Lights dim when microwave is turned on


Old 10-20-00, 09:14 AM
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I just moved in to my house and found that the lights dim when the microwave is turned on. Is this a serious electrical problem? Can I fix it with little or no electrical experience? Thanks
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Old 10-20-00, 09:22 AM
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Is this a built-in microwave, or one sitting on the counter? If it's sitting on the counter, try plugging it in to different outlets.

Bottom line is that you may have to live with this if the kitchen was wired before microwaves existed. The original wiring just never anticipated this much power being used in the kitchen. There are many people who have to remember not to run their microwave and toaster at the same time.
Old 10-20-00, 10:45 AM
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John, reminds one of "Green Acres", doesn't it?


The National Electrical Code now requires two 20 amp circuits dedicated to kitchen receptacles. The reason is, as John explained, that what one can plug in in the kitchen has dramatically changed. Older wiring was 14 gauge "knob & tube", and this size wire can only transport so much juice at once. When you're already running other electrical stuff and then turn on the microwave, if the amount of current required for everything that's on is more than the wire can deliver then the big dog on that circuit gets to eat the largest portion, and the little guys must get by on the scraps.

As far as safety, it can be made safe if it is not already. If you have old wiring it's probably 14 gauge. 14 gauge wire is designed to carry only 15 amps safely, providing it is in good condition and has not been damaged or improperly modified by a previous owner. If your main panel is a fuse box make certain that you use 15 amp fuses. Some people put larger fuses on certain circuits simply because they keep blowing 'em when too many gadgets are running at once and they become annoyed at the inconvenience. They stop blowing fuses, but they are allowing the wiring to carry more current that it was designed to handle. Doing this causes the wires to overheat, which can cause a fire. Having one's house burn down is pretty annoying too. Take your pick.

If you have a circuit breaker panel and modern wiring you can take the face off the panel and look at the wires connected to each breaker. There should be printing on the insulation of each telling you the wire gauge. If the wiring is #14, the max breaker is 15 amps. #12 wire can be on a 20 amp breaker, and if the wire is #10 you can go up to 30 amps. You are always allowed to use a smaller breaker, but never a larger one than the rating of your wire.

So, if you have fuses, with a 20 amp fuse on the circuit that the micro is on, and you switch to a 15 amp, you may blow fuses when you use it but you will not overload the wiring.

If you have a breaker panel, you may want to look into having two new 20 amp receptacle circuits installed in your kitchen. This one electrical improvement has just about the highest value in terms of a home's user-friendliness.

If your panel is an old fuse box with 60 amp main fuses, however, and you have an electric range, water heater and dryer, for instance, you will either have to live with the occasional dimming lights, or you will need a new electrical service. The wires feeding your existing panel, called your "service entrance cable", will be too small for an upgrade to a new breaker panel of 100 amps or more, so that will need replacement as well. The whole job, done by a professional, will cost an average of about $2500. Your property value will increase by at least that amount, you may be entitled to a reduction in your homeowner's insurance, and you'll sleep great at night.

I hope this was helpful.


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