Transformers

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jonny1982

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So say I have a 112.5 KVA Transformer. The primary is 135 amps (480) and the secondary is 312 amps (208). Am I thinking of the following right?

If I wanted to get the max out of it, I would get a 150 amp breaker for the primary and size wire proper for this. Then I would feed a 300 amp panel for secondary?

Lets say I only needed a 250 amp panel off the secondary. That would change the minimum size of the primary, right? What is the calculation for this (is there a ratio multiplier?)

Thanks
 
If you wanted the max out of the transformer I would suggest a 400 amp secondary panel.
Regardless of the secondary panel size, I would be reluctant to go below your 150 amp primary breaker. There will be inrush currents to take into account regardless of your secondary panel.
 
But, isn't the max for secondary of a 112.5 kva transformer 312 amps?
 
jonny1982 said:
But, isn't the max for secondary of a 112.5 kva transformer 312 amps?
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Nope. Take a look at 450.3
With proper primary protection, there is no limit to the size of your secondary panel(s).
125% of the transformer current is common.

If you needed numerous circuits it would not be a violation to have (4) 200 amp 42 circuit panels (or more) on the secondary side as long as you comply with the 125% primary rule.
 
Can even go as high as 300A primary ocpd... as long as you don't exceed 400A secondary ocpd total
 
Smart $ said:
Can even go as high as 300A primary ocpd... as long as you don't exceed 400A secondary ocpd total
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I don't understand. You are saying not to exceed 400A total on secondary, but augie47 is saying it is ok as long as the 125 percent rule is followed. Which is it?
 
jonny1982 said:
I don't understand. You are saying not to exceed 400A total on secondary, but augie47 is saying it is ok as long as the 125 percent rule is followed. Which is it?
Click to expand...
He's referring to primary only transformer protection. I'm referring to primary and secondary transformer protection.
 
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