Magnetron Torture

December 23, 2006 9:30 AM

My microwave is driving me nuts!

My apartment's wiring is run through a single 20-amp circuit breaker (six floors down in the laundry room). My microwave (which came with the apartment) draws a pretty nasty current spike when it turns on. Every now and then it will trip the circuit breaker, which means I have to go down to the laundry room to reset the breaker.

Here's the thing, though - there seems to be no correlation to the amount of power being drawn elsewhere in the apartment. I can turn off every piece of electronics, shut off all the lights, and it might still trip the breaker. I could have my computer, TV , and refrigerator all running and it won't trip the breaker.

What's the solution to this problem? How do I prevent my microwave from shutting down my apartment? Will replacing the microwave help?

Some more information:
-The microwave draws 1200W, according to the tag on the inside.
-The microwave is plugged in to the same outlet as the fridge.
-There are no other appliances in my apartment that trip the breaker (vacuum, big box fan, sound equipment all work fine).

posted by backseatpilot to home & garden (20 comments total)

From a very unscientific google, that looks to be near the higher end of microwave powers. It just sounds like it peaks too high sometimes just as it's turning on. You might look at getting a less powerful microwave (or borrowing one if possible) and see if that solves the problem.
posted by cschneid at 9:44 AM on December 23, 2006

It's possible that the breaker is defective. Try plugging the microwave into an outlet on a different circuit and see if you get the same result. Replacing a breaker is fairly easy, though you need to shut off the power at the main.

You can also try running the microwave at a lower power setting. It'll still work, but not as fast as on full power.
posted by wryly at 9:58 AM on December 23, 2006

Well, you could try hooking it up to a UPS . You might need to buy a nicer one to control for switching transients.
posted by jefeweiss at 10:10 AM on December 23, 2006

I second the UPS, or some sort of voltage regulator? IANAE(lectrician)
posted by Sonic_Molson at 11:08 AM on December 23, 2006

jefeweiss's advice is not likely to help, as no UPS you can carry will be able to substantially correct for high power factor loads like a microwave, and even if you can find one that will support big transients, you won't change the load presented to the line significantly, except to make it bigger by the amount of inefficiency created by the UPS boost/buck circuits in series with the UPS load. Most UPS devices specifically warn against connecting high power loads with even a small inductive component, like a laser printer, to the UPS, as they can damage the UPS voltage correction circuits.

The problem you are describing is common to first and second generation microwave ovens, which often had analog timers and power controls. These machines often directly switched the power to the magnetron filament at the same time as the plate supply, with a simple low voltage relay. To the wall circuit, the load looked instaneously like nearly a short circuit, as the current required by the inductive elements of the magnetron plate supply demanded high current, at the same time the magnetron was "warming up" and starting to try to create an RF field in the microwave cavity. Later designs used a series of switching elements to spread out the startup load over a couple of seconds, by first turning on the magnetron filament, then setting up the plate supply, and finally, when the tube was "warm," turning on microwave exicitation to the cooking cavity.

So, if your microwave is one of those early designs, even running the thing at lower power may not stop the problem. You could try it at power settings of 50% or less, but it is still likely that you'll be tripping your circuit breaker frequently.

Changing the microwave for a modern design with power saving features is your best bet. If a smaller 700W model is sufficient for your needs, and you don't need the box size of a 1200 watt model, the 700W countertop models are often available for well under $100.
posted by paulsc at 11:09 AM on December 23, 2006

There is no actual power setting on a microwave oven. When you run it at "50% power" what you're doing is to control the duty cycle so that it runs at 100% power for half the time and 0% power the rest of the time.
posted by Steven C. Den Beste at 11:40 AM on December 23, 2006

We have a crappy capacity power strip between our microwave and the wall that helps us prevent this kind of thing. When the power spikes, the power strip resets instead of the entire apartment's electrical system.
posted by croutonsupafreak at 11:48 AM on December 23, 2006

You might look into one of the new Panasonic ovens, which feature an inverter circuit that does true reductions in power rather than varying the duty cycle as SDB suggests. You could then run it at 90% or whatever most of the time.
posted by kindall at 11:54 AM on December 23, 2006

20 Amp circuit you said.
120 volts normally
supports safely a load of 1920 watts.

Check the watts of your refrigerator or whatever else is on that circuit and see what the total watts add up to. If you exceed the amount a 20 Amp fuse will handle then it pops.
posted by JayRwv at 12:06 PM on December 23, 2006

If all your appliances are on that ONE 20 amp circuit breaker then it is no wonder it resets. I can't tell from your description if you only have one breaker for the entire apartment or not. If you do.. no way you can run that microwave at full power and not reset the breaker.
posted by JayRwv at 12:09 PM on December 23, 2006

I agree with croutonsupafreak for the moment. It's a cheap, crappy hack, but it should work. I have a space heater at the office, and I just keep it on a power strip with a fuse that you can reset. It blows once in a while, but you can reset it without hiking 6 floors.
Long term, just buy a smaller microwave, as suggested by paulsc.
posted by unrepentanthippie at 12:09 PM on December 23, 2006

20A for the whole apartment?!? That's not enough. I think you should talk to your super and get that remedied. Further, I'm certain code requires a breaker box closer to the dwelling. Where do you live?
posted by kc0dxh at 1:35 PM on December 23, 2006

Circuit breakers have time-delay ratings (Long, Short, Instantaneous, Adjustable). If the current spike is very short in duration, you might be able to replace the breaker with one that doesn't trip fast enough to react to the spike before it's gone.

