# How can I determine why my lights flicker when the A/C is running?

My lights considerably flicker whenever a large load comes on in the house and it has been this way since we renovated five years ago. I am attempting to troubleshoot this now and have already probably waited longer than I should have. I did some reading and ultimately removed the cover of the main panel and tested across the two hot wires at the main panel. All circuits were active. The reading I received was 245 +/- 2V, meaning average of 245V but fluctuated between 243V and 247V. I then had my wife turn our whole-house air conditioning unit on and voltage dropped to 237 +/- 1V while AC was running. Is this a normal voltage drop when a large appliance is turned on or is this indicative of an issue?

• Did the voltage stay low the whole time the ac was on, or did it go back up once the ac had gotten started? – Grant Oct 18 '14 at 18:59
• The voltage stayed low the whole time the AC was on. – WeekendWarrior2345 Oct 18 '14 at 22:03
• What size service do you have? When you renovated, did you add elements to the electrical system that would increase your demand? If so, did you also do a load calculation to include the new demand and upgrade the system if required? – Tester101 Oct 19 '14 at 12:42

What, if any work was done to the electrical system when you renovated and the problem started? In addition, what, if any work was done on the house in the immediate vicinity of the electrical service entrance?

Do you happen to own a non-contact thermometer (or know someone you could borrow one from?)

If this is a bad connection (and that's my guess), it will be heating itself (more or less depending on how much current loads in the house are drawing) - ie, if your microwave draws 10 amps, it's heating itself with 20 watts (10A X 2V) when you run that - if your AC draws 30 amps it will be heating with roughly 270 Watts (30A x 9V)

Lacking a non-contact themometer you can sometimes find these by touching the grounded exterior of electrical enclosures and feeling for heat. Having one means you can also look at specific parts inside the panel (but the problem may not be there - it may be in the meter box, which you generally can't open yourself anyway.)

Ideally you'd start at some point when nothing much has been using electricity in the house, or you've actually shut off the main for several hours, go around and feel/measure temperatures, then turn on the power and turn on as many loads as possible and feel/look for an area that is getting much warmer than the rest of the wiring/enclosures.

Since you are evidently comfortable in the open service panel, you can also check for voltage differences between the incoming wire and the terminal it connects to, and/or see if the voltage you measure is very different (under load) if measured from terminal to terminal or from incoming wire to incoming wire. That checks that end of the meter-to-service cables - the other end you generally need an electrician or the power company to check.

It is important to find and solve this issue before it gets worse; you're in the early stages of "how electrical fires get started" IMHO.

• I used "renovated" very liberally. We actually replaced the house rebuilding on the existing foundation. The entire electrical system is brand new, including from the telephone pole to the house. – WeekendWarrior2345 Oct 19 '14 at 11:57
• Right. So, on the one hand, all connections were new, and on the other hand, they should all be up to snuff, size-wise, etc. So you definitely have a problem here, and my bet remains with "bad connection somewhere" though I suppose it's possible there's an undersized aluminum wire outside the house where code permits the utility to pretty much suit itself, and cheap is what suits them all too often. – Ecnerwal Oct 19 '14 at 18:07

It's tough to say for sure without seeing the setup and troubleshooting further but it could be a sign of a bad connection (corroded, loose, damaged, etc.). Does this occur only on some circuits, or the entire house? This will give you an idea of where the issue is - if its all circuits then its likely between your panel and service.

Something to watch out for is if the lights get brighter or the voltage jumps up (you'll only notice this on 120V circuits). This is a sign of an open neutral which can be very dangerous and likely requires a licensed electrician to help troubleshoot.

• Thanks, Steven. I did see a constant drop. I've also observed this when monitoring voltage at an outlet and then turning on the microwave (separate circuit). The voltage at the outlet dropped from 120V to 118V and held at 118 while the microwave was running and then jumped back up to 120. I have never seen the lights grow brighter, only dimmer when loads are added to the system. – WeekendWarrior2345 Oct 18 '14 at 20:55
• Is all of your wiring copper? Only reason I ask is that if there aluminum wiring, there is a higher chance of corroded connections – Steven Oct 18 '14 at 21:41
• @Steven - All wiring, mains and service entrance wires included, will have some series resistance. This means that there may very well be a measurable voltage drop whilst an appliance is running. The amount of drop depends on various factors including wire size, length of wire runs, amount of current drawn through the wire, temperature of the wiring and the quality of interconnections between wires, outlets and circuit breakers. So it is not correct to say that no voltage drop would be observed while an appliance is running. The amount may be small but it will be there. – Michael Karas Oct 18 '14 at 21:45
• Thanks @MichaelKaras. I've removed that part of my answer. – Steven Oct 18 '14 at 21:52
• However, this seems like a SIGNIFICANT drop, especially with a relatively small load like a microwave causing a 2V drop on a different circuit. That and the timing being "since we renovated" smells a LOT like a bad connection somewhere. – Ecnerwal Oct 19 '14 at 0:06

The fact that the voltage drops that much at the main leads pretty well says it is a power delivery problem upstream of there.

There are only a few components it could be:

• wires from the breaker panel to the power meter base
• the meter socket
• the meter itself
• wires from the meter base to the transformer
• transformer taps
• the transformer itself
• the feed upstream to the transformer

If you have other neighbors sharing your transformer, measure one of their voltages while you turn on and off your load. If there is similarly significant voltage variation, the problem is clearly with the utility. If their voltage is steady, the problem is between the transformer and your breaker box.

As far as I know, the electrical utility usually owns and is responsible for all of these but the first two (meter socket and the wires from the meter base to the panel).

You could try giving the meter a wiggle to make sure it is fully seated in its socket—especially if the meter was removed during the renovation.

Call the utility to check their connections and voltages up to the meter base and the meter itself. Since they will pull the meter to do this, try to be there to inspect the connections to your wire to the panel. Maybe the tech will measure its resistance for you.

Add a capacitor across the lines feeding the A/C unit. The capacitor will discharge in an effort to maintain the original voltage level when the A/C equipment starts. This should improve if not completely eliminate the problem.

• Hello, and welcome to Home Improvement. Adding a random capacitor across the line will almost certainly end in tears or worse; would you be more specific? And, you should probably take our tour so you'll know how best to contribute here. – Daniel Griscom Aug 23 at 2:43