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edstevn
12-02-2008, 07:08 AM
I have a friend who has a 100A GE Main Circuit Breaker Panel (10 years old) in his garage that humms for about 2-3 seconds everytime the central air conditioning starts. Then the humming stops and the air conditioning cools properly. This happens everytime the condenser runs. The condensing unit is rated for 240V/40A. The breaker that feeds the condenser is rated at 240V/40A/2P. I noticed the wires that fed the condenser were rated #10AWG TW solid. I decided to replace the wires with #8AWG THHN stranded to match the breaker size. The length of run is about 50'-0" in EMT conduit. I decided to replace the wire from the breaker panel all the way to the disconnect switch at the condenser. The wire from the disconnect switch to the condenser was already previously pulled with#8AWG TW. This did not solve the problem. Next I to replaced the 40A breaker in the main panel, but that did not make a difference. I also tightened down everything in the breaker panel and inside the meter base, still the problem persisted. I read 238 volts at the condenser. Next, I inspected the electrical 40amp breaker disconnect switch that is mounted on the exterior of the house next to the condenser. Everything was tight and it looked good. Then I inspected the electrical connections inside the condenser and found one #8 red wire had the insulation melted within an inch of the line side of the contactor. I decided to replace the contactor and remade the connections. This still did not stop the hum. I told my friend to call an HVAC company to check out the HVAC system. The technician did not find any problems. I don't know what to do next? I think it might be in the condensing unit, but the tech did not see any problems. Help!

Billy_Bob
12-02-2008, 07:27 AM
What does the name plate say?

All about this subject and "HACR" A/C breakers...
http://www.iaei.org/subscriber/magazine/00_d/johnston.htm

jwelectric
12-02-2008, 07:29 AM
Every motor manufactured will have a very high current draw upon starting.

For the sake of this discussion let’s use a 5 horse power motor. The full load amps for a 240 volt single phase 5 hp motor is 28 amps.

The amount of current required to make the shaft of that motor start turning would be 6 time its FLA. This motor upon start up will draw 168 amps.

I think this might be the cause of the panel humming. I wouldn’t worry about it too much.

My panel used to hum so I entered it on Americas Got Talent and it went on to sign a contract to be an opener at the Dunes in Las Vegas. That panel forgot all about the kindness I showed it as I worked daily trying to teach it the words.