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

[Kajunbee]

If this is a loose connection problem on one circuit it won't necessarily be at the panel. It could be any receptacle, light or junction box on that circuit.
Try to determine if the water heater is fed by this box also. I highly doubt it but there's always the possibility. Believe it or not a 40 gallon heater with 2x4500 watts only calls for a 30 amp breaker. As brad said only one element is on at a time. A 4500 watt element measure about 12 ohms. not sure if I did the math correctly . I came up with 10 amps at 120 volts for one element.

 

[c_mitra]

... I came up with 10 amps at 120 volts for one element.

That will give you only 1200 Watts. For 4500 Watts at 120V, the current will be 4500/120=37A. If both the heaters are on, the current will be 75A. They will certainly not be on a 120V supply.

It is very likely that most of the time only one heater will be on, but both heaters on (simultaneously) cannot be ruled out and the circuit must be fused so that it can pass 75A safely. Single phase 120V connections will not be provided for 75A load.

Some places (I am not in the US) provide discounted rates for water heating during off peak hours. They have their own lines and meters. The supply will be turned off during peak hours. Others may comment on this because I am not too familiar.

 

[Kajunbee]

the water heater is probably not fed from this box. But if it is being fed 120 volts from this box it's a different story. A 4500 watt element at 120 volts will draw 37.5 amps. But you wouldn't be able use a 30 amp fuse on it. But if you installed a 240 volt water heater element on a 120 volt line unknowingly the amp draw would be considerably less wouldn't it. It will still work although not very well. Recovery time would be much longer .Without knowing where the water heater gets its power it's all speculation. Just considering the possibilities.
I've seen 120 volt water heater elements hooked up to 240 volts before so I'm sure it can go the other way.

- - - Updated - - -

I guess what I'd really like to know is if the water heater and the microwave are being fed by the same fuse. It may not be a problem with the microwave but with the water heater.
Something else I've seen before is a chaffed wire bleeding voltage. Wouldn't trip the breaker but flourescent lights wouldn't work properly. This was on a steel boat I use to operate And it was shorting to the hull.

 

[Axel114]

The water heater is connected to a wire coming out of the wall - literally just a wire through the drywall, there's no plug, so I don't know where it's fused. I'm guessing outside since it's too much for that panel (isn't it?) Yes, this is the only panel, but I don't know what you would consider the outside boxes. I'm guessing that's where the dryer and water heater go. Dryer definitely but not sure about the heater. I don't know the voltage of either.

Yes, the four round sockets are the fuses. Yes, the oven is definitely fed from this panel (from the top right pull-out fuse set - I'm certain of this).

The water heater and microwave are definitely not fed by the same fuse. All the house outlets are fed from the 4 round fuses. I have tried the microwave on 3 of them with the same results. It's possible the heater is fed by the upper right pull-out but I doubt it.

 

[c_mitra]

T
The water heater and microwave are definitely not fed by the same fuse. ...

I am reasonably confident that the water heater is not supplied from the same panel (in the photograph). You are not supposed to turn on (or off) the water heater (you simply do not have a switch for that). There is no fuse for that either. So we can forget about the water heater for the time being.

If your oven is 120V (and the dryer should not be connected to the same line as the water heater), do you know the power? All the kitchen items should be on the same circuit.

 

[Kajunbee]

Go to harbor freight and buy you a cheap digital voltmeter. A simple voltage measurement would go a long way in helping the moderators isolate your problem. They are brilliant people but they can only take you so far with just a description of the problem.

 

[Axel114]

Very strange results today: The dimming has suddenly lessened significantly. The microwave interior light still dims a good bit, but not nearly as much as before, and the house lights/ACs/fans are no longer affected that much (maybe slightly). What on earth to make of this? The only thing I did before this happened was to test the dryer by turning it on and off 3 times to see the effects.

To answer the questions, the microwave is definitely 120v, but not sure about the range/oven. I've checked and am now certain the water heater is 240v with two 4500-watt elements. There are 2 pull-out style fuse boxes outside adjacent to the meter. I know one of them controls the dryer. I don't know if the other is a main switch for the house or if it's for the water heater.

 

[Kajunbee]

Did you call power company
Did you get a volt meter
Did you remove the panel covers and inspect the wiring
are you planning on going into all your panels and tightening the connections yourself
If not your waiting for the problem to go away or fix itself. Best to call and electrician.

 

[BradtheRad]

Excessive current tends to deteriorate components, perhaps suddenly. It's possible your microwave is now admitting less current. Therefore house lights stay brighter.

Does it cook things as quickly as when new? If so then the problem may have fixed itself.
If longer then it may be about to fail soon.

 

[Axel114]

Kajunbee: My mom doesn't want to call the power co now that it's doing better (although that was the plan earlier). I have a multimeter that I've used on computers and batteries, but am a little freaked about sticking the probes into this much power. By panel cover I thought you meant the one I did remove (see photo). You may be right about what's next. When I first asked I hoped it would be something obvious and the problem could be isolated, but it seems there's quite a bit of testing required that may get to levels beyond my ability. Things seem to be back to OK at the moment, but I'm guessing it won't last and the power co / electrician is inevitable.

BradtheRad: The microwave actually seems to be heating things more like normal now. When the excessive dimming was happening it would take a little longer for things to get hot.

I really appreciate all the help from everyone and at least feel like I've learned something about these things. If there's a solution or major change I'll update. For now, I think the new microwave is going back, as that doesn't seem to be the problem (that would have been simpler but oh well).