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Old 03-01-2008, 09:53 PM   #1
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How big is that transformer?


Hi all,

Is there a standard size (in KW or kVA) for those "big green" transformers you see around (residential areas)? I recently did an addition to my house and at the time upgraded to a 400 amp service. I am interested in on of those whole house tankless electric water heaters, some of which are rated at 28 to 32 kW () Anyway I am pretty sure that the meter/panel can support this load, but then I got to thinking about the transformer supplying the house, hence the main question here. A logical person would suppose that the power company would not allow the service upgrade if they didn't have the guts out in the yard to handle it.

Thanks for any help.

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Old 03-01-2008, 10:13 PM   #2
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How big is that transformer?


I would worry more about the heater actually working than the transformer size.

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Old 03-01-2008, 10:25 PM   #3
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How big is that transformer?


yep, you are NOT going to like a whole house electric water heater, unless you dont mind cold showers.
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Old 03-01-2008, 10:35 PM   #4
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How big is that transformer?


Are your responses based on personal experiences? If so, with what brand and/or stated temperature rise? Seems to me no energy calculation is more basic than putting heat/energy into water and figuring out how hot it will get. 28kw is quite a lot of juice. I guess I really wanted to believe that these products would work as advertised. The real question I have is "how can it NOT work?"
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Old 03-01-2008, 10:38 PM   #5
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How big is that transformer?


Let me ask you this, when you upgraded to 400 amp, was that with ample room for such a load as the water heater?
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Old 03-01-2008, 10:42 PM   #6
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How big is that transformer?


Quote:
Originally Posted by john-d View Post
Hi all,

Is there a standard size (in KW or kVA) for those "big green" transformers you see around (residential areas)? I recently did an addition to my house and at the time upgraded to a 400 amp service. I am interested in on of those whole house tankless electric water heaters, some of which are rated at 28 to 32 kW () Anyway I am pretty sure that the meter/panel can support this load, but then I got to thinking about the transformer supplying the house, hence the main question here. A logical person would suppose that the power company would not allow the service upgrade if they didn't have the guts out in the yard to handle it.

Thanks for any help.
Often the kVA rating is stenciled on the case. The POCO usually applies a demand factor to the xfmrs, such that they really can't deliver the current that your service is rated at anyway. But even 32 kW is "only" 134 amps, actual, and 167 A after upsizing 125%. It will probably deliver the current. Unless you have the 50 kW electric heat going too...

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Old 03-01-2008, 10:44 PM   #7
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How big is that transformer?


Chris75:

I believe so. The only real reason for the upgrade anyway was that it was not very expensive and would allow for future expansion. It is an all-electric house - hot water heat pump, stove etc. However I have not done a load calculation to make extra sure.

Last edited by john-d; 03-01-2008 at 10:47 PM.
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Old 03-01-2008, 10:55 PM   #8
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How big is that transformer?


Quote:
Originally Posted by john-d View Post
Are your responses based on personal experiences? If so, with what brand and/or stated temperature rise? Seems to me no energy calculation is more basic than putting heat/energy into water and figuring out how hot it will get. 28kw is quite a lot of juice. I guess I really wanted to believe that these products would work as advertised. The real question I have is "how can it NOT work?"
Hmmm... q = k x A x dT/s

q = energy transfered per unit time = 28 kW
k = thermal conductivity of substance = 0.58 W/m-deg C
A = heated area
dT = difference in temperature
s = material thickness

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Old 03-01-2008, 10:59 PM   #9
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How big is that transformer?


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Originally Posted by InPhase277 View Post
Often the kVA rating is stenciled on the case. The POCO usually applies a demand factor to the xfmrs, such that they really can't deliver the current that your service is rated at anyway. But even 32 kW is "only" 134 amps, actual, and 167 A after upsizing 125%. It will probably deliver the current. Unless you have the 50 kW electric heat going too...

InPhase277
This is kind of what I was concerned about. I mean a full 400A load would be 96kw right? And my transformer serves 2 households, so the full load (the neighbor has 200) would be more. Not that I expect to ever really draw 400A for any meaningful amount of time and the chances that 2 houses would both draw fiull power would be small. A better question might be an experienced point of view as to what "normal" sizes are for residential transformers - I wouldn't want dimming lights when I drew a large amount of power.
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Old 03-01-2008, 11:06 PM   #10
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How big is that transformer?


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Originally Posted by john-d View Post
This is kind of what I was concerned about. I mean a full 400A load would be 96kw right? And my transformer serves 2 households, so the full load (the neighbor has 200) would be more. Not that I expect to ever really draw 400A for any meaningful amount of time and the chances that 2 houses would both draw fiull power would be small. A better question might be an experienced point of view as to what "normal" sizes are for residential transformers - I wouldn't want dimming lights when I drew a large amount of power.
Can't say what the residential green boxes are rated at. Maybe 50 kVA? 75? Don't really know. But I can guarantee you one thing: it doesn't matter the size of the transformer, if you turn on a 32 kW load, lights are going to dim. Unless you have exceptionally large conductors for the service lateral. And you probably couldn't load the service to anywhere near 400 amps, even on a bet!

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Old 03-02-2008, 02:34 AM   #11
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How big is that transformer?


uhm might I suggest you look at the Rennai Tankless . .GAS water heater?
. . .I just wired one for a customer . .a sizable home as well . . .impressive
performance . . .quick to heat up . .no pilot . .eletrically ignited . .remore control for temp settings timing etc

VASTLY less expensive than heating water with electrons . .no tank


consider ths alternative
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Old 03-02-2008, 02:44 AM   #12
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How big is that transformer?


John-d.,,

I been allready installed few monster tankless electric waterheater and some POCO are not too thrilled with it because majorty of resdentail transformer useally are 15, 25, 50 KVA single phase [ those are most common ] but they have larger one but for pad mounted transformer most common is either 25 or 50 depending on the X numbers of the house they feed it.

they will only use just big engough to handle the load that it nothing more because they keep track of load on the system so they will know how much it actally load up the transformer.

sometime back i have one custmoer that have to upgrade to 400 amp service it was not really good for them due the cost of upgrading and they have to pay the POCO to change the transformer as well.

that why quite few of the POCO rewritten the policy on this.

and many POCO will like to be head up with larger service [ they will check your load demand very carefully to jusify the cost to replace the transformer and related parts if need to.]

But there is no question to ask with the whole house tankless waterheater
it will make the lights flickers a bit due the load cycling just like turn on 3 electric stove on fast or have central A/C kick in fast that how the light will flickers some [ it will be worst during summer time when you have few homes feed from one transforemr when they are running the A/C full tilt and turn on whole house tankless waterheater it will really compound it. ]

Merci, Marc

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