I changed out some office lighting from fluorescent to LED tubes, bypassing ballasts and hardwiring the sockets.

Tenants now say the lights flicker. I checked them out, and sure enough there was a flicker on a 2 minute time frame.

Flickers for about 10 seconds.

Read voltage at 1st power supply box, before running to switches, and saw a dip at same time of flicker. Normal voltage 124.8, dips to 123.4 every 2 minutes. Followed circuit all the way to subpanel, opening every box and looking for loose wires. Found none. Could not replicate voltage dip with voltmeter at panel. I have not tried following to main panel.

Tightened all wires in panel, tho none were loose.

It was late in afternoon, and tenants said flicker seems gone.

I came back a few days later, and they thought it was still gone, now flicker is back.

I have tried to check for equipment which may be causing this flicker, such as plug in heaters, copiers, etc..., but did not find anything which would account for the flicker.

I am thinking about trying some type of filter to provide a smooth voltage.

Anyone have any ideas on type of filter/power conditioners? I would like to try it on just this one circuit, as no other office has had problems with flickers, even those that have been changed to LEDs. Do they make a filter that could be mounted next to a 4sq box above ceilings?

I would like to hear some ideas if anyone has any.

Thank you.

My opinion on what you describe:

No sense on going further back then the subpanel, if the voltage was steady there, and the 'dip' was only on the lighting branch circuit, agree??

You called "1st power supply box", is that the subpanel, or ??

Did you check the neutrals, and the grounds?

As for some type of 'filter', Greg may have some input on that.

I would question the quality of the LED.
I am not an expert but I assume the decent LEDs use an inverter power supply and that should be very noise tolerant with a very wide mouth.
Think of the wall warts that will go anything from 100v to 250v 50 or 60 hz and still put out steady DC.

Rick,
It's strange how this flicker happens every two minutes.
Could it be something like an air-con unit calling for heat or cooling and then starts?
I like your idea of voltage conditioning, but I think you need to get to the bottom of why the voltage is sagging with such a short time-frame in this installation.
It could be something as simple as a solenoid operated valve that is starting to go bad, I don't know.
With anything like this, I would suggest putting a data logger on the installation over a week or so and taking it from there, to see exactly what sort of power problem you have.

{Edit} Rick, LED lighting is terribly voltage-sensitive, it is after all based upon a diode, that has a forward voltage drop of anywhere from 600 to 750mV.
When you changed the lighting, you more than likely opened a whole can of worms and anyone that does this sort of work, needs to be aware that this could happen if the driver you install cannot handle fluctuations in voltage, it's going to be a bad day.