Transformer overcurrent protection 1000 volts or less

[elmundogrande]

Let’s say: I have a 30 KVA step-up transformer 240 to 480 volts and no mention of thermal overloads. Using table 450.3B with primary and secondary protection is going to be used 30000/240v= 125A x 250% = 312.5 according to the table I can’t use the next size up not using x125% and no mention of thermal overload so a 300a ocpd is needed.

 

[augie47]

You are correct that when using the 250% primary protection you can not go "next size up" noting that the 250% is only allowed when the secondary protection is sized at 125%
A 30kva transformer is normally found in 3 phase not single phase in which case your primary current would be 71 amps (assuming 240 primary)

 

[elmundogrande]

You are correct that when using the 250% primary protection you can not go "next size up" noting that the 250% is only allowed when the secondary protection is sized at 125%
A 30kva transformer is normally found in 3 phase not single phase in which case your primary current would be 71 amps (assuming 240 primary)

Yes I know not in 1 phases it was a way to show the why’s that’s all

 

[augie47]