Calculation Formula for Transformerr's breaker

[Guinevere]

Dear All,

i Have Transformer 25 kVA, 480/240-120, 1 phase, 60 Hz
and having this problem of calculating the nominal for it's secondary breaker. (For MCC purpose, on Load center unit)
i already attach both calculation, and i'm confused, which one should be used/correct, and why?

Need your help...

Thanks all before...

 

[rbulsara]

Look up NEC Article 450. It's very clear there.

 

[waross]

A couple of small points. You have inadvertently selected your primary current from the table for three phase transformers.
25,000VA/480V = 52A
I see a calculation for the secondary current where you use 120 Volts. When calculating the current for a center tapped 120/240 Volt transformer use 240 Volts.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rbulsara]

Oh , I did not see the attachment and relied much on the title of the OP's post. The word "rated current" would have been better than the "breaker" in the title.

 

[Guinevere]

Thanks all,
the main thing that still confuses me is that it is only mentioned in the NEC of using 1,25 % of full load current, while there are 2 different ways of getting the load current (look up table, and by manual calculation. as attach).

Waross,do you point that nothing's wrong with calculation for the primary, while for the sencondary, it is best to use 25000/240? why is that?

Thanks waross and rbulsara

 

[waross]

The formula for single phase current is KVA/Voltage.
The formula for three phase current is KVA/(Voltage x 1.73)
Your figure for primary current included the 1.73 factor.
For a 120:240 secondary circuit use 240 Volts. That will give you the current in the windings.
If you reconnect the windings in parallel for 120 volts only, you get rated current from each winding. So when the windings are in parallel use 120 Volts. The current is double when the windings are in parallel because each winding can carry rated current.
Even though you may be only using 120 volts, if the windings are connected in series, 120:240 V, you must use a factor of 240 Volts.


Bill
--------------------
"Why not the best?"
Jimmy Carter