Ground for Transformer

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jonny1982

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How do I determine size of building steel ground and neutral bonding jumper for 112.5 KVA transformer?
 
Have you read through art 250 part II? Some places will direct you to other parts of the code as well, like your building steel will require you to see 250.66

I suggest you do that first then ask more specific questions if you have them.
 
kwired said:
Have you read through art 250 part II? Some places will direct you to other parts of the code as well, like your building steel will require you to see 250.66

I suggest you do that first then ask more specific questions if you have them.
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So lets say I have a single set of 3/0's for my primary into transformer and parrallel 250's for secondary (AL). Would I size my building steel ground for the transformer to the primary's and go with #6 copper, or would I have to size it for the parrallel 250's?

Thanks
 
jonny1982 said:
So lets say I have a single set of 3/0's for my primary into transformer and parrallel 250's for secondary (AL). Would I size my building steel ground for the transformer to the primary's and go with #6 copper, or would I have to size it for the parrallel 250's?

Thanks
Click to expand...
You size it to the conductors connected to the separately derived system - which is the secondary. Note it is based on secondary conductor size and not size of transformer, which means you could possibly have either of:
1. small kVA transformer with large secondary conductors that require a large GEC
2. large kVA transformer with small secondary conductors that require a smaller GEC even though larger capacity is available - probably doesn't happen too much but not impossible. Example 112.5 kVA transformer 208 V but for some reason you only connect a 100 amp circuit to it. GEC only need to be sized to those 100 amp conductors not the 312 amps capacity of the secondary.

If you have a transformer that steps voltage up, you very possibly have a larger equipment grounding conductor on the primary side then what the GEC for the secondary is.
 
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