Transformer wire and ocp sizing

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Joe Newman

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I need input on sizing prim and sec wire sizes along with ocp sizing for a 225 kva 3 phase 480 v prim 240 v sec transformer supplying a 400 amp panel for single phase 240 v car chargers I come up with 3\0 cu for prim. on a 300 amp breaker and 500 kcmil for sec.
 
Keep in mind Art 450 is for transformer protection and the conductors still need to be protected per Art 240. A 3/0 conductor would not be protected by a 300 amp breaker. If you elect ton use a 300 amp breaker per Art 450, you need a conductor rated 300 amps.
 
Pretty sure he is talking about the primary conductor and primary OCPD.

augie47 said:
Keep in mind Art 450 is for transformer protection and the conductors still need to be protected per Art 240. A 3/0 conductor would not be protected by a 300 amp breaker. If you elect ton use a 300 amp breaker per Art 450, you need a conductor rated 300 amps.
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What do you have for the secondary in way of equipment. What is you plan.
Then we can help more.
It may impact what you decide on the primary.
 
Ok I'm installing a transformer to feed a 400 amp panel to serve car chargers in a parking garage I did a transformer calculation and needed a 170 kva transformer, transformers go from 150 to 225 kva which is good for 541 amps but I just need 400 out of it I guess this is what's confusing me because the rise in kva is making my ocp size go up also
 
Joe Newman said:
Ok I'm installing a transformer to feed a 400 amp panel to serve car chargers in a parking garage I did a transformer calculation and needed a 170 kva transformer, transformers go from 150 to 225 kva which is good for 541 amps but I just need 400 out of it I guess this is what's confusing me because the rise in kva is making my ocp size go up also
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You size your transformer secondary conductors based on your load.
You protect your conductors.
You size your transformer based on your load.
You select your transformer primary side protection based table 450.3.
 
If you have a 400 amp M/B secondary panel, the 500s are undersized per 240.21(C).
As jim notes, you select your desired OCP device based on 450.3 and select the conductor based on the OCP rating.
 
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