Power Quality - Sustained Dips in Utility AC voltage

How was your issue detected?:
Primarily Sense Labs power quality monitor.

Did you noticed any other signs of this issue around your home?:

Lights will both flicker and noticeably dim for extended periods of time. Even relatively minor loads coming on – think less “AC” and more “TV” here! – will cause some flickering.

Screenshots from Sense Lab

  • Sense Labs graph screenshots

From the last few days:


So what I’m looking for advice on is a little different than most of the posts here, where there are wild, but transient, sags and spikes on the line.

The 1000x voltage dips screenshot (all from the last 3 days, not 30, since 1000x is the maximum) is kind of misleading, because nearly all of those are where one of the line voltages drops below the threshold of 110V. The line isn’t particularly noisy but there’s very clearly a periodic dip in the base voltage coming from the utility.

Part of this is obviously attributable to peak demand, and the day graphs above where it’s most pronounced correspond to high cooling-degree days. So what I’m trying to suss out is:

To what extent is this actually a problem vs. “working as expected?” Should I reasonably expect the base utility voltage to be 106V to neutral if it’s a hot evening?

I’ve used National Grid’s “report an outage” tool for this twice in the last two weeks – one of their outage options online is “dim or flickering lights.” Once, two weeks ago, I never heard anything back. The second time, on Tuesday, National Grid sent a rep out but all they did was tell me they were doing work down the block that would probably resolve it that night.

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Not normal.

I would use the “my lights are flickering” reason to get a truck roll, then show the trouble worker your voltage graphs. Ideally try to time the call to have them show up at 5PM. My utility shows up in about an hour after making such a call as long as it’s a clear day with no storms.


Awesome! That’s the standard that I knew must exist but had no idea how to find.

On Wednesday we had a brief power outage followed by a convoy of Nat Grid trucks going down the street, and I haven’t noticed any significant undervoltage since. Maybe they paid more attention to my report than I thought?

Wish I knew what they were up to. In any case, I’ll continue to keep an eye on it and follow your advice to put in another trouble ticket.

I did show the Sense stats to the tech who came out on Tuesday, but since they were already doing line work nearby I think they assumed it was related and didn’t do a lot with it.

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Hey Erik,

I would imagine that your ticket reporting flickering lights prompted the same response that a call would have. If you have the ability to, it would be really great to know what happened/what they fixed to resolve the undervoltage issues you were seeing! From my experience on the forums, it seems like calling the utility (vs. a digital ticket) leads to some more human interaction i.e. ability to ask questions.

Either way, thanks for sharing this!

As stated previously, I am in no way an EE but I did have a power outage a couple months ago. It affected something like 17 houses total, in the middle of two dense city blocks. The electric utility has a major truck yard about a mile or two away, so they showed up quickly. Looks like it was an issue with one transformer. I was surprised that transformers were so granular, and also that the grid was so inflexible that it couldn’t recover from one transformer issue. For the record, I don’t KNOW that it was a transformer, but I did see the cherry picker going up to transformers and poking around. I know other users have talked about transformers being the source of poor power quality.

Maybe you had a wonky transformer and they swapped it out for a good one?

The local Electric Grid is not like the Internet with a web design. It’s essential a series circuit, not parallel.

Series is where one Christmas Tree light goes out, they all go out. Parallel is where one goes out and the rest stay one. There are advantages and disadvantages of each design.

Furthermore, if there is a wiring issue, they go up in Cherry pickers and switch the transformer off. Just like you would do at the breaker box if changing wiring in your home.