Flickering lights/low voltage on one leg

[Duster1671]

This is only tangentially related to fixed lighting, but I thought it was worth running by the CPF brain trust.

I've been having an intermittent problem for several months at home with lights flickering and UPS devices switching over and over between battery and line power. I started probing outlets and breakers with a multimeter and found that one leg of the power service coming from the meter into the panel sometimes drops 8-10V, measuring 110-112V. The other leg is at 118-120V all the time.

Here are my observations:

1. Flickering lights and low voltage at outlets on one phase but not the other. Affected outlets measure 110-112V.

2. Low voltage as measured between the bus bar and hot on every other breaker in the panel.

3. Low voltage on one leg of the mains coming in the top of the panel. The affected leg measures low with or without the main (200A) breaker switched on.

4. Power company pulled the meter and ran a load test at the meter connection and detected a low leg.

Here's what has been done:

1. Electrician rewired two multi-wire branch circuits that were wired incorrectly with both hots on the same phase. This did not fix the issue.

2. After pulling the meter and running a load test (#4 above), the power company replaced ~175 ft. of wire from the pole to the house. This did not fix the issue.

So my questions are:

1. Would you agree this is an issue with the power company's equipment? I don't see how it could be a problem with my home wiring considering observation #3 above.

2. Any idea what kind of equipment failure might cause this? Typical of a transformer on the fritz, or something else?

 

[LEDphile]

If you are measuring the difference with the main breaker off, the issue is upstream of the breaker. Voltage fluctuations on an electrical service are typically due to voltage drop in the wiring (a function of the load current and the resistance of the wiring and connections), with the added bonus that a high resistance neutral connection can cause the voltage on one hot leg to increase instead of decrease (one hot leg will go down, the other will go up).

My guess is that there is a bad connection or heavily unbalanced load somewhere between your service drop and the nearest upstream transformer. The transformer output is likely not the culprit, and the load test the power company performed should have indicated if the issue was a load issue or a connection issue (connection issue would be different voltage drop with load on each hot leg, upstream load balance issue would be similar voltage drop with load on both hot legs).

 

[Duster1671]

If you are measuring the difference with the main breaker off, the issue is upstream of the breaker.

Thank you for the sanity check

Voltage fluctuations on an electrical service are typically due to voltage drop in the wiring (a function of the load current and the resistance of the wiring and connections), with the added bonus that a high resistance neutral connection can cause the voltage on one hot leg to increase instead of decrease (one hot leg will go down, the other will go up).

I don't think this is an issue with neutral from what I've seen. When the bad leg drops 8-10V, the other leg stays at a normal voltage. 120V with no load, maybe 118 or 119V with some load.

My guess is that there is a bad connection or heavily unbalanced load somewhere between your service drop and the nearest upstream transformer. The transformer output is likely not the culprit, and the load test the power company performed should have indicated if the issue was a load issue or a connection issue (connection issue would be different voltage drop with load on each hot leg, upstream load balance issue would be similar voltage drop with load on both hot legs).

The power company person didn't specifically say if he saw the same or different drops on the two legs under load. He just ran the test and stated that I had a low leg. I would think that means he saw different drops and concluded a connection issue. So then he called in a crew to replace the wire from the pole to the house.

After replacing all that wire and not fixing the issue, I think all that's left is PoCo equipment from the transformer on up, or the short run of wire from the meter up to where the overhead line attaches to the house.

 

[turbodog]

...

After replacing all that wire and not fixing the issue, I think all that's left is PoCo equipment from the transformer on up, or the short run of wire from the meter up to where the overhead line attaches to the house.

At least they confirm the problem. Should eventually fix it.

If the transformer is fed by a single wire... then it's pretty much got to be in the transformer itself. Otherwise, how could it be outputting a differing voltage. Although maybe you could be looking at a neutral issue at the transformer itself.

I had a related problem years ago. Each time the wind blew, I could hear a frying noise come from all surge protectors. Ended up being a loose overhead wire over a mile away.

ALL of this assumes you are the ONLY load on the transformer. If this is a shared unit, all bets are off.

 

[Duster1671]

Brief update: PoCo was out again last night around midnight, different technician this time.

He pulled the meter and ran a load test again, this time finding nothing wrong. We chatted for a while and I described everything I've seen and what's been done so far. He seemed diligent and interested in fixing the issue, but basically said there's not much he can do if the issue is not occurring while he's there.

However, before he left he went up the pole and inspected the transformer. He found a lug at the transformer that was loose and arcing. Did not have a replacement with him, but tightened it down as best he could. That seems like a smoking gun, but I'll keep an eye on things and see if the issue is resolved.

 

[turbodog]

Brief update: PoCo was out again last night around midnight, different technician this time.

He pulled the meter and ran a load test again, this time finding nothing wrong. We chatted for a while and I described everything I've seen and what's been done so far. He seemed diligent and interested in fixing the issue, but basically said there's not much he can do if the issue is not occurring while he's there.

However, before he left he went up the pole and inspected the transformer. He found a lug at the transformer that was loose and arcing. Did not have a replacement with him, but tightened it down as best he could. That seems like a smoking gun, but I'll keep an eye on things and see if the issue is resolved.

At midnight... your usage was a good bit lower and the transformer has had time to cool down from daytime load. Both of these could explain heat-expansion-related poor connection at the lug.

I can't believe they replaced the service wire w/o finding this already.