Hello, thanks in advance for reading. I purchased a new countertop microwave over the weekend and have a question about power consumption. The reason for the new purchase was just because we wanted a larger one, went from 0.8 cu ft up to a 2. So, I learned something new when I unboxed the microwave and looked at the info plate on the back. When a microwave advertises as being 1200 watts it is actually referring to the output and actually needs to consume 1650 watts at 14.0 amps (again, according to the plate on back). Ok, not a problem. The outlet is on 12ga line to a 20amp breaker so plenty to spare before tripping the breaker. I let curiosity get the best of me and I plugged a kill-a-watt power monitor in to see how the microwave performed. Kill-a-watt allows up to 1800 watts – 15 amps to be plugged into it so I gave it a go. I heated up something in the new microwave and the kill-a-watt meter blew past the specs on the plate and leveled off at ~1930 watts and ~17.3 amps. I tested the kill-a-watt afterwards with a 60 watt bulb and it was spot on. Anyone have any insight as to why this would deviate so much from the specs on the plate and in the manual?? My first fear is a defective appliance that is a ticking time bomb, but this is assuming the kill-a-watt is able to accurately measure the draw from a microwave. Maybe they are a different animal and require special equipment rather than an inexpensive gadget from the hardware store? I really appreciate any feedback. If this thing really is sucking up that much juice then I would rather exchange it for a smaller one for the peace of mind of living in an older house when electrical standards were different. Thanks, -Chris

 

Two factors are at play. One is that the rated input power of the microwave is based on an assumed nominal or average line voltage. In reality the voltage level fluctuates within a normal range. When the voltage changes the power will also change. Depending on what the voltage happens to be at your house on any given day may change the power consumption. The second is that the bulk of the microwave's electrical load is an AC transformer, which means that watts is not the most appropriate unit to examine its power use. It also draws a magnetic component of power called reactive power which in the case of the microwave causes the current to increase for a given real power (watts). Electric motors are also in this class of loads, and anything else that has wire wound around an iron core. Incandescent light bulbs are purely resistive loads and therefore do not consume reactive power.

Bottom line is that with a 20A breaker and #12 wire you need not worry about it. The appliance will behave just fine.

 

Power consumption ratings for appliances are specified at a specific line voltage, usually 120 Volts. If your line voltage is different, the appliance can draw more or less power depending on it's internal design.

Plus, the Kill-a-watt is less accurate on inductive loads and less accurate near the top of it's range.

I doubt you have a defective appliance and I, personally, would not return it out of worries that it is stressing a 20 amp circuit.

 

Thank you. I knew I was thinking too black & white. When I see a plate that says 14 amps I thought that's what you'd get, but reactive power that you explained and inductive loads from another member who replied explains why it isn't so absolute. Let me ask this. Why would the manual require at a minimum that the microwave be plugged into an outlet protected by a 15 amp breaker? Seems like with the fluctuations and increases in current that you described would require the manufacturer to require nothing less than 20. If people are plugging this particular model into a 15 amp line then wouldn't their breaker be tripping all the time when in use?

 

Not if it is a dedicated 15 amp circuit. Circuit breakers do not trip exactly at their ratings. They act similar to a slow blow fuse and will handle current higher than their rating for some time. Depending on how much current over the rating is being used will dictate how fast the breaker will trip. Since microwaves only run for one or two minutes at a time there is likely not enough time for the breaker to trip.

 

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