View Full Version : Blowing Fuses

Edward Loughman
01-14-2005, 8:31 PM
Antbody heard of this one? New wiring run from old fuse panel with 20 A fuse/12Guage wire - problem is old microwave keeps blowing fusees evry so often (once every two weeks or so). The micrw is the only load on the circuit. Ideas? My thought is to try a new micro. Old one draws 11A but maybe spikes due to a sporadic short. MAYBE SOMEBODY IS PUTTING TIN FOIL IN WHEN i'M NOT LOOKING - WOULD THAT DO IT?Tks for your thoughts.

Steve Ash
01-14-2005, 9:13 PM
My wife inadvertantly put tinfoil in the micro once...it fried (I think the motherboard) so that the controls wouldn't function...light would come on and the time was still there....but you couldn't use it to heat food. If memory serves me, it was close to a $100.00 mistake.
Ooops, didn't see it was your first post, Welcome to the Creek!

Rob Russell
01-14-2005, 9:35 PM

Welcome to SMC! Interesting first post :).

Fuses and circuit breakers serve two purposes. They will blow/trip based on current loads and short circuit. Your guess about an old microwave with an intermitten short is probably the answer. There is nothing about an 11 amp microwave that would cause even a 15 amp fuse/breaker to go based on amperage draw.


Ken Fitzgerald
01-15-2005, 8:13 AM
Welcome to the 'Creek Edward! I'm with Rob and you ...if your microwave is on a separate circuit and intermittantly blows the breaker....I'd guess that you've got an intermittant short in the microwave.

Randy Moore
01-15-2005, 8:40 AM
Welcome to the Creek, the water is nice most of time but it is cold on the bank right now 10 degrees above zero here just outside of Kansas City.

I would get a new microwave


Bill Turpin
01-15-2005, 7:03 PM
Take a male and female plug and wire in a pigtail light socket in series with the black wire. Screw a fuse into socket. Use this to power the microwave. If the fuse blows, the problem is the microwave. If the panel fuse blows and the microwave fuse does not then the problem is in the new wiring. Make the fuse at the oven a little smaller than the panel fuse. This can be done with a short extension cord and socket for about $6. A cheap test to save a MW that may not be bad. This handy tester has many uses around a shop. Caution: If you are not comfortable with flowing electrons then consult a friend who is.

I spent six years as a microwave repairman. All but one: the last thing cooked was popcorn. If popcorn is over cooked, with little moisture to absorb the excess MW energy, the oven goes into self destruct mode. At this point it can pull more than nameplate amperage.

Bill Turpin in WNC mountains

Edward Loughman
01-16-2005, 12:24 PM
Wow, thanks guys. Bill T, your little bypass fuse idea is great. One question, if its not the microw then what else could blow the fuse other than a nail thru the wire (which is possible since I used a 16G nail gun near the wiring. One very helpful guy suggested that corrosion at the panel socket can melt (not blow) the fuse. I am going to look at the socket Monday (its actually my mom's house) and see if it looks burnt - the fuse did not it looked like a cleanly blown fuse. I gave her another (unfortunately also older) microw to try. I think she said she was doing pizza in the oven when it went - she might have burnt it a well as like popcorn. So - the idea is that without enough water in the oven it can overdraw current (what about internal fuse - are there any?) maybe thats why my Dad used to keep a jar of water in the oven since he fried a micro mangnetron the way Bill suggessted.

Thanks for the interest guys, it can be lonely when things go wrong and your friends get this blank look as in - what' a fuse and what does 20 A mean. FYI, I'm a wood worker antiques restoration guy. Phila. PA area, it just finally got down to freezing - been 60 can you believe.

Bill Turpin
01-16-2005, 4:50 PM
Microwaves have internal fuses that are usually sized fairly close to actual amp draw. MW's pull startup amps quite similar to locked rotor amps of motors. They have resistive filaments in the magnetrons that have to heat to glowing before generating waves. The draw of filaments falls off after start-up. The heat of waves keeps the filament hot. The total amp draw falls off by about a third after start. The internal fuse is probably +-13 amps. It should blow before panel fuse.

Microwaves travel in straight lines and reflect off the oven sides until they are absorbed by the food. Fat and water are the best absorbers. When the food is over cooked the waves can reflect back up the MW gun tube and impinge on the magnetron. There are back flow filters in these tubes but with age thay can lose their effectiveness. Be sure new house wiring is not damaged. House fires are not fun. I lost mine to a loose connection in 1982.

Bill in WNC mountains

Edward Loughman
01-17-2005, 7:43 PM
Thanks, evrything is ok for now. I checked fuse box and it looked like new despite being 40 years old. It was a very old microw so I'm still suspicious of that and the cook (don't tell my Mom - its her oven and house).