Another transformor question

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kevinware

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Louisville, KY
I need help correcting what I am not understanding about article 450. This is what I have:

From a 480V three phase 400 amp distribution panel I have a three pole 50 amp breaker feeding the primary side of a 30KVA transformer with AWG #6 THHN. The secondary side of the 30KVA transformer is 120/208Y and it is being back-fed to a panel through a three pole 60 amp breaker as a main using AWG # 6 THHN. The load on this back-fed 60 amp main breaker is about 56 amps. I am being told that the 60 amp breaker can not be increased because the 50 amp 480V breaker on the primary is being used as the OCPD for primary and secondary. Please check my math now.

I calculate the flowing:

30KVA transformer 480V pri. and 120/208V sec. Z% unknown

30KVA / 831 = 36 amps x 1.25 as per table 450.3(B) = 45 amp. According to 240.6 45 amp is a standard size OCPD as per note 1 of table 450.3(B) I am not allowed to increase this breaker. Wouldn't using the 50 amp put me over 125% of the primary current rating? and require me to have primary and secondary OCPD?

The was I understand article 450 I can use the flowing:

30KVA / 831 = 36 amps x 1.40 = 50 amp protecting the primary and thus 30KVA / 360 = 83 amps x 1.25 = 100 amp protecting the secondary with AWG # 3.

Please correct me where needed.

Thanks,
Kevin
 
Do not confuse high voltage winding with primary and low voltage winding with secondary. The primary is always the incoming voltage, and the secondary is always the load voltage. In a step up transformer arrangement the primary will be the low voltage side.

oops, I think I missed the point of the OP.
 
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30 KVA 480-208Y/120 transformer.

90 amp primary protection.

100 amp secondary protection.

83 amp secondary current rating.
 
jim dungar said:
Do not confuse high voltage winding with primary and low voltage winding with secondary. The primary is always the incoming voltage, and the secondary is always the load voltage. In a step up transformer arrangement the primary will be the low voltage side.
Click to expand...

Good point, a lot of folks don't understand this. The IEEE standards are even starting to use the same terminology as IEC in that IEC does not even recognize "primary" and "secondary", they simply use HV and LV. In using both US and international standards, I acutally prefer IEC.
 
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