Lights flicker when AC kicks in...

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Old 06-23-05, 12:46 PM
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Lights flicker when AC kicks in...

I have a new (1yr) house and a 200amp service panel. The condensor unit (2.5 ton or 3 ton 12seer) sits roughly 10 ft away from the panel. It is fed via a 20amp 12ga 240v circuit....when the A/C kicks in the lights in the house do a noticable flicker...my question is if I upgrade that line to a 10ga 30amp breaker will that help things at all?...the breaker is the last one in the "chain" and it sits below the breakers for the lights etc...

thanks
Doug
 
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Old 06-23-05, 12:51 PM
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No, changing the breaker and/or wire to the A/C will not help. Moving the breaker closer to the mains may or may not help. This may be just the nature of the beast, based on the loads on the power company transformer and the size of their wires. But just to be sure, depending on how noticeable the flicker is, I'd check the connections in my panel to make sure they are all tight, and I'd ask the power company to check their connections too. You might ask the power company if the transformer is due for an upgrade. Even after you do all this, however, it might still do the same thing, but at least you can feel a bit more comfortable that nothing is wrong.
 
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Old 06-23-05, 01:30 PM
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I'm not sure I agree that increasing the wire size to the unit wouldn't help. 3 tons on #12 might be pushing it a bit, even if it is relatively short (he/she says it's 10 feet, but that could be 16-18 feet of cable). The voltage could be dropping at the unit to the point where it struggles to start and therefore makes the duration of the dimming longer (and therefore more noticeable). Of course, that is about the last thing I'd consider, but it could possibly help.
 
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Old 06-23-05, 01:41 PM
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I see your point. So increasing the wire size to the A/C might help. But don't increase the breaker size!!!! That can't help and it might hurt.
 
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Old 06-23-05, 02:11 PM
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I have the momentary dim issue in my house as well when my 4 ton condensor starts. I put the 2P 40A right up snug to the 200A main, and used 30' of 6/2 RX because I had it in stock...

It's only apparent on the recessed lighting when dimmed which I would expect. I still would rather be cool than worry about a little dim now and then.
 
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Old 06-23-05, 02:16 PM
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I have not had a house that it did not do that in. The a.c. just draws such a high load at start up. But it should only be momentarly and hardly noticable. Even had people says theres dont do it but they just never really looked close enuf.

Also you say flicker do you mean dim? To me a flicker is off on off on and a dim is well you know a dim. I would say a dim is normal but if they indeed flicker off and on then maybe not.
 
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Old 06-23-05, 09:03 PM
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The inrush current of the AC unit is causing the flicker. The sudden heavy current draw drops the voltage at the common connection point of the breaker panel. Increasing the wire size to the AC unit will only help if the wire is so undersized that the unit is having trouble starting. (#12 AWG and a 20 amp breaker does sound a little small, but I don't think increasing it will help your problem). In any event, the heavy current draw at startup is causing a voltage drop in the service wires and the transformer, which means every circuit in the house sees it.
If you look on your AC unit, you will see two ratings; a FLA (Full load amps) rating (which is what the unit draws at normal running) and a LRA (Locked rotor amps) rating, (which is the inrush at startup). Typically you will see that the LRA is 4 to 6 times as high as the FLA. So if your unit draws 25 amps normally when running, it might draw as high as 100 to 150 amps momentarily when it starts. The POCO usually tries to size your transformer and service for running load, not inrush.
Unless the flicker is low enough to re-set you digital clocks, it is not harmful....just annoying.
 
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Old 06-24-05, 08:41 AM
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Thanks guys...I guess I'll just have to live with it...It's not enough to reset clocks or anything. I will take a look at the load rating and see if I should replace the wire/breaker. The main thing is that I don't want to starve that compressor of any juice...

thanks
Doug
 
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