Dimming lights when A/C turns on


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Old 05-16-07, 02:40 PM
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Dimming lights when A/C turns on

I recently built our home and did the wiring myself. I had a nice garage already on the site, so I connected the house to it with a breezeway. Before construction began I put 200amp service in the garage, was inspected, and given service by the power co. I then ran a 100amp sub box to the house (I know thats not typical) using 2awg copper, approx a 42ft run. I had an electrician check it out, he said it was fine. The A/C is on 8awg, approx a 46ft run from the sub box. The A/C is seer 13, (I think 3 ton). I have an ariel drop but it is "mid line?" in between two poles, probably an 80-100ft run. Why do my lights dim slightly when the A/C starts?
 
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Old 05-16-07, 02:59 PM
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> Why do my lights dim slightly when the A/C starts?

Slight dimming is normal when an A/C starts; however yours is probably emphasized by the #2 feeder to the 100A panel. An easy way to test this is to see if lights fed directly from the 200A garage panel also dim when then A/C starts. If only the house lights dim, then you can be certain that voltage drop in the 100A feeder is the culprit.

Electric motors draw a substantial amount of current for about 1 second when they first start spinning. This inrush of current into the motor causes the voltage to drop briefly in the panel feeders. That voltage drop can be visibly seen by the dimming lights.
 
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Old 05-16-07, 03:23 PM
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2awg feeder

No, the garage is unaffected. The dimming isn't excessive, just annoying. Should I be concernerned with the 2awg feeder? The furnace, water heater, and stove are gas, which is why I ask.
 
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Old 05-16-07, 03:40 PM
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Even though 2AWG is the typical size wire for 100Amp service... It never hurts to increase the size. Even over a 50' or less run you will still experience a voltage drop on the line... and since your garage is unaffected it's the Sub-Feed cable.
 
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Old 05-16-07, 04:00 PM
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Dumb question time: if I pull the #8 out of the sub-box and use a small junction box so I can run it to the 200amp panel, it will be approx a 90ft run. The A/C calls for a 30amp brkr, would this be OK? The junction box will be fully accesible in the attic.
 
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Old 05-16-07, 07:32 PM
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The dimming of the lights that you are experiencing is _probably_ not a problem, although as you have noted it is a nuisance.

_All_ electrical systems will have _some_ voltage drop. _Always_. This voltage drop is caused by the various impedances in the system as current flows through them. This includes the distribution transformer itself, your service drop, the breakers in the panel, the feeder to the subpanel, etc.

When the AC kicks in, there is a substantial current inrush, which causes a deeper voltage drop, and which is also transient, so more noticeable. With everything perfectly fine in your electrical system, you will get some flicker of the lights. A problem, such as a bad breaker, poor connection, etc. will cause _more_ voltage drop with the same loading, but it is very difficult to differentiate between 'normal' voltage drop and 'fault induced' voltage drop...if you notice a significant _change_ in the voltage drop then this indicates a problem.

_Fixing_ this nuisance is a question of how much money you want to spend.

1) Lights on dimmers are often more sensitive to voltage disturbance. If you have dimmer controlled lights in your home but not your garage, then this might make you think that the majority of the voltage drop is in the feeder between garage and home.

2) In general, you may have only _one_ feeder between detached structures. This means that in general you couldn't have a feeder to the house main panel _and_ a separate feeder to the house AC. There may be some ways around this limitation, and you may be able to get permission from the AHJ, but that is the general rule.

3) You might consider looking at HVAC controls that reduce the inrush current to the AC motor; check with your HVAC supplier.

-Jon
 
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Old 05-17-07, 05:17 AM
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The HVAC control winnie mentioned would be a hard start kit or some call them start assists. It's a simple start capacitor that gives the compressor an extra boost for starting and will sometimes minimize the light dimming if not eliminate it all together. These kits are not very expensive and can be a DIY job.
 
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Old 05-17-07, 08:35 AM
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Thanks for the good info. I was unaware that a dimmer controled light was more sensitive. The offending lights are on... suprise... dimmers! I may look into that hard start kit, but sounds like everything is normal. Thanks again everyone.
 
 

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