I'm looking at wiring a Stiebel Eltron tankless electric hot water heater in a single family dwelling that requires three(3) 50 amp 240 volt branch circuits to the unit. The unit draws 28.8KW

So my question is what to install for the disconnecting means? Under the 2008 NEC 424.19 requires the disconnecting means to operate simultaneously.

Anyone done such an installation?

Thanx, shortcircuit

I think you might be in the wrong code section. A water heater is an appliance and falls under the requirements of Article 422. Article 424 applies to fixed electric space heating equipment.

Even if 424.19 did apply, you're feeding this equipment from more than one source and all disconnects are required to be grouped. Simultaneous disconnect only applies to one power source.

Tom

Thanks, so I'll use three 2-pole breakers with lock kits on them at the panel where the branch circuits originate.

422.31(B)

I installed one of these about 4 years ago and installed it as you proposed in your latest post. Had no problems with operation or inspection.

Good Luck!

(BTW, since this installation, I've strongly advised my clients to go with gas. The electric on-demand heaters require a huge amount of current and installation costs are disproportionatly high due to the wire, conduit (if required), etc. I've installed about 25 since, all gas)

You know, looking at this from a design standpoint ... I'm not all that comfortable with having three breakers and three runs of wire all the way to the appliance.

Would it not make sense to set a sub-panel, with a main? Then you could 'kill it all' with a single flip of the switch - and you would have a single run of wire for most of the way.

I saw one where the house only had a 100 amp service! Funny thing, I don't recall going back to that job for any further inspections.....hmmm!

I like to go with the subpanel at the device also with the main because of the high current use, though you can get small low flow units that service a small sink and use 30 amp, 120 VAC. Gas is greatly desired when dealing with these things instead of electric. These things can really break the bank in low power usage sites with demand meters.

I have a kitchen and bath on the other end of the house from the existing water heater and I am thinking about one of these or just a small tank to get some fast hot water. Right now you have to let the water run close to a minute, just to wash your hands if you want hot water.
Plan B would be my "attic" water heater idea. Just a large diameter pipe running at ambient attic temperature and a heat trap at the water heater. The problem is finding large diameter CPVC pipe.
Right now the water pipes run close to the ceiling drywall under the insulation, a northern idea.

Greg, I think you hit on the challenges we face with this issue.

I've seen the little under-sink water heaters, and have not been happy with them at all; the best they seem to do is make the water slightly warm. Half a dozen installs, and I've yet to find one that I like. (These were maybe 3 gallon tanks, and not true 'point of use' heaters).

Nor am I all that happy with 'instant hot water' piping arrangements; that's a lot of pipe to keep warm. IF I went that route, I might try to put the circulating pump(s) on timers, for high-demand periods.

Solar and attic heat collectors have merit, but there is always that issue of winter freezing. Before you know it, you have some pretty elaborate controls - and they're of little help for that 6AM shower.

What HAS worked is having a small (maybe 20 gallon) water heater mounted in the attic, above the bathroom. You feed this from the hot water line; its' job is to provide you the first few gallons of hot water while the line warms up. With this arrangement, there is very little 'stale' water to drain before you are getting hot water.

I tend to lean toward that small heater in the attic too.
I am getting ready to rip out that far bathroom and it shares the wet wall with the kitchen so the plumbing will be trivial. I will put a big drain pan in there so if the worst happens I will just be watering the grass. (I have a 55 gallon PVC drum I am willing to cut up)
I am thinking I can get away with a little 120v model.

Fortunately freezing is not a problem here. The plumbing is all in the attic anyway. My addition made that area of the attic easily accessible with a pull down ladder and a big platform to work from. I shoved 2 full sheets of plywood up there while the framing was still open.

Another idea are the local recirc pumps that use a temperature sensor. You push a button to start the local pump. The pump either pushes the cold "hot" water into the cold water line or thru a dedicated return line back to the input of the water heater. When the hot water line is hot enough, the pump turns off and you have hot water at the tap. No water waste.

