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Electrical Noise from AC condenser

Posted by radry (My Page) on
Thu, Jul 10, 08 at 17:28

I noticed what I thought was an electrical noise when my outside AC condensor unit first starts up. Sort of like a buzz or strain, but not like crackling arcing noise. The freon was recently checked and the unit cools very well. So I thought the problem might be in the panel. When it first started up I heard the noise somewhat by the panel, sort of a vibration noise.

I explained to the installer and they thought the breaker might be bad. They sent an electrician. Who removed old breaker, checked for signs of burn etc. Ran voltage test with his clamp on meter (a bit over 20 amps at initial start, and then about 7 amps when its running) and he said it all looked ok. The noise he felt was associated with the vibration of the wiring when it first starts up.

The electrician checked outside units ratings on the label. The draw he said was a minimum of 18 amps to maximum 30 amps. It was installed to the same breaker and wiring that the old AC unit used.....12AWG and double throw 20 amp breaker with maybe a 50 feet at most run of wire.

He said beefing it up to 10 and a 30 amp breaker might get rid of the sound, but he wasnt sure. But the way it was wired now was fine and didnt see a need to spend the money to beef it up. There is only the very minimum of light dim when the unit first kicks on, and a breaker has never tripped.

Is this good advice? Would it be normal to hear a sound as I describe when it first kicks on? Thanks.


Follow-Up Postings:

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RE: Electrical Noise from AC condenser

Heres the pertinent info from the outside unit...

LRA: 68 amps
RLA: 14.1
Minimum Ciruit Amp. 18.7
Maximum Breaker 30 amp

Is running my system with 12 awg and 20 amp breaker dangerous?

The amps measured at the breaker wasnt close to the ones on the rating plate?

Also, the strain, hum, buzz type noise on startup, does that seem normal after what I described above?

Thanks for reading.


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RE: Electrical Noise from AC condenser

If both the installer and a different electrician think it is ok, things are within the nameplate values, and you don't have any complaint about its performance, I wouldn't obsess over a little noise.

Going to #10 wire would ease starting a trifle. But it is starting ok now so why worry?


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RE: Electrical Noise from AC condenser

Yes but is the explanation, "the wire vibrate in the conduit" reasonable. Can this even occur?

When you turn your thermostat to cool, do you hear any noise besides the click of the thermostat, and then blower motor starting?


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RE: Electrical Noise from AC condenser

"Yes but is the explanation, "the wire vibrate in the conduit" reasonable. Can this even occur?"

When the unit starts up the first surge of current is up to the "LRA: 68 amps".

LRA is 'Locked Rotor Amperage."
Since the motor is NOT turning yet, it pulls a large current surge for a few cycles of the 60 Hz power.
This CAN cause wires in a metal conduit to move around.
It is less common in non-metalic conduit.
It is not a problem.

As long as the unit starts up without tripping the breaker, and starts up quickly (no more than a second or two) it is fine.

The current will drop to the "RLA: 14.1 " value or less so quickly the breaker does not trip.


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