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Microwave wiring
Old 09-20-2007, 07:36 AM   #1
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Microwave wiring

I installed a new self standing stove and microwave in place of an old combo stove/microwave. The old combo used a 220v circuit for the stove, and a 120 extension from a lighting cicuit for a fan and light on the hood.

I plugged the new stove into the 220v circuit used by the old combo, no problem. The new microwave is a 1000w 120v model, and I made an outlet inside a cabinet above the microwave, but since I didn't have a separate circuit, I used the same 15 amp circuit the old stove/microwave used for its fan and light.

Everything works fine except periodically the 15 amp fuse blows and it seems to be getting more frequent. This seems to happen even when I don't have a lot of lights on in the rest of the circuit, so I don't know if it's the microwave or the circuit. DW has run out of patience and I need to get this fixed or else figure out how to repair this problem.

Any ideas? Is it dangerous to replace the 15 amp fuse with a 20?
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Old 09-20-2007, 08:01 AM   #2
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STOP! Do NOT increase the size of the fuse/breaker! There may be a chance the wiring is large enough to carry the additional load, but, to quote one of our popular actors, "Do you feel lucky?" Check the size of the wire in question, it should be printed on the insulation. Here's an abridged ampacity chart.

Wire AWGAmpacities @ 30 C Ambient

(Table 450-5b of NEC)
16 AWG 10amp
14 AWG 15amp
12 AWG 20amp
10 AWG 30amp

My guess is that if it's a lighting circuit, it's 14 gage. Be careful.

Do you have additional capacity in your fuse/breaker box? You might want to pull a dedicated circuit from the box. If this exceeds your job grade rating, better call your friendly electrician.
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Old 09-20-2007, 08:38 AM   #3
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Right, the correct way would be to run a new 12 gauge wire from a new 20 amp circuit in your breaker box. If you don't have space in the box to add one, you can replace one 20 amp breaker with a double mini breaker that fits into the same space.

The cheap way to fix it is to figure out what else is on the same circuit and avoid using other high draw appliances (coffee pot, toaster, etc) at the same time.

Never bump the amperage on the fuse.
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Old 09-20-2007, 12:51 PM   #4
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Everything works fine except periodically the 15 amp fuse blows and it seems to be getting more frequent. This seems to happen even when I don't have a lot of lights on in the rest of the circuit, so I don't know if it's the microwave or the circuit.
Yikes, I'm only 46 years old but I haven't seen a fusebox in at least 20 years. Just how old is your wiring?

Microwaves usually run the magnetron at full power when it's energized. Even using a "lower-power" setting may just mean that the manufacturer's circuits turn on the magnetron for five or ten seconds at a time (at full power) instead of full-time. So your 1000-watt microwave is drawing as much as 10.5 amps (assuming a 0.8 power factor) steady-state on what may have started life as a 15-amp circuit. Add in a few incandescent lightbulbs and the starting surge of cycling that magnetron-- no wonder the fuses are protesting. Most of them will have a fast-blow feature to prevent a ground from heating up the wiring and causing fires, so it's equally harsh on large starting currents.

Unless you're an electrician you need to seek professional help. As others have suggested, a dedicated circuit is the best way to handle the surge. Of course you may want to use an electrician who's older (or a lot more experienced) than I am...

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Any ideas? Is it dangerous to replace the 15 amp fuse with a 20?
Generally yes, and in your house's case particularly so!
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Old 09-20-2007, 03:30 PM   #5
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Yikes, I'm only 46 years old but I haven't seen a fusebox in at least 20 years. Just how old is your wiring?
Actually, I meant a circuit breaker panel. But the message from you and others is loud and clear - I'm an idiot on house wiring and need to call an electrician.
Done, and thanx.
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Old 09-20-2007, 03:36 PM   #6
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Actually, I meant a circuit breaker panel. But the message from you and others is loud and clear - I'm an idiot and need to call an electrician. Done, and thanx.
Assuming for a second you're exaggerating a bit (idiots don't generally ask for advice or listen to it), have you considered you might simply have a bad/weak breaker switch? What I'd do is 1) cut the power to the breaker box 2) double check and make sure I'd actually cut the power to the breaker box , 3) replace the current 15A breaker with a new one to see if that solved the problem. (Oh yeah, and turn on the power to the breaker box...)
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Old 09-20-2007, 03:42 PM   #7
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Assuming for a second you're exaggerating a bit (idiots don't generally ask for advice or listen to it), have you considered you might simply have a bad/weak breaker switch? What I'd do is 1) cut the power to the breaker box 2) double check and make sure I'd actually cut the power to the breaker box , 3) replace the current 15A breaker with a new one to see if that solved the problem. (Oh yeah, and turn on the power to the breaker box...)
Thanks for the tip, sounds like a good idea. But you know what? I might just electrocute myself, which would be good for ER but not for FIRE. I used to work on nuclear weapons no sweat but get really nervous around electricity. So I think I'll call in the expert and at least get an estimate for a new circuit.
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Old 09-20-2007, 03:45 PM   #8
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Actually, I meant a circuit breaker panel.
Done, and thanx.
That's a little more within the reach of the average handiman (with below-average fear). You've already made a good choice, but for an entertaining read you could browse Amazon.com: Wiring a House (For Pros by Pros): Books: Rex Cauldwell

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What I'd do is 1) cut the power to the breaker box 2) double check and make sure I'd actually cut the power to the breaker box , 3) replace the current 15A breaker with a new one to see if that solved the problem. (Oh yeah, and turn on the power to the breaker box...)
New breakers are about $5 at Home Depot.

