Service Size, how much do you need?

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  #1  
Old 05-17-05, 07:27 AM
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Question Service Size, how much do you need?

I am considering replacing a leaking tank type hot water heater with a
tankless heater. A whole house electric tankless water heater can
require a 100 amp circuit (some are larger I'm sure, some smaller).

The breaker panel is rated for up to 150 AMP, I can't read any number
on the main cutoff so I'm not sure what the actual capacity of the service
is rated at.

What or how do you determine what your service rating should be for
the entire panel? If you simply add up all the amps from breakers on
each circuit it seems like you would be way over what is really needed.

How can I determine what the power company's service drop is rated at?
I don't want to assume that just because someone installed a 150 amo
panel that the power company drop to the house is also capable of 150 amps.

How could I estimate (so I can compare the cost of installing an electric
tankless water heater versus a gas tankless water heater versus just
putting in another tank type heater) the cost of upgrading the power
supplied to the hoiuse if the current service is not adequate?

Thanks,
Jim
 
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  #2  
Old 05-17-05, 07:41 AM
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Click on the link and find out what size you need. Then we can discuss if a service upgrade is necessary.

http://www.tanklesshotwaterheater.com/
 
  #3  
Old 05-17-05, 10:48 AM
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Sizing Electric service

Thanks for the link. The installation will be in Columbus, OH.
The site your referenced indicates we would need a 60 degree
rise in water temperature. I could probably get an exact value range
from the water company, but this seems close enough. We would like
to run 2 low flow showers at the same time. This then translates to
about a 3 gal/min capacity from the heater. The referenced site has
3 models that we might select from. On the low end is one providing
2.8 gal/min and requires 100 amps (two 50 amp circuits), in the middle
is 3 gal / min requiring 108 amps, and at the high end is 3.3 gal / min
requiring 116 amps. The spec for the model on the low end of our range
carried the recommendation "150 amp house service recommended" and
the unit at the high end of our range says "200 amp house service recommended".

I have also looked at various other sites, though I haven't checked each
manuafcturer in detail for ratings in the area of quality, support, reliability,
etc.

Here's one that I found interesting, as a feature of this unit is that it
ramps up its power needs depending on demand and inlet temperature effects. (info for an example model that might meet our needs)

MODEL EX280T2T, VOLTS 240, kW 28, AMPS/phase (3 x 40A) 120 amps, Temperature Rise F at /GPM 2.0 - 90 degrees, 2.5 - 75 degrees, 3.0 - 65 degrees, 4.0 -47 degrees, Wire Size 8 AWG (+This unit must have independent electrical circuits using correctly rated wire and circuit breakers
Unit turns on in stages, (.7GPM) turn on). My note: based on info and wiring diagram there are 3 heating elements, each to be attached to its own breaker.

What is the next step to determine if I need a new service or can use
the existing service?

As a note, the current tank type water heater is installed in a closet on the first floor. If I install a tankless water heater I would like to make its location the crawl space freeing up the closet as well as lessening any effects of a
water leak should one occur. Both water lines (cold in and hot out) are easily accessable from the crawl space, and pulling circuits from the electrical panel down to the crawl space is also fairly convenient.
 
  #4  
Old 05-17-05, 03:17 PM
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I'd follow the manufacturer's recommendation for the size of the service.

List the amperage rating of each circuit breaker in the your service panel.
 
  #5  
Old 05-17-05, 08:22 PM
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Totaling the sum of the breaker rating is useless in determining the size of your service. You could have a total 2-3 times what the actual srevice is rated for.

Even if you upgrade your service the power company may not make any changes to their wires. They work under a different set of rules.
 
  #6  
Old 05-17-05, 08:28 PM
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Service Size

Well I did finally find a rating for the main breaker.
Its 150 amps, the same as the max rating for the
panel.

So I have a service large enough to handle the
tankless water heater model that calls for 100 amps,
but nothing larger. Or do I? In the morning I will
list the existing breaker capacities as requested above.
 
  #7  
Old 05-18-05, 07:15 AM
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Originally Posted by pcboss
Totaling the sum of the breaker rating is useless in determining the size of your service. You could have a total 2-3 times what the actual srevice is rated for.

Even if you upgrade your service the power company may not make any changes to their wires. They work under a different set of rules.
I realize that!

I wanted to get an idea of what circuits are being fed from the service panel, electric heat, AC, etc.
 
  #8  
Old 05-19-05, 06:05 AM
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Existing breaker sizes

The HVAC is in the crawl space and is a high efficiency gas
heater with electric AC. Everything else in the house is electric
(range, dryer, existing hot water heater, etc.).

