Why does new 1700 watt microwave keep burning out my 20amp dedicated breaker?

[Toddvandy]

20amp outlet, 20amp breaker, 14 gauge wire. I suspect I need a larger wire but that’sa lot of trouble to go through without being sure.

 

[marconi]

First a question for the OP please? How far in metres or yards is your breaker board from the supply transformer? If your supply is overhead you ought to be able to see a round drum like object to which your supply cables connect to.

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Now to my learned colleagues. I have taken literally what the OP has said - the mcb has burned out - so it may well be the microwave oven is faulty in some way - but I suspect not since it is new. Anyway, it seems to me this MWO is a powerful model - 1700W. The Panasonic we had before disposing of it was only circa 800W.

The way the high voltage is generated for the magnetron is something like a step-up isolating transformer, capacitor C1, charging/swing diode D1 see:

https://www.researchgate.net/figure...in-a-microwave-oven-The-switch_fig1_234930316

When energised with HV the magnetron is pulsed on/off at to produce bursts of microwave radiation with a repetition rate equal to the mains frequency 50/60Hz.

Such a circuit will produce a primary current waveform which is not sinusoidal since the diode conducts on one half of the cycle and the magnetron on the other - once there is sufficient applied emf for forward biasing of the diode and circular thermionic conduction in the magnetron. You get the gist.

Such a power supply has a low power factor - I'd guess 0.65. Thus for a 1700W real power output the apparent power is 1700/0.65 = 2600VA If the supply voltage is 120V then the rms current is 2600/120 = 22A when the MWO is operated continuously at 1700W. I doubt this happens since it would quickly turn the contents of the oven to burning carbon. The magnetron's HV will be further pulsed on and off to achieve a lower average cooking power. But for the on periods the current will be circa 20 -22A - the continuous average rating of the breaker.

The cycling on and off of the magnetron is normally done on the primary side of the transformer by crudely switching the supply. No switching to minimise transients then - unless the MWO uses an appropriate solid state switch. The transient current for such a transformer rectifier combination can be up to 10, maybe 20 times the steady state current with a decay dependent on the L/R time constant.

The current waveform will have a high crest factor - the ratio of peak current to rms current - greater than the usual purely resistive (PF=1) circuits sinewave crest factor of 1.414. And because of what I said in red above will be of short duration something like this:

https://www.researchgate.net/figure...orms-in-a-Typical-Peak-Rectifier_fig1_3627656

I wonder whether then the Square D QO 20 A breaker has unsuitable overcurrent/short circuit characteristics and contact type to pass the peaky high current short duration waveform causing the magnetic trip to operate though I think the OP suggests this has not happened since the lever does not move.

Instead the contacts burn out because they are subject to a current density far higher than they are able to cope with - resulting in gradually and eventually catastrophic failure - they burn out. The Joule/Ohmic heating of these contacts depends on a square law which rises very rapidly with increasing current I/current density J eg: I/Isq = 20/400, 21/441, 25/625, 30/900, 100/10000, 200/40000. These very high peak currents albeit of short duration can cause very high local heating of the contacts. And small contacts will suffer most.....

Interestingly, thanks to Westward10's identification of the breaker as type QO, there is a specialist range of QO breakers viz QO - HM and QO -HID to handle one off switch on surge currents and cyclical surge currents respectively. See page 8 of:

https://download.schneider-electric.com/files?p_Doc_Ref=0730CT9801

QO® and QOB Miniature Circuit Breakers Special Application Circuit Breakers 8 07/2008

QO-HM and QOB-HM High Magnetic Circuit Breakers QO-HM and QOB-HM high-magnetic circuit breakers are recommended for area lighting (such as athletic fields, parking lots, and outdoor signs), when using lamps of inherent high inrush current, individual dimmer applications or other applications where high inrush currents exceed standard tripping conditions. These circuit breakers are available in one-pole 15 and 20 A ratings only. QO-HM and QOB-HM circuit breakers are physically interchangeable with standard QO and QOB circuit breakers and accommodate the complete range of QO accessories. QO-HM and QOB-HM circuit breakers are manufactured with the magnetic trip point calibrated at a much higher level than standard QO and QOB circuit beakers, as shown in Table 3.

