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#22278 - 02/21/03 08:21 AM KWH vs, Load
jdlittle Offline
Junior Member

Registered: 04/26/02
Posts: 2
A local city requires that the meter readings be collected for one year to demostrate that the new addition on a house will not exceed the capacity of the existing service. How do you get kva from kwh??

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#22279 - 02/21/03 10:34 AM Re: KWH vs, Load
Len_B Offline
Member

Registered: 01/19/03
Posts: 56
Loc: New Hampshire
Single phase

KVA = KW/PF

At unity power factor(PF=1) they are the same.
Typical residential PF probably ranges between about .85 to .95
Generally, the more motor loads the lower the PF.

edit: I have never had to do this, but I would probably assume a .95 power factor for such a calculation, especially if the new addition does not add motor, fluorescent ballast, or other inductive loads.

Len

[This message has been edited by Len_B (edited 02-21-2003).]

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#22280 - 02/21/03 11:04 AM Re: KWH vs, Load
C-H Offline

Member

Registered: 09/17/02
Posts: 1508
Loc: Stockholm, Sweden
Errmmm: He asked for conversion from kWh to kVA, not kW to kVA. There are in fact such formulas, but the uncertainty will of course be significant. Some assumption has to be made, for example that the average load on the service will be one third of the peak load. (Don't use that one, it was just an example!)

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#22281 - 02/21/03 12:02 PM Re: KWH vs, Load
Len_B Offline
Member

Registered: 01/19/03
Posts: 56
Loc: New Hampshire
C-H,

I agree, but what the city is asking for really doesn't support whether or not the service is of sufficient size. I'm sure a good guestimate on kVA can be made from kWh usage by throwing in some time and peak usage parameters; but the only true way to prove sufficient capacity is to do a load calculation on the remodeled structure.

kW and kVA are measures of POWER.
kWh and kVAh are measures of WORK.
WORK is POWER / TIME

Len

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#22282 - 02/21/03 12:57 PM Re: KWH vs, Load
Elzappr Offline
Member

Registered: 12/20/01
Posts: 273
Loc: Oregon
Isn't what the power company is asking for is a comparison between true power measurement BEFORE vs AFTER the addition? I wouldn't think that there is any concern about the proportion of reactive power changing because you added more living space to your home. Are you sure that you have to worry about how much is true power vs how much is reactive power?

Anyway, to answer your question plainly, just divide the year-long kW-h measurement by the number of hours, and you will have your kW reading average. Without knowing power factor averages for your home, it is impossible to figure out what the kVA might be...unless the power company meter already has some kVA reading ability in it. Are you sure you have to be concerned about kVA?

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#22283 - 02/21/03 01:15 PM Re: KWH vs, Load
resqcapt19 Offline
Member

Registered: 11/10/00
Posts: 2209
Loc: IL
I think that they are really looking for demand information as permitted by 220.35, but houses don't normally have demand meters. There is no way to convert KHW into a demand number to size the maximum load.
_________________________
Don(resqcapt19)

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#22284 - 02/21/03 01:52 PM Re: KWH vs, Load
txsparky Offline
Member

Registered: 05/25/01
Posts: 578
Loc: Magnolia,.Texas U.S.A.
Why wouldn't you just do a service calculation?You know whats in the house and what you are adding(I would hope)I can't imagine an ahj not accepting this,if he does,how would you build a new house in this town?
_________________________
Donnie

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#22285 - 02/21/03 02:07 PM Re: KWH vs, Load
jdlittle Offline
Junior Member

Registered: 04/26/02
Posts: 2
Thanks for your responses. I have always done a load calculation on both the existing load and the new addition. This city required that the meter readings be submitted with the plans!! I'm sorry, but I just did not know why!!

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#22286 - 02/21/03 02:20 PM Re: KWH vs, Load
txsparky Offline
Member

Registered: 05/25/01
Posts: 578
Loc: Magnolia,.Texas U.S.A.
Hmmm....I believe I would get a copy of the electric bill for the past year and ask the ahj to show you how he would go about calculating the impossible.
_________________________
Donnie

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