Microwave suddenly dims house lights -- but nothing else does as severely

[Axel114]

An update: I bought a new 1100-watt microwave (does not have an inverter, nor does the old one). It also dims the lights, but not quite as bad. The light on the extension cord dims a lot but has not flickered or gone out completely. I don't know what to think. The light inside the microwave dims quite noticeably when the magnetron comes on. Is this normal? I don't remember this happening before, but now I'm beginning to wonder. Can anyone confirm this does/does not happen to theirs?

dick_freebird: That's an interesting possibility and one that I'll check out.

 

[Axel114]

If the problem is not confined to the microwave oven, it shows up with other power appliances too, then the problem lies outside the microwave oven. It is very likely your house wiring is divided (or made up of) several circuits. Each circuit is protected by its own fuse or circuit breaker. It is easy to identify: remove each fuse and see which lights or power points go off.

The question I am asking is whether the effect is seen only in the circuit in which the microwave oven is plugged in OR all lights are getting affected (effect is seen in other circuits too). If the dimming effect is seen in all the circuits, the cause is beyond - it is close to the meter box or the connection from the meter box to the inside. You need to track the source of this "loose connection" that is failing under high current. If you have an air-conditioner, turn it off during these tests.

I have mapped all the circuits several years ago. I know which outlets are on each circuit. It is indeed dimming lights and slowing AC fans on OTHER circuits. I tried another extension cord (100 ft 12-guage). The plug's LED also dims when connected to each microwave, but doesn't flicker or go out. So now the behavior is that both microwaves cause the effect, with the newer one slightly less. I'm assuming the loose connection possibilities are beyond anything I could fix personally. I wonder if the power company should check it. There would be a limit to how far they would go though.

 

[Kajunbee]

If you have the power company check the pole connections and there's no problem have them also check voltage at meter and connections at weather head if your service is above ground. A friend of mine had issues that plagued him for a long time. Dont know exactly what they found but problem was at or near the weatherhead.

 

[andre_luis]

I bought a new 1100-watt microwave (does not have an inverter, nor does the old one). It also dims the lights, but not quite as bad

This only indicates that the new product best meets current energy efficiency standards, but from what you have indicated the problem persists. This type of equipment should not cause such an impact on your home's electrical grid.

 

[dick_freebird]

Circuit breakers can develop increased resistance as
they age. If line sag is confined to the kitchen branch
then you might look into replacing the breaker.

 

[andre_luis]

Most likely, at some point there is a bad connection at the house power entry (eg due to the exchange of a burned circuit breaker without properly tightening its screws to the wire), to which the addition of an extra load is making sparks, causing the voltage drop in the whole home. This has already happened to me. If you had a thermal camera, you could detect this in the circuit breaker box pannel, even if the spark was not visible.

 

[Kajunbee]

Is this a 120 volt dedicated circuit that the 30 amp fuse is on with #10 wire. Are is the fuse feeding lights and receptacle also. If so someone along the line has increased the fuse rating to fit the load. If the fuse is not correctly sized to the wire it is potentially dangerous. Especially in and older installation. Never a good idea to increase the fuse rating to fit the load.

 

[Axel114]

I forgot to mention that about 2 months ago the fuse for the circuit the microwave is on mysteriously blew out (microwave was on, as was a 500-watt AC on the same circuit). Still that should not have blown a 30 amp fuse. We just assumed the fuse was old/faulty. I now wonder if this could have been the beginning of the current issues or was itself a symptom of something that began earlier.

andre_teprom: I will examine the panel tonight in darkness when the microwave and other items are on. There are 4 round screw fuses, not breaker switches.

Kajunbee: I'll remember to insist the power co. check those locations. Yes it is older wiring/panel and we've been aware of this (30 being too much) for a long time, which is why we mapped the circuits so that we can carefully manage the loads. New wiring and breaker panel would be ideal, but just not financially possible now.

 

[RobertFalbo]

Is it Aluminum wiring?

 

[Axel114]

I don't know if it's aluminum. It's in the US and is from the 1955-1960 era.

 

[BradtheRad]

fuse for the circuit the microwave is on mysteriously blew out (microwave was on, as was a 500-watt AC on the same circuit).

Identical to my apartment circuits. My building was constructed before microwave ovens were invented. Hence the microwave is on a 15A circuit which also supplies bath and bedroom. These add a few hundred W if I'm careless what I leave on.

The fuse blew a few months after I moved in. It made me realize I cannot leave bathroom lights on while the microwave is running. I wanted to increase to 20A but the fuse receptacle takes a particular size and thread. So I resorted to trying heavy-duty and time-delay types, 15A. Nevertheless a fuse blows about once a year, despite my efforts to be careful not to allow total usage to go above 15A. The fuse heats and cools as I use the microwave a few times each day. After hundreds of such cycles I believe the fuse gets metal-fatigued.