QO-HID and QOB-HID High Intensity Discharge Circuit Breakers QO-HID and QOB-HID circuit breakers are for use in high intensity discharge (HID) lighting systems, such as systems using mercury vapor, metal halide or high-pressure sodium lighting units. These circuit breakers are designed to handle the high inductive loads, harmonic currents and cycling which are inherent in HID lighting systems. QO-HID and QOB-HID circuit breakers are physically interchangeable with standard QO circuit breakers and accommodate the complete range of QO accessories. QO-HID and QOB-HID circuit breakers are manufactured with larger contacts than standard QO and QOB circuit breakers to allow switching of high inductive loads. They also have magnetic characteristics similar to QO-HM and QOB-HM high-magnetic circuit breakers to allow the circuit breaker to hold in against the high starting inrush currents which are typical in HID lighting systems.

Or something along these lines... ?
====================================================================

For the OP - I am providing no advice just yet on what to do.

 

[Toddvandy]

That is Square D QO. Have you tried the breaker in a different position or temporarily connect to another 20A breaker, don't move it just connect your wire to another breaker.

Great idea. I switched both the breakers and the wires. The breaker I thought was broken does work with another wire both in the same place and different place. The microwave wire isn’t working in any breaker at any place. So it’s not the breaker I’m concluding. Even though the outlet doesn’t work I’m still getting a a strong electrical reading from the outlet. I removed the outlet and tried swapped it in a different place and it works. So it’s not the outlet. Am I right to conclude this is a wire problem? I’ve got to crawl into the attic and take a look to make sure it’ looks good

 

[westward10]

Is the microwave outlet the only accessory on this circuit.

 

[Toddvandy]

Is the microwave outlet the only accessory on this circuit.

Just got out of the attic. Found the 12 gauge from the box was wired to a14 gauge extension wire up there. The wire nut on the white wire was melted. My idea is to continue a12 gauge extension all the way to the outlet (5 feet). Otherwise, does this wiring look good? I’m running another receptacle off of the dedicated microwave box below it to plug in the gas oven which draws a trickle for sparks, light and timer.

 

[westward10]

I'm no expert on American wiring @Megawatt would know more but it looks as if you have found the issue. Is that connection unenclosed.

 

[Toddvandy]

First a question for the OP please? How far in metres or yards is your breaker board from the supply transformer? If your supply is overhead you ought to be able to see a round drum like object to which your supply cables connect to.

====================================================================

Now to my learned colleagues. I have taken literally what the OP has said - the mcb has burned out - so it may well be the microwave oven is faulty in some way - but I suspect not since it is new. Anyway, it seems to me this MWO is a powerful model - 1700W. The Panasonic we had before disposing of it was only circa 800W.

The way the high voltage is generated for the magnetron is something like a step-up isolating transformer, capacitor C1, charging/swing diode D1 see:

https://www.researchgate.net/figure...in-a-microwave-oven-The-switch_fig1_234930316

When energised with HV the magnetron is pulsed on/off at to produce bursts of microwave radiation with a repetition rate equal to the mains frequency 50/60Hz.

Such a circuit will produce a primary current waveform which is not sinusoidal since the diode conducts on one half of the cycle and the magnetron on the other - once there is sufficient applied emf for forward biasing of the diode and circular thermionic conduction in the magnetron. You get the gist.

Such a power supply has a low power factor - I'd guess 0.65. Thus for a 1700W real power output the apparent power is 1700/0.65 = 2600VA If the supply voltage is 120V then the rms current is 2600/120 = 22A when the MWO is operated continuously at 1700W. I doubt this happens since it would quickly turn the contents of the oven to burning carbon. The magnetron's HV will be further pulsed on and off to achieve a lower average cooking power. But for the on periods the current will be circa 20 -22A - the continuous average rating of the breaker.

