Residential transformer sizing 1

[douglas1965]

What is the best calculation to use to size a residential overhead transformer if I know the maximum peak KWH use in a one year period?

 

[rbulsara]

You can't. You need to know the peak kW (not kwh).

A single house is seldom fed by a dedicated transformer. Usually it feeds multiple houses. Where does this transformer get its supply from?

Rafiq Bulsara

http://www.srengineersct.com

 

[pwrtran]

kwh is the average during the interval the meter sees, as rbulsara indicated it is not your peak demand. You also need to estimate the kvar or PF to size your xfmr. Maximum kwh gives your maximum energy consumption but doesn't give enough information for the instantaneous load drawn from the xfmr.

 

[wareagle]

Are you trying to size just one transformer or are you trying to come up with a method for sizing multiple transformers?

 

[alehman]

Many utilities have tables for sizing residential transformers based on number of houses served.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[douglas1965]

Thanks for the responses.

We have a group of six houses fed off of one transformer.
We are going to split the load and put 3 houses on one transformer and three on another.

We are going to assume the power factor is one so that KVA = KW.

The maximum readings I retrieved from out automatic meter reading database are the max KWH over a one hour period during the past year.

I know there are other ways to calculate this problem.
I was interested in how others might approach the problem.

 

[jghrist]

if I know the maximum peak KWH use in a one year period?

The maximum readings I retrieved from out automatic meter reading database are the max KWH over a one hour period during the past year.

The second statement clarifies the first. The max kWh over a one hour period is the maximum one hour average kW.

This could be used for sizing the transformer if you only had one customer on the transformer and your assumption of unity pf was good. Unfortunately, with three customers on the transformer, the maximum one hour average kW is not the sum of the three maximum readings because they don't necessarily occur at the same time. The sum of the three customers' max one hour kW would be a much better value to use than the average kW over one year.

The sum of the max hourly kW for the customers will probably be close enough for transformer sizing. It will be a conservative estimate of the peak transformer kW, but since the pf will be less than one, this will make it less conservative as a measure of peak kVA.

 

[PHovnanian]

You migh want to factor in the type of loads your residential customers are likely to have. General lighting and power have differing diversities than electric heat or air conditioning. You also need to consider the transformer derating needed if the peak occurs during hot wweather (air conditioning) or overload factor if the peak occurs when its cold (electric heat/heat pumps).

This may not be that big a deal for 3 houses, but for larger collections of customers, these factors become important.