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Old 02-26-2009, 12:41 PM   #1
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Voltage dropping from 121 to 117


In my bedroom I have noticed that my air purifier's motor intermediately slows down for a few seconds and then pick pack up. It is totally random and when it happens it doesn't last very long or there doesn't seem to be any pattern to it. It happens a few times an hour but it doesn't seem to happen every hour. Also, I think that it might be confined to that circuit, but I can't be sure, and I haven't noticed any lights dimming when this happens.

I hooked up a multimeter to the other receptacle on the duplex and when it happened voltage dropped from about 121 to 117.

I am going to call an electrician, but I was wondering what kinds of things could cause this?

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Old 02-26-2009, 12:54 PM   #2
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Voltage dropping from 121 to 117


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Originally Posted by Caliski View Post
In my bedroom I have noticed that my air purifier's motor intermediately slows down for a few seconds and then pick pack up. It is totally random and when it happens it doesn't last very long or there doesn't seem to be any pattern to it. It happens a few times an hour but it doesn't seem to happen every hour. Also, I think that it might be confined to that circuit, but I can't be sure, and I haven't noticed any lights dimming when this happens.

I hooked up a multimeter to the other receptacle on the duplex and when it happened voltage dropped from about 121 to 117.

I am going to call an electrician, but I was wondering what kinds of things could cause this?
(121-117) divided by some 10A load somewhere that goes on and off = 0.4 ohm = 80' of #14 Romex or 130' of #12.

No 10A load? Excessive resistance, maybe a bad wirenut.

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Old 02-26-2009, 01:48 PM   #3
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Voltage dropping from 121 to 117


When this happens, does the voltage drop usually last for about the same amount of time, or is it completely random?
Does the voltage fluctuate during this time, or just drop to 117, remain there and then jump back up to 121?

I would suspect another load turning on, unless the drop is erratic in that it jumps around during the time of voltage drop.
I would think a loose wirenut would be a much more erratic fluctuation.

It's only a 3% drop, so it's really not excessive.
I had a situation where, when the AC in my bedroom turned on, the UPS on my computer would switch to battery for about 1 second.
I measured the drop to be about 5%, and realized that I had an excessively long run of #12/2 wire.
I rewired the circuit, running cable a much shorter route, and no more trouble.

Check the circuit to find how long a run you've got. An electrician should be able to help you here.

FW
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Old 02-26-2009, 02:46 PM   #4
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Voltage dropping from 121 to 117


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Originally Posted by KE2KB View Post
When this happens, does the voltage drop usually last for about the same amount of time, or is it completely random?
Does the voltage fluctuate during this time, or just drop to 117, remain there and then jump back up to 121?

I would suspect another load turning on, unless the drop is erratic in that it jumps around during the time of voltage drop.
I would think a loose wirenut would be a much more erratic fluctuation.

It's only a 3% drop, so it's really not excessive.
I had a situation where, when the AC in my bedroom turned on, the UPS on my computer would switch to battery for about 1 second.
I measured the drop to be about 5%, and realized that I had an excessively long run of #12/2 wire.
I rewired the circuit, running cable a much shorter route, and no more trouble.

Check the circuit to find how long a run you've got. An electrician should be able to help you here.

FW

It seems to be about the same amount of time, a few seconds. And it does not seem to flutuate, it dips and then returns to 121. I don't have any big loads on this circuit that I would think would cause this. On other circuits I do have a fridge and a deep freeze, but other than that everything else in the house is small and energy efficient. I have HVAC, but this does not occur when heat kicks in.
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Old 02-26-2009, 04:49 PM   #5
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Voltage dropping from 121 to 117


Quote:
Originally Posted by Caliski View Post
In my bedroom I have noticed that my air purifier's motor intermediately slows down for a few seconds and then pick pack up. It is totally random and when it happens it doesn't last very long or there doesn't seem to be any pattern to it. It happens a few times an hour but it doesn't seem to happen every hour. Also, I think that it might be confined to that circuit, but I can't be sure, and I haven't noticed any lights dimming when this happens.

