I just underwent a major renovation and now when my water heater is maxed out, it trips the washing machine breaker on its own dedicated circuit. I upgraded to 400A service with one of the 200A panels supplying only a Rheem tankless electric water heater (240V 150A using 4x 40A), dryer (240V 30A), and washer (120V 20A Square D PON AFCI/GFCI).

The washer works fine by itself, and all is good when the water heater is only serving a light load. But if I set the heater too high, or fill a tub faster than the heater can work, it shows “E5” (can’t keep up with demand) and the washing machine circuit trips.

Interestingly, when the water heater is fully taxed, there is a slight electric hum at the breaker panel.

  • It sounds like that dedicated circuit isn't as dedicated as you thought. You need to re-think the "major renovation" because something didn't go as planned or somebody didn't plan correctly.
    – gnicko
    Feb 19, 2022 at 23:03
  • The washing machine breaker trips even if the washing machine isn't on? I assume both panels have their own main circuit breaker?
    – rogerdpack
    Feb 19, 2022 at 23:12
  • What type of circuit breakers feed the tankless? I ask because the monster tankless units I have installed have contactor to pull in with heavy/ high flow hot water , it’s a inductive kickback thing or contact arc that Tripp’s GFCI’s and or AFCI’s when all of the elements are firing.
    – Ed Beal
    Feb 20, 2022 at 2:00
  • 1
    Are your panels QO (skinny breakers) or HOMeline (fat breakers)? Feb 20, 2022 at 2:04

1 Answer 1


You need to determine what type of trip you are getting: Overcurrent, GFCI or AFCI. If it is overcurrent then you very possibly have something miswired.

But my bet is on AFCI. AFCI detects arcing, but it does that by watching for certain patterns. My hunch is that the tankless water heater, because it draws so much power, is inducing a fault into the washing machine circuit. The humming is also an indication of a problem, and really shouldn't be happening if everything is installed correctly. Keep in mind that your tankless water heater, at full load, is drawing close to 100A. That is a lot of power to one device.

You may be able to solve the problem simply by moving the washing machine breaker farther away in the panel from the tankless water heater breakers. If you've got a typical big 200A panel and only have the water heater and washing machine in it, put the water heater next to the main breaker and the washing machine all the way at the other end. But if the wires are parallel for a significant portion of their run then you may have to actually separate the wire runs (obviously easier to move the 1 x 20A washing machine circuit than the 3 x 40A water heater circuits) in order to alleviate the problem.

  • I thought the same but the mis wire lost an upvote, contactors trip both AFCI and GFCI drop the mis wire and I could upvote, see my comment as I wrote it before reading on.
    – Ed Beal
    Feb 20, 2022 at 2:03
  • I'll change it. But while I fully understand how AFCI would be affected by other circuits, I am a bit confused as to how a pure GFCI would be affected. Feb 20, 2022 at 2:17
  • 1
    The inductive kickback & arcing from motors and contactors can trip GFCI’s and regularly trip AFCI’s sometimes on a different circuit, when I was doing more residential I saw this more often than today, they may be getting better but I am not sold on AFCI’s except for bedrooms, my belief based on failures is backstabs are the biggest problem , well other than failed AFCI’s & GFCI’s that have to be replaced.+
    – Ed Beal
    Feb 20, 2022 at 16:39

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