Hum and lights dimming - Electric Tankless Water Heater

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  #1  
Old 01-19-20, 10:58 AM
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Hum and lights dimming - Electric Tankless Water Heater

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

Any ideas?
 
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Old 01-19-20, 11:25 AM
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Welcome to the forums.

That Rheem water heater is 18Kw or 75 amps at 240vAC.
It requires TWO circuits. Each circuit is a 2P40 breaker with # 8 cable.

It seems to be more noticeable when the hot water is cracked,
A 75A load coming on line can certainly cause some lights to dim.
I would also consider a light hum normal.

Rheem manual (pdf)
 
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Old 01-19-20, 11:35 AM
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When you surround high ac currents with steel/iron expect some amount of hum. Can u hear this hum 6 away?

also, load centers will cause a certain amount of conductor separation. This increases the mag field that causes sheet steel to shake. Twisting conductors together minimizes the effect. Is it the cover itself the source of much of the hum?
 
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Old 01-19-20, 11:50 AM
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@telecom guy - No, the hum is really only heard when I'm a few feet or closer.
@PJmax - Yeah, I've got two dedicated lines running, as you noted. The dimming is not even though, like when our AC compressor kicks on in the Summer it is one initial hit and it seems relatively even afterwards. This fluctuates..

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, the other fluctuates. When it's medium, the second element is on 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?
 
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Old 01-19-20, 12:01 PM
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Old 01-19-20, 12:06 PM
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@AllenJ - I don't understand why the noise and dimming isn't noticed when the hot water is fully on though. If it were a loose connection wouldn't it be the same all of the time? Also, wouldn't I hear crackling/arcing, or see it?
 
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Old 01-19-20, 12:29 PM
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Did anyone perform a demand load calculation before you installed this energy hog? Your service may now be too small. The power company transformer may be too small also.
 
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Old 01-19-20, 12:31 PM
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A rapid light blink for any duration would certainly be annoying. Is this what u are seeing?
 
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Old 01-19-20, 01:28 PM
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@pcboss - We have a one bedroom house, 600sqft, not running anything besides the gas furnace blower, a couple led lights, a fridge, laptop, and radio. Even in the highest demands of summer, the calculated load should still be no more than 120 AMP.. If we were also running the dryer, washing machine, and a guitar amp, we should still be ok, under 180 AMP.

@telecom guy - it's more irregular than blinking.. Kind of like a wave against the shore, if that makes sense.
 
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Old 01-19-20, 01:42 PM
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one element is on, the other fluctuates.
This will be your lights blinking. Turning on and off high power loads does that. And this is definitely a very high power load.
 
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Old 01-19-20, 03:16 PM
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I'm not sure where you are measuring but that unit uses 240v elements. They are either on or off.
When you measure 120v it may be that the element is actually off as it can't generate any heat without 240v.

Based on the flow.... it may cycle one or both elements on and off.

When a load analysis is done.... the expected load is given to the power company so they can see how it fits into the street setup. You have a 200A service more than likely fed with a 100A aerial drop. That right there will allow for some dimming. If you are quite a ways from the transformer you will have more voltage drop there. The power company tries to put as many customers as possible on one transformer.
 
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Old 01-19-20, 04:25 PM
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I just measured our drop service cable and it looks like it's for 150A using this article as reference (https://www.thebuildinginspector.net...vice-capacity/). We are right off of the transformer first house actually. Any way to tell the rating of the transformer?
 
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Old 01-19-20, 04:33 PM
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Usually a double digit number is stenciled on the exterior. Like 50. For 50kVA
 
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Old 01-19-20, 07:16 PM
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The service cable from the head down to the meter needs to be 200A cable.
The cable between the service head and the pole is typically 100A cable. This cable is in "free air"
and is allowed to be much smaller in size than used inside a home.

I've upgraded many homes to a 200A service where the existing 100A service drop was not changed.
The only time an actual drop gets replaced is if the customer requires close to the 200A with a continuous load.
 
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Old 01-20-20, 06:08 PM
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What did your power company say when you told them you were going to add 18 KW load? You did tell them before installing the heater.....right? It's always the customer's responsibility to tell the power company when any new large loads are to be added to their system. Without tell them you could be held responsible for any utility equipment failure that might occur.
 
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Old 01-21-20, 06:06 AM
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... - I don't understand why the noise and dimming isn't noticed when the hot water is fully on ...
Upon re-reading your original post neither do I. So I deleted my comment.

... It's always the customer's responsibility to tell the power company when any new large loads are to be added ...
Would you shut off one of the two breakers for your new water heater and live with the consequences* until you notify the power company and get their approval?

* Before using hot water, (a)Turn off air conditioning or other electric heat.. No clothes drying at the same time. No use of electric stove or oven (or microwave oven) at the same time. (b) Only one person showering at a time with a GPM restrictor (typically red round insert with small hole in the middle) in or behind the shower head to keep the hot waer flow slow enough for adequate heating.
 
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Old 01-21-20, 07:51 PM
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@CasualJoe - I've never heard of this rule. Where does it come from? Seems impractical, especially since we've switched several large appliances from electric to gas (oven and dryer (should have switched this one too)). Those two would nearly equal this load... When I contacted the power company (Ameren MO) with the problem they replied with the following:Ryan,

I have entered a trouble ticket for someone to come out and check your service. You didn't attach a phone number so I used the number on file for your account. Once we check the service you will be notified of our findings.

Thanks for contacting us.

They did not reply with a reprimand for adding this load.


 
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Old 01-21-20, 07:56 PM
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@AllanJ - I don't think I want to live with this thing in general. Looking at an ultra high efficiency gas tankless right now. About to return this junker and cut my loss on this wire run.. maybe a sub panel for the future.. ha.
 
