Check this calculation

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brother

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This is for a transformer, 150 kva 480 primary 120/208 secondary. NEC 2008

150kva = 150,000
150,000/(480 * 1.73)= 180.635 amps

150,000/(208 * 1.73) =416.851 amps

primary breaker was 250 amp and the secondary was feeding a 600 amp.

for the primary Table 450.3(B) currents 9 amps or more 125% *180.64=225.8

for secondary 125% * 416.85= 521.06 so next higher size 600 amp for the Article 240.6

But they have have a 250 amp on the primary, I thought they had to use a 225 amp breaker for the protection, Can they actually do this at just 225.8 amps in the calculation??
 
brother said:
But they have have a 250 amp on the primary, I thought they had to use a 225 amp breaker for the protection, Can they actually do this at just 225.8 amps in the calculation??
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I think that is ok. 450.3(B) will allow 250% for protection on the primary if the secondary is protected at not over 125%.
 
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brother said:
This is for a transformer, 150 kva 480 primary 120/208 secondary. NEC 2008

150kva = 150,000
150,000/(480 * 1.73)= 180.635 amps

150,000/(208 * 1.73) =416.851 amps

primary breaker was 250 amp and the secondary was feeding a 600 amp.

for the primary Table 450.3(B) currents 9 amps or more 125% *180.64=225.8

for secondary 125% * 416.85= 521.06 so next higher size 600 amp for the Article 240.6

But they have have a 250 amp on the primary, I thought they had to use a 225 amp breaker for the protection, Can they actually do this at just 225.8 amps in the calculation??
Click to expand...
Yes, note 1 to Table 450.3(B) permits rounding up to the next standard size and 225.8 is not a standard size. I see nothing that says you have to round down in this case.
 
Now I know this is completely different application but, I was talking with the POCO line man today and he was saying they routinely run transformers at up to 180% of there capacity. We live in a coastal area so the temperatures don't get extreme. The only reason I added this in is just for a perspective.
 
I've heard them say the same thing.

The kVA rating of the transformer is really more of a recommendation of where you should run it if you want it to last very long. Overloading it will overheat it and heat = age, so you're just aging it more quickly.

I found an equation once for estimating the rapid aging of a transformer if it was overheated. At my last job, our pad-mounted transformer faulted and got really hot. I calculated that we aged it 7.5 years in 30 minutes. The DGA actually verified this.
 
But don't forget the secondary wire (between the xformer and the breaker) probably needs to be sized for the full 250 amp secondary breaker.

Steve
 
charlie b said:

Would you, by change, be refering to the "Arrhenius Equation"?
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Nope. I just looked it up again. I have an old (copyright 1947) transformer textbook that says:

...experience and tests indicate that the rate of deterioration of transformer insulation approximatley doubles for each 8 degrees (Centigrade) increase in temperature.
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When I applied that to the transformer I referred to earlier (I knew the maximum temperature at the time), I calculated, as I said, that it had aged 7.5 years in 30 minutes. The transformer was actually 12 years old. When Weidmann returned the DGA, they estimated it was 20 years old. I was only off by 6 months. :)
 
charlie b said:
Would you, by change, be referring to the "Arrhenius Equation"?
Click to expand...
drbond24 said:
Nope. I just looked it up again. I have an old (copyright 1947) transformer textbook that says:
Click to expand...
drbond24 said:
...experience and tests indicate that the rate of deterioration of transformer insulation approximately doubles for each 8 degrees (Centigrade) increase in temperature.
Click to expand...
Click to expand...

You will note that a rate that doubles for every unit increase tells you that the equation is one of exponential order. In this case, among the relevant coefficients is the ?activation energy? of the material of interest. And that brings us back to Arrhenius. :wink:
 
charlie b said:

You will note that a rate that doubles for every unit increase tells you that the equation is one of exponential order. In this case, among the relevant coefficients is the ?activation energy? of the material of interest. And that brings us back to Arrhenius. :wink:
Click to expand...


I agree. I just had never heard of Arrhenius until you typed it. :D He was a little bit before my time. ;):grin:
 
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