearest circuit. I would try to find where it was added (assuming unfinished basement or at least access to the ceiling or other relevant areas), and add a junction box and a new line back to the panel, and give it its own 20A circuit.

Next best is to run a new circuit for the sump pump. You'll still have the issue with anything plugged into the basement outlets, but at least that's more controllable.

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"your best bet is to run a new circuit for the microwave" - Sump pumps are recommended (and in some placed, required) to be on their own circuits, so what @Jeffrey should really do is put the sump-pump on its own circuit. – BlueRaja - Danny Pflughoeft Sep 9 '13 at 21:26

Circuit breakers can wear out over time, so probably the first thing to do would be to replace it (check your local codes to see if it's something you can do yourself or if you need to call in an electrician).

If you do have an electrician look at it, you might also consider splitting that circuit into two or more. A 1000W microwave will take about 9A (1000W / 115V for the magnetron, plus some overhead for the motor and the electronics), and the first few search results for "sump pump" show current ratings of around 10A. You're already close to the limit for a 20A circuit, even without any basement lights on!

I don't know much about sump pumps (don't have one myself), but electric motors generally take more power the harder they have to work, so if there's something clogging its intake or outlet, clearing it might solve your problem.

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It seems especially likely to throw if the microwave is used after a big rain (when the sump pump is active).

As well as the load as mentioned by Niall, any moisture in the air can affect electrical circuits. So if your breakers are in the cellar - where it's damper - that might be enough to trip it.

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I have that problem with my microwaves throwing the breaker all the time so maybe I do need to replace the breaker but what I do that works, I plug my microwave into my stove outlet. I have had three microwaves over the years and all of them would throw the breaker. I've also thought I could put a higher powered breaker into the fuse box.

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Oversizing a fuse or breaker is dangerous, don't do it. – Steven Jan 12 '13 at 20:16
Breakers are sized according to the wiring in the walls. If you oversize a breaker without upgrading the wiring in your house, you've created a fire hazard. – BMitch Jan 13 '13 at 22:44

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