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Old 01-21-20, 09:21 PM
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The call center employees may not know whether you advised of the new load or not. As far as the rules of needing to advise the power company, it would be very hard to design an efficient system without knowing the loads expected to be imposed on it. Transformers and wiring do not have infinite capacity.

the abandoned cables will be of limited use for a subpanel. They do not contain a neutral.
 
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Old 01-21-20, 09:48 PM
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@pcboss - I used 8/3, just in case of future plans, and going through the trouble of running wiring.

Also, are you saying this is normal practice to call the power company and tell them my plans when I'm barely using half of my approved 200 AMP service? I am just not aware of that being a thing. Especially after removing 110 amps of 240v energy hogs already. I'm new to home ownership, and understand this WH is a big hit for a bit, but we cook a lot at home, and the electric stove/oven was a huge energy abuser before we took it off.. and again, the previous 30A 240V water heater and dryer.. All of those things added up would be more than the current water heater. I'm just confused why I would need approval from the power company. I might understand if I was adding this without removing the previous appliances.. that makes sense, even though I had ignorantly not considered it before. But for this situation, seems silly.
 

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Old 01-22-20, 05:39 AM
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OT: A gas instant tankless water heater may or may not need a heavier gas service (fatter incoming pipe). The installer or the gas company should be able to give you some hints.

OT: As all electric cars become more popular there will be more in-home charging stations drawing typically 25 amps @ 240 volts at a shot, usually overnight but a continuous (multi-hour) load. Many will be installed without notifying of the power company.

OT: Since (1/21/20 when) you mentioned you have removed several other heavy loads on your already approved 200 amp service, perhaps I should question the arbitrary need for y ou to notify the power company and question your liability for the consequences if you did not do so. A load analysis (sample rules at the back of the NEC code book) prior to any changes you made and a load analysis as of today both indicating no need to upgrade the service would suggest you would not need anything more than following the city electrical permits and rules.

... 100 amp feed going to a 200 amp service ...
OT: Every wire run has an amperes rating for acceptable temperature and an amperes rating for acceptable voltage drop. Overheating depends on the conductor size and material and insulation material, and how it is installed (in free air, in a stud bay, etc.). Voltage drop depends on the conductor size and material and the length of the run.
 

Last edited by AllanJ; 01-22-20 at 06:15 AM.
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Old 01-22-20, 11:12 AM
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@CasualJoe - I've never heard of this rule. Where does it come from? Seems impractical, especially since we've switched several large appliances from electric to gas (oven and dryer (should have switched this one too)). Those two would nearly equal this load... When I contacted the power company (Ameren MO) with the problem they replied with the following:Ryan,

I have entered a trouble ticket for someone to come out and check your service. You didn't attach a phone number so I used the number on file for your account. Once we check the service you will be notified of our findings.

Thanks for contacting us.

They did not reply with a reprimand for adding this load.
I am quite familiar with AmerenMO, I've dealt with them for years and know the ins and outs. Just check their Service Manual and everything I told you is there.

104. CUSTOMER OBLIGATIONS Inform Company as to the size and characteristics of the load that is to be initially and thereafter served, the location of the Premises, the need date, and any special circumstances or conditions affecting the supply of electric service by Company. Customer is responsible for securing information from Company regarding available facilities at a particular location. Equipment is to be installed in a condition acceptable to and approved by the governmental inspection authority having jurisdiction in the territory in which the Customer’s Premises is located or, where no authority exists, in accordance with Company’s standards and the requirements of the latest edition of the National Electrical Code. Be responsible for any damage, alteration or interference with company metering or other electrical equipment on Premises. Refer to Standards and Qualifications for Electric Service for additional information.
https://www.ameren.com/-/media/corpo...9D71BDAD573802

You may have a 200 amp service, but Ameren, like most power utilities, provides service based upon calculated load and NOT based upon the size service in your home. There is a very good possibility your 600 sq ft home is served with no more than 60 to 100 amps. Unless I miss my guess, your home probably has a #6 aluminum service drop, but definitely no larger than #4 aluminum.....maximum. I have a 200 amp service too, but with gas heat and gas water heater my service drop is just #4 aluminum, good for 100 amps. Adding an 18 KW load typically triggers an upgrade to a 1/0 service drop and many times a larger transformer too.

Just curious, what district are you in?
 

Last edited by CasualJoe; 01-22-20 at 11:27 AM.
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Old 01-22-20, 03:06 PM
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@CasualJoe - This is interesting. I feel like this is the first time I'm seeing the service manual. Wow, what a doozy; a lot to look through.Thank you for sharing. I'm in St. Louis. Ameren was here today. My wife said they had power off for an hour and the repairman said he made some changes outside because "there were a lot of wires that weren't being used and he streamlined what was coming in". The problem is still here however. I scheduled for J Bathe to come out next week.
 
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Old 01-22-20, 05:58 PM
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Both J. Bathe and M.R. Bathe are good residential electrical contractors, my old company has subbed worked to both of them (mostly to M.R. Bathe) on loft apartment buildings down in the City in the past. The Service Manual generally is only used by contractors, but I think it can be helpful to customers as well if they have a pretty good understanding of the mechanics of an installation. Please understand, I wasn't trying to bust your chops on this, just help you find a solution. Still just curious what area are you in. I live in North County. St. Charles by chance?

I had an idea of something you might try if you have a meter and think you can check a voltage. I would recommend you remove the panel cover and check the voltage across the 2 main lugs on the 200 amp main breaker without the water heater running. Then, turn on your hot water faucet all the way to establish good flow and check the voltage again after the water heater is running adding the full 18KW load to the service. I suspect you will see a substantial voltage drop. If so, you should share this information with both Ameren and the J. Bathe electrician when he comes out.
 
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