You probably want/need (depending on local laws & your comfort/competence-level around AC wiring) an electrician involved in doing this.
posted by Crosius at 1:35 PM on December 23, 2006

We have the same sort of microwave with the same power rating as backseat pilot, although that's not the only circuit in our house. We do what croutonsupafreak does, and we don't run the coffee maker, toaster nor any other device on the same circuit until the micro is finished. The good thing about having that micro is it takes far shorter times to cook/warm/thaw things in it than a 700W model, like our old one.
posted by Lynsey at 3:17 PM on December 23, 2006

Nnd'ing kc0dxh above - 20a is almost certainly not enough for your entire apartment - Hell, each BATHROOM is supposed to have a seperate 15a leg, and kitchens are supposed to have two 20a legs with GFCIs according to national code(IIRC). Make this the super's problem.
posted by Orb2069 at 5:58 PM on December 23, 2006

Interesting. To answer some questions, yes, it's a 20A breaker for the whole apartment. There's definitely some dimming of the lights, etc., when high-draw appliances are turned on. The breaker is located right next to the meter, which is in the laundry room of the six-floor building.

I'm not really sure how to determine what generation the microwave is. It seems to be relatively new - it's got a digital display and all those one-button program modes that I never use. I was thinking about replacing it anyway, since it's really massive and the only place it fits is sitting on top of the fridge.

The one thing that's interested me the most about this whole thing is that it's only the microwave that kills the power. Is the instantaneous draw from the radar range that much more than any other appliance?
posted by backseatpilot at 8:02 PM on December 23, 2006

inrush current limiters exist to address this problem (inrush current explained, but I haven't read the article). However, unless you are comfortable tinkering (like adding an inrush limiting thermistor to a cheap power bar - as it will be AC connected, it is not a job to be undertaken lightly), a new microwave is probably the more reasonable solution. If it doesn't solve your problem you can always take it back and try a different tack.

Meanwhile, putting the microwave on a 15A breakered power bar is a very good idea. Breakers wear out, so it is much better to let the cheap and easily replaced power bar one do the work.

And sure, one 20A breaker for the whole place is pretty terrible.. Certainly something to think about long term..
posted by Chuckles at 2:55 AM on December 24, 2006

"... Is the instantaneous draw from the radar range that much more than any other appliance?"
posted by backseatpilot at 11:02 PM EST on December 23

The problem with most microwave ovens isn't the actual cooking wattage they draw, it's the additional reactive (imaginary) power they need to store in the magnetron's power supply components, to create the microwave field. Typically, larger microwave ovens have very poor power factors, which leads to high inrush currents at startup, as well as a higher apparent current demand than the cooking wattage rating would imply. Even though the reactive power is not "used" by the device (or is returned to the line during shutdown), it is "real" in the sense that it is power the utility must supply, and that therefore must pass through your circuits, and breaker, adding to the RMS value of the load.

It is possible to correct for low power factor by adding capacitive components to the magnetron power supply, but such capacitors are large, bulky and expensive devices that themselves can eventually fail, leading to warranty issues. For the most part, in a cost sensitive appliance sector like microwave ovens, such features are not well supported by the market.
posted by paulsc at 12:22 PM on December 24, 2006

it is "real" in the sense that it is power the utility must supply,

Better to say that it is real in the sense that it is current that the utility must supply. Current which can trip breakers, overload power lines and saturate transformers. It is covered better in the talk page than in the actual wikipedia article:

Most of the reactive power flows back and forth between the source and load such that "On one half-cycle, the source supplies energy to the energy-storage element, and on the next half-cycle the energy-storage element returns energy to the source....currents required to supply the stored energy produce losses in the generating and transmission system..." Scott, Ronald E. (1960). Linear Circuits. Reading, MA: Addison-Wesley. “Low power factor means more current and greater I2R losses in the generating and transmitting equipment.” Fitzgerald, A. E.; Kingsley, Charles Jr. and Umans, Stephen D. (1983). Electric Machinery, 4th ed., Mc-Graw-Hill, Inc.. ISBN 0-07-021145-0.

posted by Chuckles at 12:44 PM on December 24, 2006

If you've got an electric range the plug on the range is fed from the 40A range curcuit. Plugging the microwave into that outlet would probably solve your problem.
posted by Mitheral at 7:39 PM on December 26, 2006