Here are all the circuit breaker ratings:

220 Volt
two 30 amp
two 40 amp

110 volt
five 20 amp
five 15 amp

Thanks,
Jim
 
  #9  
Old 05-19-05, 08:27 AM
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It's getting iffy. How many tons is your A/C, and how many square feet is your house? The answers to those two questions will tell us whether you can do this with your existing service.

Oh, and also tell us if you have any other unusual electric stuff, such as a pool, hot tub, welder, kiln, or jacuzzi.
 
  #10  
Old 05-19-05, 10:19 AM
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I had a friend that just installed one and wished she hadnt. It was expensive and doesnt seem to work all that well. It would be great for a church where a little bit of hot is used on Sundays but for a home the response time is not great and there is a delay at the faucets. As John said, you have electric everything, I certainly wouldnt be putting another super hi draw electric appliance on if I didnt have to. Where water is used often daily I dont see an advantage and if I did use one it would be gas. It would drive me crazy thinking this thing was firing up every time I washed hands.
 
  #11  
Old 05-20-05, 09:39 AM
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AC size

The AC is a Carrier unit, model 38TKB036300, which is rated at
3 tons (36,000 BTU). The info plate on the condenser unit
shows a minimum circuit of 30 amps but a 19.9 amp draw (if I am reading it
correctly). The house is 2 story, with a total of 1,800 sq ft heated and
cooled. There is no basement, just an unheated crawl space beneath.
Typical construction from 1970's for Columbus, Oh, so the window sizes
are minimal and the insulation is adequate (but not exceptional). The
yard has at least one large tree shading the roof on all but the south
side. The south end of the home is a 1 story, 2 car garage (not
conditioned).

I actually would prefer to have a gas tankless unit due to their reputation
for greater capacity, however their cost, especially with installation which I could not perform, is 3 times higher than an electric unit (if I install the electric unit). I do feel capable of replacing the current tank type water heater, or installing an electric tankless water heater (including new circuits, and relocation, but not including a change in the house service size).

The major electrical appliances are; tank type water heater, electric AC (whole house forced air), clothes dryer, clothes washer, range with convection oven, dishwasher, refrigerator 18 cubic ft, refrigerator 4 cubic ft, 35 inch TV, and 3 personal computers. No hot tubs, saunas, et. al.

Thanks,
Jim
 

Last edited by JTeller; 05-20-05 at 11:01 AM. Reason: Add information
  #12  
Old 05-20-05, 11:45 AM
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I'll run the numbers when I get home, but I think the information now presented is going to preclude you adding this heater.

Just an aside. We had a poster here a while ago who was having trouble selling his house. The home inspector pointed out that the electric tankless water heater overloaded the service. The seller was going to have to either go back to a tank water heater, or upgrade the service, before he was going to be able to sell. Either proposition was going to be expensive.

Tankless gas water heaters make some sense to me (not a whole lot, but some). Tankless electric water heaters make little sense to me for most people.
 
  #13  
Old 05-20-05, 08:35 PM
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I agree with John about them making sense. My personal choice is gas tank type. Nice during outages to have gas appliances. I had a girlfriend that had a small house and it had 60A service which was in good shape but the lights bounced a bit sometimes and it had a stupid 120V water heater and some circuits were not very well routed but the wire was in good shape, the well also ran on 120. She does some remodel and I tell her,, gas water heater, she had gas heat and cooking already, between taking the water heater off and changing the well to 230V it freed up a couple spaces to split up a couple wires and helped the bounce from the well start, balanced the loads a bit and the 60 was plenty for that place. Took about an hour or so and fixed what seemed to be marginal to pretty fair service. If you already have access to gas it only makes sense to take the load off of the electric, just like getting a free service upgrade.
 
  #14  
Old 05-20-05, 08:49 PM
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According to my calculations, your current demand load is already 147 amps. You have no room to spare for a tankless electric water heater. Either upgrade your service or replace the tank water heater. There's no question about which of those two I would choose.
 
  #15  
Old 05-20-05, 09:41 PM
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Thanks

Thanks to all.

I felt like the service was too small for a properly sized electric tankless
but didn't know how to crank the numbers.

I can't say I have any personal experience nor any friends that have
used tankless heaters (electric or gas). I take it to heart that this
series of posts included two negative takes on electric tankless heaters
and none positive. The gas tankless, properly sized runs about $1000,
through the wall vent kit another $150, and I would feel comfortable only
with a licensed install that would certainly run over $200. Even with increased efficiency the payback would be a long time coming. My only real loss putting in another tank type is that I won't gain the floor space in the
closet. Not a biggy.

So I'll find an efficient 50 gal tank type and install same. The hardest part of the task is then only taking the old tank to the dump.

Thanks again,
Jim
 
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