This is first time for me dealing with instantaneous water heaters. I have a quadraplexes type building that will be divided into 4 tenants. They want instantaneous water heater in each tenant rated at 24KW at 240V. I'm coming up with a total load of about 38KW in each tenant, which puts me at 200AMP 240V single phase panel at each tenant. The issue is that this particular area is mostly residential where this is being built and everything is pole mounted transformers. Should i provide an 800AMP single phase service out that will probably require a pad mounted transformer from utility? Or possibly do a 600Amp 208V 3 phase service that will use a pole mounted transformer but will derate the waterheater by 25%. If you don't mind sharing your opinions on this and what you think is a good options to deal with this. Thank you!!
Instantaneous water heaters and required service
- Thread starter Cartoon1
- Start date
- Status
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
First question that comes to mind is the water heater listed for 208v? Sure it will work but some inspectors might call you on it if it were only rated 240v
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
I would also contact the power company engineer and ask them what they will allow.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
If you end up going 208 volts beware reduction of 25% in watts may be too much for an on demand type unit and will require upsizing the units. Actual performance really depends on how closely they were sized to expected flow rate and incoming water temperatures. You can get away with going 208 volts on storage tank heaters and the lesser watts more easily than with on demand units.
Spoke to the mecahnical engineer. It will work but he will have to increase the size of it because of the derate. So 208V is not a good option i believe.
Agreed. 240V is the way to go with this i think.
What worries me is in winter, if all tenants are taking a shower at the same time in the morning and you have your A/C heaters running and few other things. I know it is highly unlikely but what if!! lol. I can probably get away with 600A single phase service easily, but those water heaters pulling close to 100A a pop worries me a little.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
I wouldn’t say it is highly UNlikely, I think your scenario is highly likely! Remember, Murphy was an optimist…
Reasonable concern and Art 220 may not approach instant water heaters properly. I would not be surprised in the end if some "value engineering" bid doesn't end up with a 400 amp service and POCO a 75kva transformer 
In answer to your question as Dennis says in Post #4, your next step would be to see how the power company approaches it.
In answer to your question as Dennis says in Post #4, your next step would be to see how the power company approaches it.
Reasonable concern and Art 220 may not approach instant water heaters properly. I would not be surprised in the end if some "value engineering" bid doesn't end up with a 400 amp service and POCO a 75kva transformer
In answer to your question as Dennis says in Post #4, your next step would be to see how the power company approaches it.
Unfortunately the local utility here is not responsive, it will probably take a month just to hear back from them. At 400A service it will definitely trip sooner or later. Heck, even if 3 of 4 water heater come on at same time i think it will trip, when you consider all the other misc electrical demand they have. I believe 600A will be sufficient. But 800A service will help me sleep better at night lol.
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
Does whoever spec'ed instantaneous electric water heaters understand their downside as far as the electrical demand and service, and made an educated decision that the tradeoffs are a net plus? If not, perhaps you need to educate the specifier so they can come up with a better solution.
Cheers, Wayne
Cheers, Wayne
I tried!! but it seems like the owner have made up their mind.
hbiss
EC, Westchester, New York NEC: 2014
- Location
- Hawthorne, New York NEC: 2014
- Occupation
- EC
Conventional tank storage WH?
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
I said that somewhere and I about got my head chewed off. They insisted that the electric was the better choice. Certainly electric is 100% efficient but for some reason they are not thrilled with it. Not sure why..
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Incoming water temp is a big factor in how many watts is needed. OP being in FL might have better results with a electric on demand heater than people up north, particularly those on private wells with an incoming water temp that may be only 55F. If on municipal water that incoming temp may have a chance to rise some in the piping system before it gets to your water heater, but then in a dry summer if there is high demand for say lawn watering that temp may not rise that much at all. Then on top of that hard water will increase maintenance intervals needed, does with strorage tank heaters as well but they do have more room to store mineral deposits than a tankless does.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
They might have been unaware of the effect it would have on their electric bill! That’s what I see and hear around here.
“Why are my power bills so high?”
Has anything changed?
“All we did was change out our old gas water heater for a new electric instant hot water heater.”
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
If you change from gas to electric then yes electric should go up, but at same time gas should go down. How severely can depend on usage habits, rates, etc. In theory the tankless should use less energy overall (gas or electric) because as long as things are functioning properly should nearly eliminate idle time losses. Now if in a location where heating systems run more than cooling systems, heat lost by tank type heater isn't totally lost and does lessen heating load. In location where cooling runs more than heating it is the other way around though and tank losses increase cooling load.
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