Lights dimming under load

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Old 09-21-09, 06:48 AM
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Lights dimming under load

Hello all,

I have a 1965 south Florida home with a 150 AMP service. The main panel is a Square D QO located on the east side of my house. The sub-panel with the branch circuits is a GE PowerMark Gold and is located on the west side of my house in the garage. The sub-panel is fed by 2/0 aluminum conductors (maybe 50-75 ft). The main panel was installed about 1 yr ago and the sub panel in the garage with the branch circuits was installed about 2 yrs ago (both were Zinsco change-outs).

Here is what I'm noticing. When the A/C comes on (1 yr old 3.5 ton Trane XL19i with the two stage compressor), when the different compressors come on, I notice the lights in the house dim for a second or two and then come back to full brightness. I notice this at night (that's the only time the lights are on) when the electrical load in the house is otherwise low.

Additionally, in the kitchen, when I turn on the garbage disposal, the kitchen lights dim briefly (maybe 1/2 a second). I am 100% certain that the kitchen lights and disposal are fed from different circuits (15 amp for the lights, 20 amp for the disposal)

Is there a way to eliminate this light dimming? Is it something to be concerned about? I've checked all of the screws in both panels (except for the screws securing the feed from the electric meter - which was changed to copper when the panel was upgraded) and they're all tight.

Thanks!

- Joe
 
  #2  
Old 09-21-09, 09:37 AM
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Originally Posted by joec2000 View Post
Is there a way to eliminate this light dimming? Is it something to be concerned about? I've checked all of the screws in both panels (except for the screws securing the feed from the electric meter - which was changed to copper when the panel was upgraded) and they're all tight.
It sounds like the dimming you experience is within normal limits. There is very little you can do to eliminate it completely. One option is a "staggered starting kit" which an HVAC technician could add to your A/C unit which may help somewhat, but is unlikely to eliminate the dimming completely. This is basically a small timer that puts a few seconds of delay between the start of the air handler, compressor and compressor fan. You'll probably see three small dims instead of one bigger one.

The reason is an A/C unit that size can easily draw over 100A for a moment when the compressor first starts. Anything short of really massive cables and a big power company transformer will still experience some dimming under that kind of load.
 
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Old 09-21-09, 09:47 AM
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Thanks, Ben. I'm assuming the same holds true for the disposal too, right? Even though it is a much smaller motor (I think 1/2 HP), the start-up current is still sufficiently large to affect other circuits in the home.

The strange thing is, I haven't seen this happen in other peoples' homes. Maybe I'm just more sensitive to these things in my "castle"

Thanks,

- Joe
 
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Old 09-21-09, 10:26 AM
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Originally Posted by joec2000 View Post
Thanks, Ben. I'm assuming the same holds true for the disposal too, right? Even though it is a much smaller motor (I think 1/2 HP), the start-up current is still sufficiently large to affect other circuits in the home.
Yes that's true. The disposal also has a cheap low efficiency motor (because of the rare usage), so it can be as big of a start up as the high-e air conditioner.

The strange thing is, I haven't seen this happen in other peoples' homes. Maybe I'm just more sensitive to these things in my "castle"
Trust me it happens a lot. Other common culprits are the vacuum sweeper and the cloths iron.
 
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Old 09-21-09, 11:35 AM
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Are you saying that even if I oversize all my branch circuit wiring, including A/C and A/H, I could experience the same dimming?
 
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Old 09-21-09, 12:05 PM
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It will dim to a lesser extent, but yes it will still dim. The wires outside your house to the pole and to the transformer and the transformer itself also contribute and you really can't do anything to upgrade those short of bugging the power company. If the voltage drop is less than 10% there's no chance of getting them to do anything about it.
 
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Old 09-21-09, 12:35 PM
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Interesting... I wonder if the length of the sub feed conductors (2/0 aluminum, 50-75 ft) is contributing significantly to the problem? I know the conductors in my aerial feed are super small - they don't look any larger than #2. I suppose that given the exposure to the environment for cooling, they probably can be a little smaller though.

Would there be any benefit to hooking the fluke meter to an outlet to see how low the voltage drops when the A/C kicks on?

Thanks,

- Joe
 
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Old 09-21-09, 01:36 PM
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Originally Posted by joec2000 View Post
Interesting... I wonder if the length of the sub feed conductors (2/0 aluminum, 50-75 ft) is contributing significantly to the problem?
Anything less than 100' subpanel feed is not a significant contribution to voltage drop. Also #2/0 is a pretty good sized conductor. Your home wiring seems to be appropriately sized.

I know the conductors in my aerial feed are super small - they don't look any larger than #2. I suppose that given the exposure to the environment for cooling, they probably can be a little smaller though.
This is more likely the primary issue. The power company often installs much smaller conductors in the air than would be allowed in the walls. Convection cooling keeps the cable cool under high load, but it's really the resistance (ohms) of the conductor which is the factor in the dimming effect. You may have several hundred feet of aerial cable to the pole and transformer which several houses share. Depending on how many of your neighbors A/C are on at the time the transformer may be heavily loaded further contributing to the drop.

Would there be any benefit to hooking the fluke meter to an outlet to see how low the voltage drops when the A/C kicks on?
Not really for any purpose other than curiosity. Most meters are not fast enough to detect a motor start-up, but some of the higher end meters can capture it.
 
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Old 09-21-09, 02:58 PM
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I took a look outside... it looks like my neighbor and I are the only ones on this transformer and it appears to be the last transformer on the block. I remember losing power a few weeks ago and everyone to my west had power and every one to the east didn't (my house included). It was a little weird!

Think it is worth a call to the utility company?

Thanks again for your assistance and advice!

- Joe
 
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Old 09-21-09, 03:26 PM
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Originally Posted by joec2000 View Post
Think it is worth a call to the utility company?
You could give it a try, but I doubt they'll do anything about it.
 
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Old 09-21-09, 03:29 PM
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You're probably right. I filled out their online power quality reporting form (this is Florida Power & Light) and they said they'll look into the issue within 5 business days. I'll let you know what (if anything) comes of this.

The last time I called one of the utilities to report a seemingly small problem, I had 5 guys digging up my front yard later that day replacing a telephone junction box. We'll see how FPL compares to AT&T

Thanks,

- Joe
 
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Old 09-25-09, 10:42 AM
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The utility came out yesterday and couldn't find a problem. They did a load test using one of those hair-dryer sounding machines and the voltage drop was about 4 volts - which the utility said was fine.

The technician did recommend a new meter be installed because apparently the one I have is well known for burning up the meter attachment points. The tech said he would put a call in for a new meter but he wasn't sure if they would accept it.

- Joe
 
 

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