Transformer Load Capacity

[katyo]

Here at an electric distribution coop, we have always used a guide probably from the old REA days for sizing transformers. I’m wondering, though, why does load capacity go down with more customers?

Example: for a 25 kva transformer, the first column is number of customers and the second is the correlating transformer load:
1 7500
2 8000
3 8400
4 8400
5 8200
6 8000
7 7700
8 7200
9 7200
10 7000

I know as more customers are added (1-4) capacity increases because of more diversity, so why doesn’t the load capacity continue to increase?
I tried looking at the IEEE C57.91 standard, but couldn’t find anything.

 

[jghrist]

Assuming that the "correlating transformer load" is watts per consumer, it looks like someone tried to use the old REA Bulletin 45-2 Demand Tables nomograph for kW demand with about 1700 kWh/mo/consumer and extrapolate below 5 consumers. This method is for many customers on a feeder, not for transformer loading. The extrapolation may explain why the watts/consumer does not follow the expected pattern below 5 consumers; the nomograph stops below 5 consumers.

 

[katyo]

To clarify, the second column is in kWh.

The kWh for 1 consumer also doesn't increase linearly:

First column is transformer kva size, the second is kwh, and again this is for just 1 consumer:
3 300
5 500
7.5 1000
10 2500
15 4000
25 7500
37.5 12500
50 16000

Does this guide assume different load factors?

Thanks in advance for helping me sort this out.

 

[wareagle]

Kayto
I think your chart is wrong. Assuming the customers are similar in their load, the load per customer would be the same. However, when you add more customers to a transformer the diversity of load occuring at any monent does not follow a straight line. The chart should look something like the following:

1 7500
2 8000
3 8400
4 8600
5 8800
6 9000
7 9300
8 9500
9 9800
10 10000

 

[k2panman]

I'm not sure where your chart is from, but I calculate a loading based on the house sq ft and construction, and use a diversity factor for each transformer - the more consumers you connect to one transformer, the less transformer kva you need because the load diversifies - not everyone is at peak load at the same time. Add up the calculated peak loads for all houses on a transformer and multiply by the DF to get transformer kva

# of houses DF
1 1
2 .86
3 .78
4 .73
5 .69
6 .67
7 .65
8 .63
9 .62

Also, most transformers can be loaded to 140% of nameplate. This will shorten the life of the transformer some and losses are higher.

K2ofKeyLargo

 

[buckman]

I concur with K2panman.