I hooked up a multimeter to the other receptacle on the duplex and when it happened voltage dropped from about 121 to 117.

I am going to call an electrician, but I was wondering what kinds of things could cause this?
My air filter in my bed room does the same thing. I would not worry much. I suspect it is related to the start up of an appliance that draws high inrush current, (compressor motor - fridge)

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Old 02-26-2009, 05:21 PM   #6
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Voltage dropping from 121 to 117


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a bad wirenut
Quote:
a loose wirenut
My feeling are hurt
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Old 02-26-2009, 06:22 PM   #7
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Voltage dropping from 121 to 117


It is possible that electrical equipment such as heat pumps in your neighbors' homes (served by the same pole transformer) are making the voltage drop.

You could satisfy your curiosity by measuring the voltage at different places about your electrical system, including in the panel, as your appliances, etc. come on. Get a length of 20 or smaller gauge insulated wire and connect this to one voltmeter test lead and tape it up well. You can use this to probe the big lugs on your main breaker with no more danger than measuring voltage at receptacles. (Connect the other lead to a neutral part physically several inches away.)
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Last edited by AllanJ; 02-26-2009 at 06:25 PM.
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Old 02-26-2009, 06:56 PM   #8
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Voltage dropping from 121 to 117


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It is possible that electrical equipment such as heat pumps in your neighbors' homes (served by the same pole transformer) are making the voltage drop.

You could satisfy your curiosity by measuring the voltage at different places about your electrical system, including in the panel, as your appliances, etc. come on. Get a length of 20 or smaller gauge insulated wire and connect this to one voltmeter test lead and tape it up well. You can use this to probe the big lugs on your main breaker with no more danger than measuring voltage at receptacles. (Connect the other lead to a neutral part physically several inches away.)
I don't like that; making measurements on the main lugs. I realize that the #20 or smaller wire will pop like a fuse if he shorts it out, but it's going to get extremely hot in the process, and could cause severe burns, and possibly copper in your face, or worse; your eyes!
You don't have to probe the main lugs. Just touch a probe to one of the circuit breaker load screws. If you think there could be a bad connection at the circuit breaker, try removing it (with the mains off), checking for corrosion on the bus, and on the breaker tabs, then replace.

Check suspect breaker against others for voltage, while the appliance is running.

It is more likely though, that there is not a problem at all. Just that it's probably a fairly long run of #14 wire.
Upgrading the branch to #12 and replacing the breaker with 20A would help, but of course make sure the entire branch is #12.

FW
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Old 02-26-2009, 09:45 PM   #9
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Voltage dropping from 121 to 117


Try changing the breaker, it may be faulty or about to go out. I would also make sure that, yes, loose wirenuts could happen.

Many house wiremen, not all, wire their homes the quick way, stabbed in the back... I would open all receptacles, lighting and switches and make "pig tails" to each device and leave the stabbing to OJ..i mean Jason Vorhees....

Also check the panel for:
Loose neutrals at the bar
more than 1 neutral under a lug
all connections on the service are tight
check the bonding to cold water, ground rod, if sparks, loose neutral..

When it does it again, see if you can catch it at the panel before and after the main breaker if you have one. Also check it at the branch circuit breaker. Check with the neighbor to see if they may have a problem, if so, Utility Company problem on 1 phase or incoming neutral.
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Old 02-27-2009, 12:12 PM   #10
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Voltage dropping from 121 to 117


You can measure the voltage at the main lugs by measuring the voltage at an electric dryer outlet with the dryer switched off. Pull the plug partially out to get assured contact with the test probes.

The source resistance test at the main lugs: if you switch off a 30A load the voltage should increase less than 30x10 milliohms = 300 millivolts (out of 240v). More typical would be 30 mV.

Last edited by Yoyizit; 02-27-2009 at 12:15 PM.
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Old 02-27-2009, 04:53 PM   #11
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Voltage dropping from 121 to 117


If you measure voltage in two different locations between a load and the service entrance while the load is turned on and off, and the measurements mimic each other, chances are you do not have a loose connection between those test locations. For 120/240 volt locations, measure both sides of the line.

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