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Old 01-06-2009, 11:36 AM   #1
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power loss

I'm fairly handy for a home owner weekend warrior, but skills in electrical are rather novice (installed subpanel & various 15-20A single pole circuits). Whenever my 5-ton central air conditioner cycles "on", there is a "buzzing" noise and the power throughout the entire house decreases (lights dim & all other electrical devices pause) for a few seconds, then everything runs normally. The AC unit is the only significant electrical load in the house other than lights & refrig. All cooking applicances and clothes dryer is nat gas. This power decrease always occurs, no matter what loads are in the house. Can someone provide guidance as to what problem is and how it can be rectified. I do know the 5-ton condensing unit (outdoor unit) is on a (220-240 Volt) 2-pole circuit from the main 200A panel.


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Old 01-06-2009, 11:39 AM   #2
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Your compressor is struggling. Probably bad start/run capacitor.

It would be much easier to call an HVAC guy in this situation. It's what I do.


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Old 01-06-2009, 11:43 AM   #3
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What he said^
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Old 01-06-2009, 11:53 AM   #4
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Is the buzzing sound at the panel? There could be a loose connection there which will only get worse as time goes on.

A loose connection where a breaker clips on to the live power bars (busses) underneath can destroy the panel.
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Old 01-06-2009, 11:57 AM   #5
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Originally Posted by rcassidy View Post
there is a "buzzing" noise
I'd use a garden hose to isolate the noise source. You need someone on the distant end to signal you when the hose end is on top of the noise.

Assuming it is not a 60 Hz hum, it sounds like a relay armature oscillating, possibly from a high resistance connection upstream of the compressor.
The connection holds until heavy current is drawn, the voltage drops, the relay drops out, the voltage rises again to an approximately normal level, etc. The heating of the connection due to the heavy current draw causes it to "heal."
You get the same result, a rapid clicking noise rather than a buzz, when you try to crank a car engine with high resistance battery connections.

Knowing the length of wire and gauge supplying the compressor, you can figure out how much voltage drop you can reasonably expect with a known load at the compressor end of the cable that supplies it.

The problem is finding a heavy, 240v known load; most toasters/hair dryers want to see 120v. Two identical hair dryers in series might work to give you a known 10A load.
A cooktop element also works but you'd have to find a way to hold the thing for the few seconds it takes to measure the voltage drop.

Connect and disconnect the load a few times. You're looking for the difference in voltage.

Last edited by Yoyizit; 01-06-2009 at 12:15 PM.
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