The cycling on and off of the magnetron is normally done on the primary side of the transformer by crudely switching the supply. No switching to minimise transients then - unless the MWO uses an appropriate solid state switch. The transient current for such a transformer rectifier combination can be up to 10, maybe 20 times the steady state current with a decay dependent on the L/R time constant.

The current waveform will have a high crest factor - the ratio of peak current to rms current - greater than the usual purely resistive (PF=1) circuits sinewave crest factor of 1.414. And because of what I said in red above will be of short duration something like this:

https://www.researchgate.net/figure...orms-in-a-Typical-Peak-Rectifier_fig1_3627656

I wonder whether then the Square D QO 20 A breaker has unsuitable overcurrent/short circuit characteristics and contact type to pass the peaky high current short duration waveform causing the magnetic trip to operate though I think the OP suggests this has not happened since the lever does not move.

Instead the contacts burn out because they are subject to a current density far higher than they are able to cope with - resulting in gradually and eventually catastrophic failure - they burn out. The Joule/Ohmic heating of these contacts depends on a square law which rises very rapidly with increasing current I/current density J eg: I/Isq = 20/400, 21/441, 25/625, 30/900, 100/10000, 200/40000. These very high peak currents albeit of short duration can cause very high local heating of the contacts. And small contacts will suffer most.....

Interestingly, thanks to Westward10's identification of the breaker as type QO, there is a specialist range of QO breakers viz QO - HM and QO -HID to handle one off switch on surge currents and cyclical surge currents respectively. See page 8 of:

https://download.schneider-electric.com/files?p_Doc_Ref=0730CT9801

QO® and QOB Miniature Circuit Breakers Special Application Circuit Breakers 8 07/2008

QO-HM and QOB-HM High Magnetic Circuit Breakers QO-HM and QOB-HM high-magnetic circuit breakers are recommended for area lighting (such as athletic fields, parking lots, and outdoor signs), when using lamps of inherent high inrush current, individual dimmer applications or other applications where high inrush currents exceed standard tripping conditions. These circuit breakers are available in one-pole 15 and 20 A ratings only. QO-HM and QOB-HM circuit breakers are physically interchangeable with standard QO and QOB circuit breakers and accommodate the complete range of QO accessories. QO-HM and QOB-HM circuit breakers are manufactured with the magnetic trip point calibrated at a much higher level than standard QO and QOB circuit beakers, as shown in Table 3.

QO-HID and QOB-HID High Intensity Discharge Circuit Breakers QO-HID and QOB-HID circuit breakers are for use in high intensity discharge (HID) lighting systems, such as systems using mercury vapor, metal halide or high-pressure sodium lighting units. These circuit breakers are designed to handle the high inductive loads, harmonic currents and cycling which are inherent in HID lighting systems. QO-HID and QOB-HID circuit breakers are physically interchangeable with standard QO circuit breakers and accommodate the complete range of QO accessories. QO-HID and QOB-HID circuit breakers are manufactured with larger contacts than standard QO and QOB circuit breakers to allow switching of high inductive loads. They also have magnetic characteristics similar to QO-HM and QOB-HM high-magnetic circuit breakers to allow the circuit breaker to hold in against the high starting inrush currents which are typical in HID lighting systems.

Or something along these lines... ?
====================================================================

For the OP - I am providing no advice just yet on what to do.

Thanks for your time. I don’t understand most of it. As you can see in my recent reply with photos, the nut was melted where it connected to an extension in the attic. They ran a 14 off a 12, so I’m going to replace it with a 12 in a box. But very odd that changing the breaker would have solved this problem the first time. I agree this microwave draws a lot of power. Lmk if you think 20amp breaker and outlet are still sufficient.

