Transformer ocpd

[jbfan74]

Here is the situation.
15kva, 480/208 feeds into a 100 amp main.
A 55 amp two pole heat source was added and the primary breaker(20 amp) tripped. The heat source was added based on the 100 amp main, and without considering the transformer. Now that it has tripped, I have confirmed the transformer is rated 41.6 amps per phase.
We now need to replace the transformer, and I am looking at a 45kva.
The problem I am having is the ocpd.
At 54 amps on the primary, based on 450.3(b) states I need 250% or 135 amp breaker.
The secondary is 125*1.25= 156 amp
The other loads are 1 2 pole 25 amp ac, and the rest are single pole computer loads.

Am I right to go with a 45kva? Are my numbers right? Do I need to change the 100 amp main?
Can I go with a 30 kva?
I haven't had to figure a transformer since I took my test in 94.

 

[jim dungar]

Here is the situation.
At 54 amps on the primary, based on 450.3(b) states I need 250% or 135 amp breaker.

Not really.

450.3(b) says the primary can not exceed 250%, not that it must be equal to 250%. You are free to use a smaller breaker if you want, however anything smaller than 125% may trip when the transformer is "firing up".

 

[micromind]

Most 45's I've done use an 80 on the primary, and 150 on the secondary.

Rob.