So, I just installed an electric water heater and I'm getting a light hum from the water heater and the breaker panel. It's also dimming some lights.. unevenly. It seems to be more noticeable when the hot water is cracked, but it's annoying, because I've checked everything so many times now. I've contacted the power company an an electrician to come out next week, but any other ideas?

200 AMP service. Murray breaker box. New Murray breakers (2 x double pole 40 AMP). 8 Guage Stranded Romex (40' run). Rheem RTEX 18.

All connections good and tight. Small 600sqft house, 2 people, not much load otherwise. Cleaned bus bars. Breakers not hot, not loose. Set to 110 degrees.

I've tested breakers and power at the WH. Everything appears to be fine.. However, going back to the idea of the problem being apparent with lower water, I checked the heating elements and noticed that when the hot water is on low, one element is on. When it's medium, the second element engages at 120, and at full the second element is at 240. Could the on/off, uneven volts be part of the problem? Is there a way to remedy this?

Any ideas?

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    That's very possible, and one of those things that's wild about home improvement; once you do something you are hyper conscious of your work and decisions afterwards. Plus it was an electric tank water heater before, and I never paid attention to when it was heating or not.. But the lights dimming is definitely new. Commented Jan 19, 2020 at 18:09
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    ... It would help if you could be more specific about under what conditions the lights dim, and what voltages you measure on the circuits when that happens. Commented Jan 19, 2020 at 18:45
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    @Peter Duniho - Correct, this is only when hot water is on. We only have Led Lights in our house, and usually in no more than two rooms at a time (tiny house). No big appliances running electricity besides the heater blower right now, and the fridge. The lights dimming are kind of uneven. Unlike our ac compressor in the Summer, where it one consistent thing at the beginning, this kind of wavers. Commented Jan 19, 2020 at 18:58
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    Ok, You said the second element get 120 volts.at medium. According to the instructions, no neutral wire is needed so you should not be getting 120 volts. Check both sets of breakers to make sure both sets of feed wires are 240V.
    – JACK
    Commented Jan 19, 2020 at 19:25
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    "200 AMP [panel]" - assuming there's nothing actually funny gong on in your house's wiring, I suspect that when the power company comes out they'll replace your inadequately sized service drop (?). Other than that, LED lights freaking out when you draw almost 40 amps? Yeah, probably ;)
    – Mazura
    Commented Jan 19, 2020 at 20:07

2 Answers 2


18KW is a 75 amp load at 240volts , which is a HUGE addition in draw for a very small house. For comparison, if you replaced a tank type water heater they typically have 2 4,500 watt elements but only one is on at a time for a current draw of just under 20 amps. You say you ran 8 gauge Romex, I assume that's 2 runs one for each 240volt breaker.

LEDs esp. with dimmers can be very sensitive to voltage changes. I have some in my home, on the same dimmer that dim at slightly different rates even though they are on the same circuit/dimmer.

Last, I agree with Mazura, your power drop from the POCO may have been adequate before, but now, with that much larger load, isn't anymore.


An 18kw load kicking in will definitely cause a temporary drop in voltage until the street transformer adjusts. You see it even with the air conditioner. That suggests with this new load that the utility company might need some new equipment. You need them to monitor the voltages for a period of time to verify this. Other item of concern is seen in one of the photos, must have been a huge sale on 20 amp breakers. I even see 14 awg wire on 20 amp breakers. Question: when you were "cleaning" the bus bars, did the breakers plug back onto the bus with ease or with effort? If there was no effort then they might be at their end of life. Need to verify with an infrared photo while under load to identify bad connections.

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