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Transformer Load Capacity

Transformer Load Capacity

Transformer Load Capacity

(OP)
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.

RE: Transformer Load Capacity

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.

RE: Transformer Load Capacity

(OP)
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.

RE: Transformer Load Capacity

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

RE: Transformer Load Capacity

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

RE: Transformer Load Capacity

I concur with K2panman.

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