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Old 12-06-2008, 10:22 PM   #1
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lights dimming when using the vacuum


For whatever reason my house is wired so that the living room, kitchen lights, and a small office are all on the same 15 amp circuit. I have instructed everyone in my home that when using our 12 amp vacuum they should make sure that everything in those rooms are off, other than a light if needed. However, when using the vacuum on this circuit with everything off except the light, the light will dim while the vacuum is run, and stays dim during the duration of use. Is this something to worry about?

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Old 12-06-2008, 10:29 PM   #2
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lights dimming when using the vacuum


With that kind of load from your vacuum, the voltage drop can be significant enough to notice the lights' dimming.

Sometimes a loose or marginal connection in one or more devices (receptacle outlets or switches) can aggravate this condition. If you have a loose or bad connection in a device, then a heavy load such as a vacuum or blow dryer can cause significant heating, and possible burnout of the circuit at that device.

I'd plug that vacuum into a kitchen or dining room outlet if possible.

How old is your house?

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Old 12-06-2008, 10:34 PM   #3
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lights dimming when using the vacuum


1996, shouldn't the breaker trip before burnout? What exactly do you mean by "burnout of the circuit at the device"?
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Old 12-06-2008, 10:47 PM   #4
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lights dimming when using the vacuum


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1996, shouldn't the breaker trip before burnout? What exactly do you mean by "burnout of the circuit at the device"?
Breakers trip when there is a short or when there is too much current that is drawn.

Where ever there is a connection, it is a potential point of resistance, when the connections are made well, there is low resistance and it is not a problem. If a connection is less than ideal it can be a point of higher resistance and it work just like your toaster, and heats up. The breaker has no way of knowing of nor detecting this problem. When you run a heavy load like the vacuum, it is putting a heavy load onto the electrical wires. If the connections are good, they handle the load and don't excessively heat up.

Seeing the lights dim is not normal, and a poor connection would be a common cause. The outlet heating up and burning out is a possible result, or worse.
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Old 12-06-2008, 11:42 PM   #5
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lights dimming when using the vacuum


So what about the case when lights momentarily dim or flicker when you initially start an appliance and then operate normally during the use of an appliance?
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Old 12-06-2008, 11:53 PM   #6
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lights dimming when using the vacuum


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So what about the case when lights momentarily dim or flicker when you initially start an appliance and then operate normally during the use of an appliance?
Most of the time that is normal, and is hard to get rid of. Sometimes new wiring and isolating your outlets on separate circuits from the lighting will help. It is caused by a small drop in voltage caused by the appliance (normally a motor load). Motors use a huge amount of power when they start. Larger gage wire can help as well. i.e. upgrading to a 12gage / 20a circuit for a general use outlet. (if we are talking about a general outlet for a appliance such as a vacuum). If this is a central AC causing the flicker, the capicator can be changed to minimize the problem.

The problem with flicker can also be use to the power company. If the problem happens more seasonally, such as in the summer with high ac use, it is more likely a power co problem.

On my back porch I installed a new outlet on a separate 20a circuit. For the first time ever when I used a power miter saw I did not see any light flicker in that room at all.

The flicker is annoying, but normally isn't dangerous or harmful. Flicker isn't nearly as noticeable when you use CF bulbs, that is about the cheapest and easiest fix to try.
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Old 12-06-2008, 11:57 PM   #7
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lights dimming when using the vacuum


Is it very dim? If its not much you should be fine.
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Old 12-07-2008, 12:07 AM   #8
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lights dimming when using the vacuum


If I had to guess I would say about 5 to 10 percent in dimming. I just hate that these circuits are 15 amp. Doesn't leave much room to play when you have a 12 amp vac. I would like to upgrade the circuit to a 20, but I dread to think what it would cost to have an electrician come in and rewire the whole thing.
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Old 12-07-2008, 12:16 AM   #9
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lights dimming when using the vacuum


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Originally Posted by farmerde View Post
If I had to guess I would say about 5 to 10 percent in dimming. I just hate that these circuits are 15 amp. Doesn't leave much room to play when you have a 12 amp vac. I would like to upgrade the circuit to a 20, but I dread to think what it would cost to have an electrician come in and rewire the whole thing.
Flickering = Annoying, but not a hazard

Dimming = Could be a poor connection, that could be a fire hazard and should be checked out.

