Breaker panel help


Old 08-30-08, 06:45 PM
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Breaker panel help

I will be installing a new on demand water heater to replace the electric tank i currently have.

The new heater requires 2 60A double breakers. According to specs, the heater actually uses 100A at full load (240v)

So im looking at the breaker panel. It says 125A max on copper wire.
I am assuming 125A on EACH of the 120v Lines right?

The other big breakers using the 240v are the one for the furnace and the AC. The rest are the small breakers

So if i add the big ones i have:

Range = 50 Amp
AC = 40 Amp (this at the breaker box, but on the AC itself there is a 30A breaker so i guess it is using less than 30A)
Water heater = 20A (Current tank to be replaced)

The rest are 20A and 15A breakers for the lights, computers, television etc, totaling 85A distributed on left and right line of the panel.

So i am calculating something like this:

The range... which is never used at full capacity, at most just 2 elements at the same time out of the 3 and the oven which i never use..

The AC is on most of the time, as i am in south Florida. I believe is drawing less than the 30A breaker built in.

The new water heater, at full capacity in winter will draw the 100A... my guess.

So my math tells me something like this:

Range around 25A + AC lets say 28A + 100A Heater = 153A or 76A on each line. Plus the rest of the electrical stuff 85A which probably uses less than half of that.. but lets assume 50A to be safe... = 76 + 25(50 divided in the 2 rails) .... 101A on each line of the beaker if i am at full load at any given time in my apartment.

Now ... correct me because i could be WAY totally wrong.

In short.. the question is..
Is my breaker panel OK for my needs if i install this water heater?

Thanks a lot.
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Old 08-30-08, 08:19 PM
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NO, your panel/service is not large enough.

Electric tankless water heaters are probably the biggest addition to any residential panel you'll ever see. From what I've read, you might want to reconsider and install a high-efficiency water heater instead.

You should have an electrician perform an analysis of your service, meter, and panel and let you know what you would need to do. You might find that you might need a 225A or larger service to accomodate these.

Here is one thread from the pros at :

Old 08-30-08, 09:06 PM
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So im looking at the breaker panel. It says 125A max
Do you have a single main breaker? If yes, what is the rating?

You most likely have a 125 Ampere 240/120 volt service. This is a total of 30,000 watts. Your proposed water heater is rated at 24,000 watts. This leaves 6,000 watts for all the rest of your electrical usage. Your air conditioning probably a maximum of 5,760 watts which would leave you with only 240 watts to run all the rest of your electrical needs.

Electrical instantaneous (tankless) water heaters that are sized for anything more than point-of-use almost always require a minimum of a 200 Ampere residential service. I would never have a whole-house electric tankless water heater.

I agree with Williswires that your best bet is a new high-efficiency tank-type electric heater or if natural gas is available a gas tankless heater would be a better unit than an electric model.
Old 08-30-08, 09:46 PM
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I agree. In most cases, an electric whole-house tankless is impractical. Gas is more practical, but you'll need to install a much larger flue.
Old 08-30-08, 11:38 PM
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Just curious in a case like this could you just up grade to a larger drop or lateral and a double lug meter and then install a separate panel just for the on demand water heater.
Old 08-31-08, 12:19 AM
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I allready hook up few electric tankless waterheater and for myself it is a mixed result some are fine and some are not worth a crap.

However.,, if you going to use full house electric tankless waterheater the exsting 200 amp service may I say may squeak by but normally it useally involded with upgrading to 320 metering class { resdentail 400 amp service }

Which it mean the meter socket is rated for 320 amp contounus 400 peak amp and you will keep the exsting 200 amp main breaker box and just add a second 200 amp breaker box next to it.

This useally be done by electrician when you upgrading to the 320 class metering device and you will have to get a hold of POCO as well to see if thier transfomer can handle the load or not. { Many POCO are getting strict with this now due some of the tankless heater manged to blow the transfomer out pretty good }

The cost for 320 metering and breaker box will varies a bit depending on the set up and local codes but expect to run in few thousand dollars { I can not tell you the excat price due too many diffrent variations on price }

But the other option is get gas fired tankless heater this will useally somehow little more cheaper than straght electric tankless heater but of course you may end up changing the gas meter as well { that part you may want to check with your gas company to verify it }

Old 09-02-08, 09:40 AM
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I agree with the other posters in saying that you would need to have an electrician upgrade your service to a minimum of 200A; possibly 320A to accommodate the tankless heater. A formal "demand load calculation" is used to determine the exact size. If you search Google for that term, you will find a few websites that describe the procedure in detail. Also, many how-to books on electrical work cover the procedure.

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