225 KVA transformer Calculation

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infinity

infinity

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Saw this today on a drawing:

225 KVA:
Primary 3-#4/0-225 amp OCPD
Secondary (2) sets of 4-250 kcmil-500 amp OCPD, in separate raceways

Sounds good?
 
assuming 3 phase 480 to 208Y/120, I would be a bit worried abot my primary OCP being undersized. What about the system bonding jumper ?
 
Yeah, I'd be at 400A primary, 800A secondary. Why limit yourself? Seems like a waste.

I'd also be worried about inrush nuisance tripping.
 
What is the load on the xfmr? If they are only using 200 kva then they are good. Everything is relevant when cost counts.;)
 
So although the design might not be the best given the parameters it's still code compliant? What calculation would you use to determine the minimum primary OCPD and conductor size when given the 500 amp secondary OCPD?
 
Your asking makes me nervous :D
450.3(B) would allow a primary OCP up to 250% since your secondary is protected at 125% of less
225000/831.26X250%=600 amp max OCP
But you know all that... what am I missing ?
 
Nothing missing, just seemed odd to be to use considerably less of the transformers capacity than is available with a 225 Kva. Maybe the engineer was saving money on the conduit and wire.
 
A couple of possibilities based on my experience: (a) They had the transformer available, (b) The gear was such that a 225 amp breaker was the largest it would accept (only space available)
 
225 KVA transformer Calculation

elect-rick said:
What is the load on the xfmr? If they are only using 200 kva then they are good. Everything is relevant when cost counts.;)
Click to expand...
If cost was the main consideration they could have spec'd a 150kVA transformer. With those primary and secondary OCPD's, they'd get the same output with higher efficiencies.


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Pharon said:
With those primary and secondary OCPD's, they'd get the same output with higher efficiencies.
Click to expand...
Could be. Iron losses would be lower with the smaller transformer, but the copper losses would be higher.
And a very rough rule of thumb is that at full load iron and copper losses in a transformer would be about equal.
 
infinity said:
Saw this today on a drawing:

225 KVA:
Primary 3-#4/0-225 amp OCPD
Secondary (2) sets of 4-250 kcmil-500 amp OCPD, in separate raceways

Sounds good?
Click to expand...

This question, as it stands, cannot be answered due to the fact there is not enough information to accurately draw any type of conclusion whatsoever.
 
infinity said:
What calculation would you use to determine the minimum primary OCPD and conductor size when given the 500 amp secondary OCPD?
Click to expand...
Gus already provided the calculation for the maximum OCPD. But you are asking about the minimum. My answer is 15 amps, without needing to resort to a calculation. My basis is that that is the smallest standard breaker rating. The NEC rules are based on providing overcurrent protection, not on making the systems work efficiently (or work at all, for that matter). I can achieve the overcurrent protection using a breaker that is at or below the maximum rating. There is no minimum requirement. If the owner wants to use this setup, including the 15 amp primary OCPD that I suggest, to serve a secondary load of 30 amps, it will be a waste (as has already been said), but it will be compliant.

 
infinity said:
What calculation would you use to determine the minimum primary OCPD and conductor size when given the 500 amp secondary OCPD?
Click to expand...

The minimum primary OCPD would be sized just large enough to carry 100% of the non-continuous load plus 125% of the continuous load.
 
charlie b said:
Gus already provided the calculation for the maximum OCPD. But you are asking about the minimum. My answer is 15 amps, without needing to resort to a calculation. My basis is that that is the smallest standard breaker rating. The NEC rules are based on providing overcurrent protection, not on making the systems work efficiently (or work at all, for that matter). I can achieve the overcurrent protection using a breaker that is at or below the maximum rating. There is no minimum requirement. If the owner wants to use this setup, including the 15 amp primary OCPD that I suggest, to serve a secondary load of 30 amps, it will be a waste (as has already been said), but it will be compliant.
Click to expand...


As per Kingpb's post this is a 480 Delta to 208Y/120 WYE transformer.

So basically the primary OCPD size is irrelevant as long as it does not exceed the calculation in Augie's post. The primary could have 15 amp conductors and a 15 amp OCPD even if the secondary is feeding a 500 amp distribution panel and the connected load was as per David's post.
 
infinity said:
As per Kingpb's post this is a 480 Delta to 208Y/120 WYE transformer.

So basically the primary OCPD size is irrelevant as long as it does not exceed the calculation in Augie's post. The primary could have 15 amp conductors and a 15 amp OCPD even if the secondary is feeding a 500 amp distribution panel and the connected load was as per David's post.
Click to expand...

:thumbsup:
 
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