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So I have a main panel (3 phase power), and a 100 amp sub-panel off of that. The sub-panel includes an ~55 amp draw tankless water heater. When it's not used everything is normal. However, when the tankless water heater is engaged, various electronics around the house show side effects:

  • The power bar for the TV / entertainment center buzzes loudly
  • Many dimmable LED lights on different circuits flicker / fade
  • LED lights in the master bathroom buzz loudly

All affected devices are on separate circuits and not tied to the sub-panel or the tankless circuit.

This only happens when the sub-panel tankless heater is active. What's going on? Is it dangerous?

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    How well balanced are your legs? A large imbalance can cause this, I have even seen neutral voltages climb when there is a very large imbalance. NEC Code states well balanced but no requirement on what well balanced is. – Ed Beal Feb 5 at 0:47
  • @EdBeal good question, I am not sure tbh as most of the work has been done by contractors in the past. – Haney Feb 5 at 0:51
  • Throwing out some ideas ... Is the Neutral bus in the sub-panel bonded to the sub-panel itself? It shouldn't; the Neutral and Ground should meet at only one point in the service; at the main panel, otherwise you can get circulating ground currents. Buzzing could be the result of a poor contact at the breaker-to-bus connection or the breaker-to-conductor connection. And I'm assuming there's no Neutral conductor as this is a dedicated and balanced (fixed) load. Did you torque them? – Chris Taylor Feb 5 at 1:04
  • I would get an amp clamp meter, at work for small stuff like this I use a fluke t5-600 it can measure AC amps up to 100. voltages to 600 and a ohm meter that only goes to 1 k. But at under 100$ it is a handy tool that I carry all the time for basic trouble shooting it is solid I have several that I use daily over 6-7 years old. There are less expensive meters with AC DC amp and better ohm meters but the cases don't hold up as well as the fluke mastech ms2108 only cost about 60$ can do 600amp AC DC but it is not a tool I carry regularly as I have broken 1 in 3 years it works well for the price – Ed Beal Feb 5 at 14:32
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    Continued using a clamp measure all 3 legs coming into the panel. What you may find is 2 legs heavily loaded and 1 very lightly loaded. Any large imbalance can cause your neutral voltage to climb. I worked on industrial lasers for many years 3 phase, the company that built the system put 1 laser ok l1 l2, and the second laser on l2 l3, these were large loads then the robot and chiller on l1 l3. There was a 50 amp imbalance and this created a neutral voltage of 13v and was blowing out electronics, moving loads and retapping all 15 systems fixed the problems that was caused from imbalanced loads – Ed Beal Feb 5 at 14:46
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Could be several things, among them:

  1. There is a loose neutral connection in the heater circuit or elsewhere and when high current flows to the heater, the loose neutral is causing flicker and it is causing what's called "common mode noise" (electrical noise related to ground) that is getting picked up by your sound bar.
  2. The overall added load of your water heater is too much for the service and is causing a voltage drop, in which case you need to call the utility about making sure you are being given enough capacity (i.e. the size of their transformer).
  3. Similar to #2, but is being caused by you have under sized conductors that are causing the voltage drop under load.

My first choice would be #1, given that it is getting picked up by the sound bar.

Is it dangerous? Possibly. Loose connections and/or voltage drops under load are never good things and will not "go away" by themselves.

  • Loose at the heater or at the breaker? I guess I should check both right? – Haney Feb 5 at 0:37
  • Could be loose anywhere. – JRaef Feb 5 at 2:05

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