Old Engineers Formula for kWh to kVA

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steve66 said:
Good point - the square root of KWH doesn't sound like the right dimensional units.




Again, the square root just doesn't make sense to me. Assume you have the exact same situation with 400 houses. Now the peak demand only goes up to about 130kW. Doesn't make sense to me. Especially if everyone has air conditioning, and its a hot, humid 110 degree Saturday.
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It's an empirical formula, based on curve fitting. The units don't have to make sense. In 1963, in rural America, nowhere close to "everyone" had air conditioning.
 
jghrist said:
It's an empirical formula, based on curve fitting. The units don't have to make sense. In 1963, in rural America, nowhere close to "everyone" had air conditioning.
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True.

But we did have a brand new '63 Impala SS convertible.

In '63 we also had extremely inefficient appliances. I live in the house my grandparents built and found an old electric bill from the 60's. It was about the same kWh as what I am paying now, and I have A/C and they didn't. The total was less than 8 bucks. About 1/5 of what I am paying today.

Think of how much power a TV built in the early 60's used. Same for the fridge and the deep freeze. Throw in incandescent bulbs and an inefficient blower motor on the furnace and you can see that the load demand per person hasn't really changed much in the last 45 years. We do, however, get more for the same amount of power. In addition to A/C, we have 4 TV's instead of 1, but with newer TV's, the load is much smaller. Our lights are more efficient, we have sensors, and so on, which let us get more for our money, something our grandparents didn't have.
 
GSEng01 said:
To be more specific - The formula is used to determine the approximate maximum load on an in-service transformer, based on the only data we have available - the kWh readings from the connected consumers. We are not attempting to determine the kVA for a particular customer, just whether or not the combined load of all consumers is still within the safe operating range of the transformer. I had misgivings about the formula from the start, but was giving it some credit due to the reputation of the engineer who developed it. I had previously concluded that the constants in the formula were likely based on empirical data - but the SQRT(kWh) was puzzling, as I couldn't think of any formula where that particular function is used. I am skeptical of the formula's results, hence my posting on this forum. The responses I've received have reinforced that skepticism. Now the difficult part will be developing a replacement formula that will have some hope of approximating the load on the transformer.
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This kind of situation is all too common. Someone develops a formula based on a number of assumptions which might have worked really well as long as the result was viewed through an understanding of the assumptions that went into the formula. The formula is simplified down for ease of use to the point where it does not make much sense (sqrt(kWhr)?) and looking at it reveals little information about its origins. No documentation is made. Years go by and the person who developed it forgets the assumptions or moves on and someone new takes over with no documentation to tell them how to use the formula. Unfortunately you are now left with the task of deriving this formula from scratch again. So starting from kWhr how can you approximate the kVA load? Start assuming! ;)

I also agree that it looks like the results of curve fitting based on what is probably very old data.
 
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al hildenbrand said:
I'd ask an additional clarification: Is the type of "multiple customers on a transformer" restricted to transformers in residential neighborhoods? Mixed light commercial and residential? Etc.
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We typically do not have mixed light commercial and residential fed from the same transformer, even if they are adjacent to one another.

Typically, this formula is used for residential or small retail situations such as a "strip mall" or small building with 5 or 6 suites. For applications larger than that we use a Demand meter which records the actual peak demand of each consumer.
 
GSEng01 said:
We typically do not have mixed light commercial and residential fed from the same transformer, even if they are adjacent to one another.

Typically, this formula is used for residential or small retail situations such as a "strip mall" or small building with 5 or 6 suites. For applications larger than that we use a Demand meter which records the actual peak demand of each consumer.
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OK. This makes a little more sense.

What you call small retail, or small 5-6 suite building, I would consider small commercial . . . this is just a jargon usage. (Toe-may-toe; toe-mah-toe)

In the early '70s I was involved with a project to extract alerts for possible transformer bank overloads from the monthly billing data. The computer billing system was extremely limited, so most attention was paid to the demand metering readings.

Given the changes in the load profile of a "typical" residential dwelling, I would submit that the curve that your predecessor's formula attempted to "fit" has probably altered a bit.

The effects of today's switching power supplies, used in CFLs, computers, video equipment, gaming systems, low voltage lighting, fluorescents, offers a reactive component unlike anything in the almost entirely resistive / inductive load of the Sixties.
 
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