Both of our microwaves need to be on its own circuit.

 

[betwixt]

If nothing else, this demonstrates the advantage of 230V AC over 115V.
Double the voltage, halve the current, halve the losses!

Brian.

 

[c_mitra]

I forgot to mention that about 2 months ago the fuse for the circuit the microwave is on mysteriously blew out (microwave was on, as was a 500-watt AC on the same circuit)[\quote]

I have not heard about an AC so small; but every time you replace a fuse, it is time to take a look into why it went in the first place.

Yes it is older wiring/panel and we've been aware of this (30 being too much) for a long time, which is why we mapped the circuits so that we can carefully manage the loads. New wiring and breaker panel would be ideal, but just not financially possible now.

In the (good) old days, engineers used to install wires double the required capacity. Today they would not space any excess capacity. Your problem is almost certainly due to a loose contact near the main junction box. Most likely at or near the point the cable enters the building. Once that is fixed, everything will be fine again.

 

[andre_luis]

I will examine the panel tonight in darkness when the microwave and other items are on. There are 4 round screw fuses, not breaker switches

Even if you are not easily able to notice any heating, as in the case that the electrical connections are hidden behind any panel plate, you can still determine the existence of a problem just by hearing some buzzing typical of electrical sparking. You can build a simple conic hornet with A4 paper to amplify the sound from the exact spot inspected, but not needless to say, you must be careful to not touch anything.

 

[Kajunbee]

If you do not feel comfortable working with live wires see if the power company will remove your meter so that you can check and tighten all the connections. Check to see if there are any fees that might be added to your bill also.
You may want to remove the panel cover a day before they get there so you can get and idea of what tools you might need. Flathead and Phillips screwdriver is a given but you may also need a set of Allen wrenches or wrench to tighten lugs. You may also encounter some rusted or corroded fastener on the panel cover. Better to fight with it before they get there. Don't stick any wrenches in the box to see if there the right size until the meters removed. Just do a visual inspection to get and idea of what sizes you might need and familiarize yourself with the different connections.
If you felt confident enough you could remove one wire at a time and clean up the bare copper end to improve the connection also. This is a lot more time consuming and can also lead to its on set of problems. Good luck and be safe.

 

[Axel114]

Last night I observed the panel and the outdoor connections in darkness and listened carefully. There was no visible sparking or unusual noises that I could hear.

Some info that may be helpful... I can't believe I didn't think of it earlier. I tried turning the oven on (NOT the microwave) and the electric range/stove while the ACs were also running. That's quite a bit of additional power I'm sure. There was a very faint reduction in the AC fan, but it was almost imperceptible. Nothing like what happens with the microwave. Doesn't this indicate something about the location of the problem? If total load/volts of the house current were the issue, wouldn't there be more dimming when a big oven element and range burners were turned on? Also, our water heater is electric. I know that's a huge wattage draw (two 4500 watt elements?), yet there don't seem to be occasions of random dimming that would be associated with it. I also checked the reaction to the clothes dryer. It was a slight blink upon startup, but again, nothing like the microwave. The dryer however is wired separately and connects to its own box outside the house. Not sure about the water heater though.

I'm definitely not comfortable working with live wires and am not really sure what I should tighten if the meter were disconnected. Here's a pic from inside the panel from last year when we were replacing fuses.

 

[betwixt]

I know it doesn't help your problem but if an installation like that was found in the UK it would be condemned as unsafe. Here there are strict regulations on cable types and sizes, from the picture it looks like you have fabric (cotton) covered wires and they have a cross section of about 6mm. Here they have to be double insulated PVC and the incoming 'tails' have to be a minimum of 25mm. Given that your appliances draw twice the current of equivalent UK ones, I would think it's time to seriously think about a complete house rewire.

Brian.

 

[Kajunbee]

is this the only distribution panel in the house. Is your dryer and water heater 120 volt appliances. I see two 230 volt pullout disconnects. One is probably main disconnect. That only leaves one 230 volt circuit which is probably feeding the stove. So either your water heater and dryer are 120 volt appliances are you have another distribution point. I'm guessing your meter pan is on the side of the house. Is there another panel box attached to it.
I also noticed the second fuse from the left has two wires connected to it. Is this the circuit that gives you problems.

 

[c_mitra]

Last night I observed the panel and the outdoor connections in darkness and listened carefully. There was no visible sparking or unusual noises that I could hear.

You have provided a photo; are the four round sockets for the fuses (they are HTC fuse, I think that is what they are called)? Are you sure that the water heater (it is very likely not two 4500 watt= 9kW; they will be fed with their own wiring circuits) and the oven are fed from this box?

 

[BradtheRad]

water heater is electric. I know that's a huge wattage draw (two 4500 watt elements?)

It is not easy to discover when or which elements are energized. Both might be on when heating a cold tank to 120 deg F, then it maintains that temp by turning on only one element, I believe it's the lower one.