Mr. james64 (Electrical)(OP)10 Jul 23 00:42
"....We have a unit new 75kVA dry transformer on site now. I would like to know how many of the existing transformers can handle more load in the future. After checking the existing demand load, around 50kVA was used up to date....."
1. What is your existing transformer kVA rating? If it is say 50kVA, you are at full load. You can not add more load unless the existing pf is low. You can add some load by improving the pf.
2. If your existing transformer is say 75kVA, you can add an additional of 25kVA to you existing load of 50kVA.
3. Your new 75kVA transformer can take up another load up to 75kVA, irrespective of your existing loads.
Che Kuan Yau (Singapore)

Two ways to go:
1. Do the demand calculations according to code. You may not be able to add load based on these calculations.
2. If it can be shown that the existing load, etc.
There are two ways to show to the AHJ that the existing load is low enough that more load may be added.
a} Recording demand meters on the existing loads for a time acceptable to the AHJ.
This may be 6 to 12 months.
b) If this is a main service transformer, ask the accounts payable department for copies of the last two years of power bills.
Look at the maximum demand each month for the last two years. The AHJ may allow you to ignore the highest demand as an outlier, it depends.
Note: when adding KVAs, PF matters. If the added load is at a significantly different power factor than the average PF of the existing loads, the actual combined KVA may be less than the simple sum of the existing and added KVAs.
That said, simple addition is always safe.
Don't worry about the PFs.
But, If the measured KVA after the addition of new loads is slightly less than the calculated KVA, this is probably why.

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Ohm's law
Not just a good idea;
It's the LAW!