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Old 07-10-2008, 07:11 PM   #1
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Wiring tankless water heater

I know automotive wiring, but some home wiring practices have me stumped.

The original tanked heater was on a 240v / 25a circuit, This is odd to me, as the rating for the heater easily exceeds 25a. The heater is 9000w which calcs. to 37.5a. It seems like it should be on a 40amp circuit.

Does the heater rarely, if ever, work at the maximum load of 9000w?

Next, I need to ask a code question

The tankless heater has three heating elements rated at 40a each and they are required to be on 3 separate 50a circuits.

I do not want to run the new wire in the walls. I would like to run the wire from the box, through the wall and into a conduit that will run along the interior garage wall to the heater. The obvious reason is I do not want to open up 30 feet of drywall. What considerations do I need to look out for to meet code requirements?



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Old 07-10-2008, 07:27 PM   #2
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First off, your old heater was NOT 9000 watts. It was 4500 watts.
The two elements are NEVER on at the same time.

Your new heater will draw a TON of power for short durations. What size is your service? There is a very real possibility that you may need to upgrade to a 320A service if you have a decent calculated load right now. Dealers love to leave that little tidbit out when selling these energy hogs.

Codes are the same for this as any other circuits in your home. NO different being a water heater.

If you are running conduit as you describe, it must run from the panel to the unit. You CANNOT run individual wires in the walls.

If you run conduit all the way from the panel to the heater then you can run six #8THHN and one #10 ground for the whole thing.
You may also need disconnects at the unit if it is not within sight of the panel.


Sometimes I feel like if I answer any more questions it is like someone trying to climb over a fence to jump off a bridge and me giving them a boost.
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Old 07-10-2008, 08:10 PM   #3
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I see about the elements not being on simultaneously. Thanks a ton for that info.

I did a lot of research and I chose a heater that exceeds the household needs and didn't require an upgrade to the main service. I have 200a service. I consulted the manufacturer and was assured 200a service was sufficient given that there is nothing else in the home that is out of the norm.

Even with the A/C, stove and the water heater running at full load, I am at about a current of 160a. There really isn't a scenario we could think of where we would be pulling anything that would trip the main breaker. I believe I would literally have to purposely attempt it to accomplish the feat.

Thank you for the help. I really appreciate it.

Last edited by TheBossQ; 07-10-2008 at 08:15 PM.
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Old 07-11-2008, 12:07 AM   #4
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The other thing you have to watch out when you run large tankless electric waterheater{s} is the POCO they will really raise the eyebrows pretty fast due some of the POCO transfomer may not handle extra load and burn them up.

I allready see it few time so far that why my POCO did rewrote the regulations in the book so that way if other customers want to use the tankless they have to inform them ahead of the time to change the transfomer if need to other wise it will be pretty expensive bill if it is smoked.

Yeah one thing it may bother you a bit with large tankless heater is lights flickering.

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Old 07-11-2008, 12:05 PM   #5
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Update - System up and running

Now that I have the system installed, I think I made a good choice for my particular situation. I was getting worried after reading several threads blasting the electric tankless systems. However, I think it's important to gauge whether or not a system like this is appropriate for your particular situation. I did a ton of research and it paid off. I really wish I had a gas system so as to avoid these potential electrical issues, however I am satisfied with the results.

The heater I have is controlled by a microprocessor that engages each individual heater coil (3 total) in stages. This prevents the unit from instantly drawing max amperage. I also live in Arizona and the inlet water temp to my house is currently 82f, well above the manufacturers recommended nominal inlet temp of 55f. So the heater only activates the coils enough to heat the water to the temp selected, which I have it set to 115f. It's max setting is 125f.

So far, I have not noticed any lights dimming whatsoever. Being that I have nothing else in the home that is out of the norm, my 200a service seems to be plenty sufficient.

Total cost of self installation was $800.00. The "Home Warranty " company wanted $750.00 to do a like kind swap.

I will certainly post any updates I have if there is a change in the performance or behavior of the system either way.

Thank you for your help gentlemen.
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