AC tripping breaker, help

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Stevenfyeager

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Location
United States, Indiana
Occupation
electrical contractor
Customer's AC unit when it kicks on, dims lights and sometimes trips the breaker. And when we turned on the breaker today, it immediately trips several times. Circuit cap. 19.5 a, max breaker: 45 amps. on label. It is on a 40 amp 20 yr old breaker run with #8 copper wire. Sometimes the breaker hums when we turn it on.
He says it started doing these two things after we worked there. He is getting ready to sell and had asked us to make the house code compliant. Several twins (tandems) were in the panel that did not allow twins. Plus someone had added a wide variety of breaker brands over the years. And this 200 a panel was one of the most crowded packed panels I have seen. We installed a sub panel.
Today we checked voltage and amperage at the outside AC while it was running 118v and about 17 amps on each leg. The 40 amp breaker felt warm but not hot. We disconnected the AC outside at disconnect, and of course the breaker never tripped when turning it on. Owner says he had his HVAC guy check it and said it was fine.
The lighting circuit that dims each time the AC starts up is in our new subpanel. Just for experiment, we temporarily ran a 12-2 from the sub over into the main panel and powered up the lights using the main panel and turned on the breaker, unfortunately the bedr and liv lights still dimmed.
I'm going to install a 45 a breaker and run a coil of #8-2 (that is in my shop) from the breaker, out the door to the AC the next chance I get. If my new wire temporary run and new breaker makes it good, I guess it has to be the AC or a nick in the house wiring somewhere...?
Any ideas ? Thank you.
 
Check the condition of the breaker's contact and the bus stabs.

The A/C breaker should be in the main panel, near the main breaker, if it isn't.

Do fall-of-potential measurements during compressor start-ups.
 
The most likely cause for the circuit breaker tripping is overcurrent on startup. I would get a meter that is fast enough to catch the instantaneous current reading when the motor starts up.

Could be lots of reasons for that but you need to run it down and figure out what's going on.

You could have an intermittent short in a winding, bad capacitor, problem with the motor, etc. You were just going to have to work your way through all the possible things that could be wrong. My best guess is either a bad run or start capacitor, or a shorted winding of some sort. Those kind of motors are pretty cheap. It might be cost effective to just replace it rather than spend a lot of time trying to debug it. Even if you prove it's not the motor, the cost of replacing the motor might be less than the cost of proving the motor is okay. At least with a new motor you know the motor is all right and it's something you can cross off your list of possible failures.
 
Change out the start & run capacitor, some are combination style. This is the most common problem on condensing units, especially in the summertime. With our maintenance program we automatically replace them every two years, they are not expensive and keeps service calls down. With high summer heat they dry out quickly and change value.
 
mtnelect said:
Change out the start & run capacitor, some are combination style. This is the most common problem on condensing units, especially in the summertime. With our maintenance program we automatically replace them every two years, they are not expensive and keeps service calls down. With high summer heat they dry out quickly and change value.
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I’m sorry. I don’t know enough to change those out
 
106 amps is not all that much for the start of an A/C unit. How long that lasts is another issue.
I agree with the start/run capacitor replacement but those usually will have some sort of visual indication. Bulged, leaking or obviously blown. A continuity test can help, but not always. Research how that is done if someone doesn't chime in.
 
I've checked light dimming on the A/C starting with a Fluke 87 which does fast capture min max recording. What I saw was incandescent lights can dim very noticeably even as the Voltage never records less than high 90's Voltage, meaning it was a non issue imo. You could see the lights dim but maintaining high 90's Voltage as the recorded transient low at locked rotor did not concern me.

If you could count to three seconds while the lights were dim that would be an issue that needs fixing.

Also have seen A/C starting trip the old existing 100 A Bryant main. I suspected that breaker was built cheaply with an instantaneous trip but not much of a calibrated time delay (feature). They kept the old main.

Definitely change the old breaker for the stated MOCP and something of known quality (process of elimination, change the cheap stuff first). I would absolutely probably not go further than that if the conductor size is sufficient and would view servicing the old unit as asking for trouble. I would not run the temp feed as it's probably non paying and the result does not tell me what I want to know.

I suspect the regular A/C guy charges an arm and a leg to service the unit. And needing service twice annual, the unit likely needs a good service tech as well as a new breaker.

Stevenfyeager said:
He says it started doing these two things after we worked there.
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I would likely ignore someone saying that to me, to my peril. You could always grimace and say "this place is ready to smoke, you got fire extinguishers handy". I had a boss I saw do that to the old ladies after he got them downstairs but before he opened the panel.

Running the temp test circuit would be all no fun for no pay. He should let you have some fun and enjoy the work.
 
Majority of these just have PSC motor for compressor, there is no starting capacitor. Those typically don't have enough starting torque if attempting to start before refrigerant pressure has equalized after the last run.

If said run capacitor is bad, it likely never runs at all though, and apparently this one does run at least some of the time.

Could possibly have had a hard start kit added to it, which is basically a potential relay that controls switching of a start capacitor. Should start capacitor stay in circuit too long for whatever reason it will draw high enough current that eventually something has to give, hopefully either the breaker or the motor overload protector.
 
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