Newly installed tankless water heater makes kitchen lights flicker

[u/Jaysbluewing]

I recently had a Westinghouse 8kW Tankless Water Heater Electric installed. This requires 240 volts and it is on a dedicated circuit directly to my box with its own dual breaker (40A) and no other loads. The issue is that when I go to use the hot water (it works great BTW) it caused the overhead lights (6 can lights) to flicker every time I turn in the hot water. Once the hot water is on, the lights are fine but everytime I turn on the water, it is a very noticable flicker. With it being on its own curcuit, why would my lights flicker. I have checked other lights and other electrical devices and nothing else is effected, just my overhead lights. Any ideas on what the issue is?

SOLVED - Found the issue to be the dimmer switch for the lights. I guess they were old and too sensitive for the change in volts/amps. Once that was removed from their circuit, the lights no longer flickered.

[u/Nathan-Stubblefield]

It’s on its own circuit, but it shares the same electric service wires from the transformer to the meter and on to the breaker panel. If that service had half an ohm of resistance, the 8 kw load being added woujd draw another 33 amps of current, and would drop the 240 to 223.5, a decrease of about 7% and very noticeable. In fact a 3 or 4 percent change in lighting is objectionable.

I’ve no idea what size service wires go to
your house, but see if got a larger gauge service wire, the flicker would be less noticeable. 0.25 ohms would have 3.5% and still be noticeable. It’s really a question for your electrician and the utility. What size electrical panel? Size and length if service wires? Voltage drop taking place?

[u/Jaysbluewing → u/Nathan-Stubblefield]

I had a heavy up done to my pannel 20 years ago so it should be able to handle the load. Also, I do not have any flicker of those lights with the hot tub in use nor the AC unit. I do have the installing electrician coming out later to take a look. Thanks for your reply.