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#22278 - 02/21/03 12:21 PM KWH vs, Load  
jdlittle  Offline
Junior Member
Joined: Apr 2002
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 02:34 PM Re: KWH vs, Load  
Len_B  Offline
Member
Joined: Jan 2003
Posts: 53
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).]


#22280 - 02/21/03 03:04 PM Re: KWH vs, Load  
C-H  Offline
Member
Joined: Sep 2002
Posts: 1,497
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!)


#22281 - 02/21/03 04:02 PM Re: KWH vs, Load  
Len_B  Offline
Member
Joined: Jan 2003
Posts: 53
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


#22282 - 02/21/03 04:57 PM Re: KWH vs, Load  
Elzappr  Offline
Member
Joined: Dec 2001
Posts: 270
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?


#22283 - 02/21/03 05:15 PM Re: KWH vs, Load  
resqcapt19  Offline
Member
Joined: Nov 2000
Posts: 2,148
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)

#22284 - 02/21/03 05:52 PM Re: KWH vs, Load  
txsparky  Offline
Member
Joined: May 2001
Posts: 552
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

#22285 - 02/21/03 06:07 PM Re: KWH vs, Load  
jdlittle  Offline
Junior Member
Joined: Apr 2002
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!!


#22286 - 02/21/03 06:20 PM Re: KWH vs, Load  
txsparky  Offline
Member
Joined: May 2001
Posts: 552
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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