Cable sizing

[mree2002]

Hi there
I am facing a problem in checking size of power cable that fed from 480/208-120V, 75kV transformer.
The cable is 3-1/C 95mm, I wanna know the current capacity of the cable to check either we should replace it or not, because we are adding more load on the transformer.

 

[egurdian3]

95mm?

95mm?

Clarify if that is area, -as in mm^2 cuz 95mm=3-3/4".

 

[Smart $]

Hi there
I am facing a problem in checking size of power cable that fed from 480/208-120V, 75kV transformer.
The cable is 3-1/C 95mm, I wanna know the current capacity of the cable to check either we should replace it or not, because we are adding more load on the transformer.

Part of the issue here is that the NEC does not recognize metric-sized wire... though it does allow for ampacity determination under engineering supervision. As such, you have the necessary engineering data?

 

[charlie b]

If this were a US-based project, and if it used the NEC for wire sizes and ampacity, I would say that the wire is undersized. I don?t mean that you have a code violation on your hands, but rather that you are not taking advantage of the transformer?s capabilities. A 75 KVA transformer with a 208 volt secondary has a full current rating of 208 amps. I would protect the secondary with a breaker that is up to 125% of that value, and would pick a 250 amp secondary overcurrent device. That would lead me to select a secondary conductor sized at 250 MCM. The approximate metric equivalent would be 150 mm2.

The conductor you have, at 95 mm2, is roughly equivalent to a #3/0 AWG. That has an ampacity of only 200 amps. Although that is only slightly lower than the transformer?s full current rating of 208, it is well below what you could be using.

So the choice is yours. The question of whether you should replace the 95mm2 wire really depends on the amount of load you will be serving with it, and not the capability of the transformer. If for example the present loading of the transformer is only 100 amps, and if you wish to add another 75 amps, then you would still be well within the ampacity of the 95mm2 wire.

 

[mree2002]

Thanks guys for your contribution

The secondry side of the transformer is connected to a 100A DSW ( Disconnect Switch), and my load is 70kVA continoues

so while 95mm^2 is equivelant to 3/0AWG as you have said and my load will draw approximatly
195A ( I=(kVA*100)/(V*1.73)), the cable is under sized and I need to go to the next higher size.

Am I right?

Note that the distance is only 10 meters ( approximatly 33 foot)