 

[Toddvandy]

Thanks for your time. I don’t understand most of it. As you can see in my recent reply with photos, the nut was melted where it connected to an extension in the attic. They ran a 14 off a 12, so I’m going to replace it with a 12 in a box. But very odd that changing the breaker would have solved this problem the first time. I agree this microwave draws a lot of power. Lmk if you think 20amp breaker and outlet are still sufficient.

Oh, and the breaker box is about 15 meters from the transformer

 

[pc1966]

Just got out of the attic. Found the 12 gauge from the box was wired to a14 gauge extension wire up there. The wire nut on the white wire was melted.

That looks rougher than a badger's proverbial!

My idea is to continue a12 gauge extension all the way to the outlet (5 feet). Otherwise, does this wiring look good? I’m running another receptacle off of the dedicated microwave box below it to plug in the gas oven which draws a trickle for sparks, light and timer.

I would see if @Megawatt is around as he knows way more about USA practice than most of us (UK based).

However, if it were me I would look at cutting back the good 12-guage by 6 inches or so to make sure it has no thermal stress, then join it to more 12-guage using some good connectors and an enclosure so the sheath is clamped and all connections and primary insulation is covered.

This side of the pond those "wirenut" style of connectors where gotten rid of in the early 60s and no one has looked back! We prefer the likes of the spring-loaded Wago terminal as they are reusable/disconnect-able for test, but the spring-force means they don't loosen with time/vibration/thermal cycling like so many screw terminals.

Also you should be looking for the other joint, assuming you don't see 14-guage at the socket outlet, since there might well be another similar rubbish connection just short of failure/fire starting.

 

[pc1966]

Just to add, the wires at the socket outlet look as if the copper conductor was badly nicked when it was stripped. Probably someone (ab)using wire cutters. I would strip them again using a proper tool so there is no weak point to break in the future.

 

[marconi]

Just got out of the attic. Found the 12 gauge from the box was wired to a14 gauge extension wire up there. The wire nut on the white wire was melted. My idea is to continue a12 gauge extension all the way to the outlet (5 feet). Otherwise, does this wiring look good? I’m running another receptacle off of the dedicated microwave box below it to plug in the gas oven which draws a trickle for sparks, light and timer. View attachment 88404

Sorry for stepping on Megawatt’s toes but this photo seems odd/wrong but that may be because of the angle it was taken from. Why are the Black and white of one cable connected to white and black respectively of the other cable causing the hot and neutral to be reversed?

 

[westward10]

One connector, the black is hard to see but I believe the two whites are together it's just that one is burnt black.

 

[freddo]

Nice to see whoever fitted that joint was so lazy they couldn't even be bothered to cut the stripped back sheathing off.

 

[Toddvandy]

Sorry for stepping on Megawatt’s toes but this photo seems odd/wrong but that may be because of the angle it was taken from. Why are the Black and white of one cable connected to white and black respectively of the other cable causing the hot and neutral to be reversed?

Optical illusion. There were connected correctly

 

[Toddvandy]

Nice to see whoever fitted that joint was so lazy they couldn't even be bothered to cut the stripped back sheathing off.

Oh my gosh I’ve found all sorts of lazy shortcuts and cheapness in this house. Our basement fluorescent lights were “plugged in” to outlets hidden above the drop ceiling. The black and white wires were just shoved into the slots if the receptacle. Amazing.

 

[Toddvandy]

One connector, the black is hard to see but I believe the two whites are together it's just that one is burnt black.

It did turn black. What I’m not clear about is if it had something to do with 12 gauge connecting to a 14 gauge and overheating with the amount of power the microwave was drawing, or if when I remodeled and reset the box the wire in the attic moved and pulled out of the nut just slightly? Either way, it doesn’t explain why replacing the breaker worked-/at least until it didn’t. After running another 12 gauge off the wire to the microwave yesterday, everything seems to be working normally now. I’ve even got the gas stove plugged in to it. Hope it stays this way, and I’m using the breaker I thought was broken. Thanks so much for your help and I’ll reply again if there are further issues.