Off topic:

I'm just curious.

These devices are a sort of standard over here (Germany). But we can wire them on 3 x 230/400 VAC with typically 32 - 40A breakers and 6mm² wire.

Internally they are wired in delta. This does not allow for using them on 1-phase 240 VAC as you usually have. (Split phase 120 not being relevant here)

Is there a link to an US installation manual?

Wolfgang,
I've worked on the showers that you speak of.
Personally I've only ever seen them in Gyms and Health Clubs.

With the price of a gym membership these days, you'd want to have a damned good shower.

But yes, you need a very substantial supply to any building that uses these appliances.

I looked at a tankless WH at Lowes yesterday. They want two 40a 240v circuits. I didn't see the KW rating but a max would be 15KW or so @ 80%.
I may be missing something but it looks like that translates to 110 degree/gallons a minute. If you need to bring it up 60 degrees that is lsss than two GPM but that is just counting on me fingers based on 3400BTU/KWH. Assuming I rounded them all up it is maybe a tad over 2GPM at 60 degree delta. That is not much hot water for the "whole house" that it is marketed for. It will put most people up into a 320/400a service too.

Here we reckon that 18 kW is sufficient for a sink, 21 kW is acceptable for one shower/tub. If you wanna feed more than one faucet you'd opt for 24 or 27kW. Less than 18kw is just poor.

High tech is the model which is remote controlled to a desired faucet temperature( If demand is higher than available power, these models decrease the flow by a extra valve. So you don't mix hot and cold and there are no surprises.

The problem with tankless systems is the huge power required on demand. Which, with the necessary heavy wiring and switching also adds up to lots of dollars. I have a traditional longhouse, [aptly named- it's about 144 feet long!]. With bathrooms at either end, now that the old stables, hay barn and machinery shed now have us both esconced in them, a single hot water supply would have been problematic, to say the least. I fitted 200 litre [52 USG] chauff eaux, [hot water tanks] running at mains water pressure of 45 psig near each bathroom and tapped off to the kitchens. [Yes, we got 2 of those as well, due to the remodel]. Power is only 2kw per unit, [230V @ 9A], run on cheap night and 2 hours midday tarif, which you can over-ride if required. These can be got for c. US$ 450 or less here, sizes up to 300 litres/ single unit. The heating is oil fired hydronic, which is not used for our domestic hot water. We use an average of 110 liters [26 gallons] of potable water a day, discounting the toilets, which are flushed with pumped well-water, biologically clean but makes lousy tea!
My units are foam insulated and stay hot for days if turned off, which one of them usually is unless the ankle-biters turn up for a free vacation. They are marketing solar-aided chauf eaux here now. When I looked at the price tag, the only thing that got hot was under my collar!

I am looking at a small wall hanger 1500w storage heater that will keep a couple gallons hot. That will be enough to buffer the water in the pipes for a shower and be plenty of hot water if you are just washing your hands. This should fit under a bathroom vanity.

I wanted to help having spec'd these out a few times, but my memory is shot and I could not find the projects in the lists of hundreds I have worked on. For some reason A.O.Smith sticks in the head as having 1.5kw 2, 5, and I think 10 gallon water heaters.

Wolfgang, here is a link to the heater that I will wire...

http://www.stiebel-eltron-usa.com/products.html

The job will us the Tempra 29 Plus which uses 28.8KW

shortcircuit

Thanks a lot for the link.

In short form:

1. Models differ completely from European models.

2. In Europe today blank wires are used for water heating. The insulation is achieved by mounting those blank wires in a plastic pipe. The distance to the copper part of the device and the resistance of the local water therefore define the unavoidable leakage current which is typically far below 3,5 mA (limit value)

3. With 400V in delta and blank wire technology we can get 27 kW out of a small housing of the 10kW type on the American market.

4. In certain parts of Europe the low voltage grid (230/400V) is stiff enough to support this technology without any problem. But not in F(rance) or E(Spain) for instance.