Calculating transformers primary current

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netaguy

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Providence, RI
I am trying to determine the primary current rating of three 300KVA single phase transformers wired for three phase four wire output as attached.

Would I be correct that "each" primary phase could draw 12 1/2 amps?

amps = va / phase to phase voltage X 1.73 = 12.56

or would I not use sq root 3 as primary windings are single phase?

Thanks for you help.

* Edit use 13.8kv for primary and 480/277V for secondary in photo and swap H taps with X taps. The wiring is typical.
 
Last edited:
101024-1523 EST

If you had a single 300,000 va primary at 13,800 V what would be the approximate full load primary current? Use this information to double check your answer.

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(3) single phase 300kVA transformers = (1) 900kVA three phase transformer

Amps = kVA/(VLL*[FONT=&quot]√3)

[/FONT]
 
gar - I wouldn't use 1.73 in the equation and would have 21.73 amps?

jim - I calculate 62.82 amps.

If I am sizing individual fuses for each 13.8 phase, should I base the fuse size calculation on 21.73 amps or 62.82 amps?

This should be so simple, I dont know why I am having such a hard time with it.

Thanks for your help
 
netaguy said:
If I am sizing individual fuses for each 13.8 phase, should I base the fuse size calculation on 21.73 amps or 62.82 amps?
Click to expand...

Neither. I don't know how you came up with 62.82 from[FONT=&quot] Jim's equation, and you would apply [/FONT][FONT=&quot]√3 in gar's suggestion.

The primary rated current would be 37.65A
[/FONT]
 
Last edited:
101024-2036 EDT

netaguy:

The point of my question to you was to make you think about your answer. If you have one single phase transformer rated at 300 KVA at 13.8 KV and you calculate the full load current to this for a resistive load, then what is the approximate primary current at full load? It is VA divided by Voltage. In this case 21.7 A.

The next part of the thought process is that if three of these transformers are connected delta on the primary, then can the current in one line be less than 21.7 A. It should be intuitively obvious that the line current can not be smaller and should be larger. Your calculated value was smaller. How much larger should it be? Rather than just use some formula someone gave you I would hope that you do some study and understand how to derive the formula.

Too many individuals just use formulas with no understanding of their origin or meaning.

For example is the unloaded output voltage of a ferromagnetic transformer exactly related to the primary voltage by the turns ratio?

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