Real electricians replace breakers by opening the breaker they're replacing, popping it out with a naked metal-bladed screwdriver, opening the breaker they're installing, slamming it in bare-handed, and then shutting the new breaker. With a cigar hanging out of their mouth, probably swilling a soda, and leaning against a path to ground the wall.

That way it doesn't power down the whole house and make them miss lunch hour inconvenience the homeowner.

Come to think of it I haven't seen that many old electricians...
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Old 09-20-2007, 04:12 PM   #9
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... With a cigar hanging out of their mouth, probably swilling a soda, and leaning against a path to ground the wall.
I'm guessing the purpose of the cigar is to prevent biting his tongue in half while proving he's a good conductor...
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Old 09-20-2007, 04:25 PM   #10
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I'm guessing the purpose of the cigar is to prevent biting his tongue in half while proving he's a good conductor...
Well, yeah, I guess that works for the assumption that the electrician is a guy too...
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Old 09-20-2007, 05:16 PM   #11
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Agree with first replacing the breaker with a new one. Cheapest fix if it works. I like cheap.

The spring in the breakers fatigue due to heat when pushed at the limit for extended periods. If it takes less effort to reset the old breaker than a new breaker that is a good indicator the old one is defective.
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Old 09-20-2007, 05:38 PM   #12
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Agree with first replacing the breaker with a new one. Cheapest fix if it works. I like cheap.

The spring in the breakers fatigue due to heat when pushed at the limit for extended periods. If it takes less effort to reset the old breaker than a new breaker that is a good indicator the old one is defective.
Mmm.... OK, I'm listening. Question. So I remove the metal cover off the box, there's a row of breakers on both sides, with the main breaker at the top. I assume I flip that bad boy off before I even remove the cover. Once the cover is off, and the main breaker off, are the two bars on either side, feeding the individual circuit breakers, off, or can I get zapped? I mean, can I still get zapped with the main off, if I touch either of those bars feeding the individual cb's?
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Old 09-20-2007, 05:55 PM   #13
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Soon, the main breaker at the top of the panel will NOT kill power to the other breakers in that same panel. At least not in my experience.

You need to go upstream of the box containing your problem breaker and turn off the main breaker that feeds power to the box with the problem breaker. In my case, I have a breaker box on the outside wall of the house that feeds the breaker box inside my utility room. That utility room breaker box contains the all the individual breakers to my wall plugs, appliances, etc., and I suspect you will have a similar set-up.

Once you're sure you've killed the power to the box containing the problem breaker, you remove the screws from the breaker panel to expose the individual breakers and you can remove and replace the breaker with no possibility of getting zapped.
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Old 09-20-2007, 06:12 PM   #14
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Soon, the main breaker at the top of the panel will NOT kill power to the other breakers in that same panel.

You need to go upstream of the box containing your problem breaker and turn off the main breaker that feeds power to the box with the problem breaker. In my case, I have a breaker box on the outside wall of the house that feeds the breaker box inside my utility room. That utility room breaker box contains the all the individual breakers to my wall plugs, appliances, etc., and I suspect you will have a similar set-up.

Once you're sure you've killed the power to the box containing the problem breaker, you remove the screws from the breaker panel to expose the individual breakers and you can remove and replace the breaker with no possibility of getting zapped.
Got it, RE. Just went outside and found that breaker box outside the house. It has a clip on it from the power company that says "do not cut or remove." That's good enough for me, so I guess I either remove the questionable breaker veeery carefully or call the electrician.
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Old 09-20-2007, 06:20 PM   #15
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Soon, if you can't cut the power to the panel containing your problem breaker, calling an electrician is the only option I'd recommend.
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Old 09-20-2007, 06:55 PM   #16
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Let us know what the electrician finds.
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Old 09-20-2007, 07:12 PM   #17
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Let us know what the electrician finds.
Spoke with the electrician tonight. I asked him if he thought replacing the cb might fix the problem cheap. He said he'd replace it if I want, but the code calls for a separate circuit, and having other lights and wall receptacles on the same circuit is probably the problem. Since my box is a straight drop down from the kitchen, in an unfinished part of the basement, he said he'd do the job for $250, otherwise it would have been 500-600. I told him OK, not like I have a lot of options. Hard to get any work done for under 500 in the DC area.
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Old 09-20-2007, 07:23 PM   #18
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Good decision Soon. Know what you're limits are and move on.
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Old 09-21-2007, 09:10 AM   #19
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Look for easy stuff first. The wire to your breaker for that circuit could just need to be tightened. Shut the breaker off, tighten and try it.
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Old 09-21-2007, 09:59 AM   #20
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Have you found out what is actually on that circuit? Man, I would do that before I spend $500 for a separate circuit. It's easy, turn off that breaker and see what else goes off.

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This seems to happen even when I don't have a lot of lights on in the rest of the circuit,
Can you set it up so there are NO lights on? One 100W lamp is going to draw about one amp out of the 15 you have available. If nothing else, try replacing them with CFLs - they use less current.

Just to re-iterate what others have said - breakers do get more sensitive if they have been tripped numerous times. See if it is practical to keep the other stuff OFF when you use the uWave, or move that other stuff to a different circuit.

That and maybe a new breaker may be all you really need.

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