I have two zones of central a/c installed three years ago. This problem has occurred since start up, but since we haven’t run the air much until this summer, I neglected to address it. When the compressors first kick on there is a short buzzing noise that sounds like it’s coming from the conduit between the panel and the compressors. Also some of the lights in the house dim briefly. The 200 amp service panel and wiring were all installed when the a/c was put in. The wiring and breakers are appropriate for the units. One has a 40 A 2 pole breaker with 8 AWG wires, the other has a 30 A with 10 AWG. The run from the panel to the units is about 75 feet.
The electrician who installed all the wiring came out to look at it, said that the 75 foot run should be fine. He double checked the wiring requirements for the units and couldn’t find any problem. He’s going to come back and try running a temporary line with larger conductors, to see what happens. Do you all think this will solve the problem?
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The first two things I would check based on your description are; first, that the conduit fittings are all tight and there is a continuous equipment ground and second, that there is no short in the conductors. Either of these things could cause a major problem. Hopefully the problem is no more than the size of the conductors. The wires can move, sometimes violently, when they are put under a high load or shorted.
If you haven't drawn blood today, you haven't done anything.
No problem really, assuming probably EMT conduit with one ior more intermediate box.
The inrush current on startup is high enough that the magnetic forces generated by the wires cause a slight movement of the wires since both are not exactly the same spacing from the conduit or box and they are pushing themselves apart. This happens in my own house that I wired myself, have been meaning to go to the tie box and put in some tie wraps and more conduit fill to eliminate the buzz, but haven't gotten around to it <G>. As the compressor starts, the instantaneous current in the wires is going in the opposite direction, which pushes the wires apart against the conduit - this relaxes and starts 120 times a second, you could tune your piano by the buzz. The force is proportional to the current squared. If really curious, go to physics textbook and lookup Gauss's equations.
If your lights did not dim briefly, you would have one of the rare houses where the compressor startup current did not dim lights and the power company is losing money supplying core losses to too big of a transformer out on the pad or pole. Light output drops by the difference of the cube of the voltage drop, and the power company always puts the smallest most highly reactive transformer on the grid they can (good ecology and conservation, not power company greed), that and the line into the house is where the dimming voltage drop likely occurs.
Yes, it is EMT with two junction boxes on the run, they are both outside, under the deck. So you're saying you are going to open up the j-boxes and tie wrap all the wires together to keep them from vibrating?
You might talk to the AC man about installing a 'hard start' kit on the compressor. Normally only used on older units to keep them going for a few more years they can lower the in rush current when the unit starts.
He should also check to make sure the unit is not overcharged. Overcharging can cause the unit to constantly fight the pressure head.
If it doesn't happen when the unit has been shut down for an hour or so but does when it cycles the compressor may be getting 'slugged' by unevaporated refrigerant. A minor case of this can cause increased current draw. A bad case can wear out the compressor pretty quickly.
A defective compressor or one being forced to cycle too quickly, perhaps causing it to fight against back pressure, can cause it to struggle starting and this can cause high starting current and the symptoms you see.
A good and experienced AC man can often run through this list of problems pretty quickly. Some of the issues by simply listening to the unit start. I would think they need to be checked.
On the electrical side low voltage, damaged or corroded connections at any point between the transformer and AC unit could cause similar symptoms. A bad breaker may contribute to the problem. As may a loose connection in a disconnect or the main connections within the compressor. The later two are not uncommon. Mostly a situation where observation and checking the terminals with a screwdriver will serve.
Of course some current surge is normal and the sound may be because of something as simple as a conduit inadequately supported. Wires move when they take a load. If the conduit is, or is being used as a ground path a weak ground fault, a strong one would blow the breaker, can act as a choke on the incoming current increasing the size and duration of the current surge.
I'm not sure any of this answers the question. A few ideas to consider.
I'll have the a/c man come out, I'm still under a 3 year warranty for free service calls. Although I did have them come in June and they checked the charge, said it was fine.
I'll have to see if it buzzes after its been off a while.
Not sure if I mentioned it but even among exactly the same make and model of breaker some will make more noise than others. If your panel has one of the same amperage try using it to feed the AC unit. A simple replacement is an option. Breakers, at least the usual brands, are relatively cheap. Replacing a breaker is a simple and quick job.