Lights dim when AC kicks in...

Whenever our air conditioning kicks in, the lights dim for a second. We had our service upgraded to 200 amp with a new panel and that didn’t help (the voltage drop also used to happen with the old 100 amp service and panel, and it still happens despite the upgrade to 200 amp service with a new panel.
Is this likely a problem with the air conditioner? It doesn’t look that old (but we’ve only lived in our house for two years), and it works well enough after the brief–but disconcerting–near-brownout it causes each time it kicks in.
Would a new air conditioner solve this? Or is something mis-wired?

What you describe sounds normal. Air conditioner compressors draw a lot of current to start up. As long as the drop isn’t causing any serious problems (like making your computer restart), I wouldn’t worry about it.

And if it is causing your computer to restart, install a UPS on your computer to smooth out the bumps.

Out of curiosity, is the air conditioner on a dedicated circuit? Or is it on the same circuit as the lights? That could probably make a difference.

Was the service line (the wires between the breaker box and the transformer) also replaced during the upgrade?

I remember they had to replace the pipe coming down to the house with one larger in diameter, but I don’t know if they ran new lines all the way to the transformer.

It’s possible the AC has some problem. And you’ve already done what any sensible person would do, upgrading your service.

I think I’d do several more things, in your shoes. First, I’d talk to the utility company. I’d want to know how old the electrical supply (outdoor) wiring is. I’d tell them the whole story, see what they have to say. Next, I’d try to find out more about the AC - how old it is, and stuff like that. If it’s over 10 years, fershure I’d think hard about getting it checked by a (reputable; one that won’t automatically say you need a new AC) HVAC company, if your local utility company doesn’t do that, or charges astronomical fees. Finally, I’d budget to get a UPS (Uninterruptable Power Supply) for the computer. One will protect it much better than a simple surge protector, and they’re much cheaper than they used to be. I think you can get one now for about $100.

I’m not sure. You’d think it would be on its own breaker, but maybe not. I’ll check that when I get home.

Is there any sort of device that can act as a buffer for this? Some sort of big capacitor or battery that can give the AC some or most of the power it needs just for that second when the compressor starts?

The AC is most likely on its own circuit.

I would suspect the problem is undersized service wires (the wires between the transformer and the breaker box) and/or excessive contact resistance at the terminal block where the service wires connect inside the breaker box.

Most hermetic compressors already have a start capacitor, some have a start/run or split capacitor. If that part were bad, the motor overload would trip out, and you’d hear the compressor trying to start every 5 minutes or so.

Since you said that the conduit (big pipe) was upgraded in size, as it would have to be to handle the larger SE cable you’re good as far as the connection to utility aerial. Depending on the regulations of your utility, and the tools that the contractor owns, the connection up at the weatherhead is either some type of split-bolt, assembled with anti-oxidant, or an irreversible crimp, those connectors having anti-oxidant already inside. Although it’s not impossible, both kinds are difficult to screw up, particularly by someone who is familiar with their installation.

As Crafter Man mentioned, undersized aerial can cause the problem you describe. The other possibility is underbuilt distribution grid. Today’s homes are requesting many times more electricity than when the grid was originally constructed. Shortly after buying my first home, I experienced lighting dip when the refrigerator came on, and I’d already installed a 200 amp panelboard, SE, and meter can. After chatting with my Dad, who was an engineer with the local power utility, they came out and evaluated my line voltage. After looking at the pole/transformer prints, they decided that between my house and the other group drawing, a transformer should be bumped to a higher kVA rating. A line crew showed up within a few days, and the problem went away.

The start assist capacitor on the compressor might be blown out (if this is a new phenomenon) or the service wiring might be undersized (if this has happened for as long as the A/C has been there).

Okay, I’ve got some things to go on, now. Thank you for the responses. I’ll post the source of the problem once I nail it down.

I think your dimming is a result of something in the house wiring which most likely isn’t hazardous.

Why do I say it’s something in the house?

Well, dimming was there before the exterior wiring was changed and it is still there in exactly the same form after. The common element in the two cases is the house wiring.

Too small a service lead from the power transformer to your house is not likely to be the cause because the OP implies that the dimming occurs irrespective of the electrical load (other than the AC) in the house. The service lead in residences rarely is carrying its rated capacity irrespective of the size of the main panel breaker.

It can’t be the starting capacitor of the AC compressor motor because if it were bad the motor wouldn’t start at all. The motor capacity isn’t there for the purpose of storing energy, it’s there to provide a shift in electrical phase between two windings on the motor so as to get the motor started.

Why is the dimming not hazardous? Well, it has apparently been there since house-day one. It only lasts during motor start-up and so overheating isn’t a serious concern.

It seems you just have to learn to live with it, or rewire the house. It’s probably a shared ground between the light circuit and the AC circuit. In my house, when the AC goes on the lights in one of the bathrooms brighten momentarily.

Go figure.

Have someone who knows about electricity measure the input voltage from the service lead at the main breaker and start the AC during the measurement. If it doesn’t change during the start it isn’t the service lead. When a licensed electrician contractor does such an upgrade he should insure, by calling the utility or checking the size of the service lead himself, that the lead is adequate for the upgraded house service.

Article 310.15(B)(6) 120/240-Volt, 3-Wire, Single Phase Dwelling Services and Feeders determines sizing, unless superceded by the AHJ or Utility. A 200 Amp service would call for 4/0 Aluminum or 2/0 Copper conductors.

The ampacity of the service aerial on the utility side is sized differently, as the cable is not enclosed in a raceway. I believe those tables are in the National Electric Safety Code, IIRC.

Where I come from the service lead up to to the house wiring, including the connectors, belongs to the utility and nobody else touches it on pain of serious consequences. Do you year me? Serious Consequences. So when the service was upgraded from 100 to 200 Amp. the utility must have made the disconnect from the old and reconnect to the new. At that time if the possible house demand, 200 Amp., had exceeded the service lead capacity they doubtless would have said something and you probably would have to pay to have a bigger one installed. Why would you have to pay? Well, you already had an adequate service lead for the original service and if you want more I think you probably would have to pay for it.

I took a look at the size of my service lead and it looks like it is at least #1 wire which is .29 in. dia. #12 wire has an area of 6530 circular mils. #1 has an area of 83690 circular mils. #12 copper is rated for 20 Amp. under conditions of poor ventilation so #1 should be good for 256 Amp. under the same conditions. It would appear that the writers of the electrical code are being super cautious in their sizing.

The dimming is something in the house wiring and might be an annoyance but shouldn’t be a hazard.

Are the affected lights on dimmer switches? I had an electrician out to the house recently on an unrelated matter, and he noticed the lights dim when my refrigerator kicked on. He mentioned that he gets a lot of questions about that, but it’s unavoidable that lights on dimmer swtiches will be affected in this way.

There are a usually a couple of capacitors on the A/C compressor motor- a start assist capacitor and a run capacitor. The start assist capacitor shifts the phase more to provide more torque at start-up. It is usually not absolutely necessary, but is needed for reliable starting.

If it is blown, the motor will draw more current on starting. I saw the results of a blown start assist capacitor at my friends house just last month, which is why I thought of it. His A/C still worked, but it dimmed the lights on starting. On opening the service panel on the condensing unit, we saw a charred hole in the side of the start capacitor and on the adjacent casing of the unit where the dielectric broke down and there was arcing to the casing.

(I’m a P.E. that designs HVAC systems, but for the most part, I see condensing units, air handlers, etc. as black boxes. I wouldn’t have thought of this internal part unless I had just seen it recently.)