Upgrading the whole circuit means changing all of the wire to 12 gage.

You could add in a few strategically placed 20A outlets for a lot less money than rewiring everything. Then use the 20A outlets for heavy loads like the vacuum.

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Old 12-07-2008, 08:10 AM   #10
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lights dimming when using the vacuum


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Dimming = Could be a poor connection, that could be a fire hazard and should be checked out.
I disagree. Dimming is simply voltage drop and most of the time it is just normal. Rarely is it a "poor connection".
I see this in brand new homes wired beyond code.

My own home is wired with all #12, even the lighting, and quite a few circuits. I still get a dip in the ceiling lights when the fridge or A/C kicks on, and they are totally unrelated circuits.
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Old 12-07-2008, 08:22 AM   #11
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lights dimming when using the vacuum


Quote:
Originally Posted by farmerde View Post
If I had to guess I would say about 5 to 10 percent in dimming. I just hate that these circuits are 15 amp. Doesn't leave much room to play when you have a 12 amp vac. I would like to upgrade the circuit to a 20, but I dread to think what it would cost to have an electrician come in and rewire the whole thing.
Better solution would be to install a few strategically placed 20 Amp outlets for use with your vacuum, while ignoring all your existing outlets. Such as a couple in the hallway, where you could vacuum most of the house using only those outlets.

Or simply install a central vacuum system, fed from its own individual circuit.
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Old 12-07-2008, 09:26 AM   #12
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lights dimming when using the vacuum


"circuit burns out at a device"

Often happens where a wire is connected to a receptacle or switch; the wire is poked into the back and does sticks in place but for one reason or another the connection isn't that great. Screwing the wire on is much better.

Were it not for such back stab connections (you would need to go through your entire house and redo/rescrew if you have them) the existing outlets will serve your vacuum cleaner fine so long as you turn off most other things (stereo you are listening to loud while vacuuming) or use a different circuit so as not to trip the breaker.

If lights on different circuits also dim, chances are that there is no problem inside your home. But feel around the plastic parts in the main panel including the main breaker and poke a (wax) crayon at metal parts inside to check for unusual heat which would indicate something abnormal there.

Last edited by AllanJ; 12-07-2008 at 09:34 AM.
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Old 12-07-2008, 10:48 AM   #13
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lights dimming when using the vacuum


Incandescent brightness is very sensitive to voltage, but more than a ~4v drop probably means a problem.
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Old 12-07-2008, 11:10 AM   #14
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lights dimming when using the vacuum


Physiologically some people are more attune to light changes than others. I have the same issue with very slight dimming of lights when I turn on my high amperage devices. My wife and others can't see it but it is real noticeable to me.

I always assumed that it was normal since I have been aware of it since I was a kid. It has only been in the last 10 years or so that I realized that not everyone can notice it.

It's similar to hearing where some people can here a cat walking on the leaves and other can't.

I would not worry about the slight dimming of the lights when running high amperage devices. It's pretty normal in every house I have even been in.
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Old 12-07-2008, 11:51 AM   #15
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lights dimming when using the vacuum


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Originally Posted by Yoyizit View Post
Incandescent brightness is very sensitive to voltage, but more than a ~4v drop probably means a problem.
At only a 4v drop, would you expect to noticed dimming the entire time the device is running?

I've actually read alot on the subject, and done some testing, but as certainly open to learning more or being incorrect.

I have personally not perceived sustained dimming (other than the 1 second or less) unless I run a circuit past capacity with a very high load.

For example, the other night, Had the living room lights on, tree lights, flood lights in kitchens, and had a chop saw that was pluged into the same circuit. When you ran the chop saw (just doing quick cuts for say 10 seconds) the lights really dimmed down. I was curious why they dimmed so much. I put a clamp meter on the 15a circuit and realized without the saw we were pulling 14.8A continuous load. Virtually all of that saws power was being drawn in excess of 15a. We must have been just inside of the trip curve because the breaker didn't trip.

I have not noticed dimming unless a circuit is very overloaded like that. And have never seen anything other than flicker on other circuits in the house when running a high draw load.

There was some information on Mike Holts site that discuessed the possibilty of a power co issue (undersized drop or undersized / overloaded transformer) that can be responsiable if you see dimming while running motor loads.

Intresting..
Jamie

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