transformer rated

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hoodf

Member
Hello I have a pole mounted transformer the primary voltage is 7620/13200Y and the secondary voltage is 120/240 volts and trying to see what ampacity is for it

Thanks Frank
 

augie47

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Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
transf:

transf:

if I undestand your question, the kva value of the transformer is needed.
 

hoodf

Member
transformer

transformer

Its a 75 KVa
I was told the you take the amp reading from both phase and add them together and then divide by .12 and that will tell you the size of the transformer that is needed
 

charlie b

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Location
Lockport, IL
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I really don?t understand your question. So let me present two of my own:
(1) What information do you already have?
(2) What information are you looking for?

You talk about wanting to know ?ampacity,? and you talk about measuring amps. Is this thing already installed, so that it is possible to measure amps? If so, then the size of the secondary conductors must already be determined.

As to this rule of thumb you were given, it is nonsense. You can divide amps by volts, and get resistance, using Ohm?s Law. But that is seldom applied in our daily jobs. There is no other useful formula in the electrical industry that starts with amps and divides that value by anything else. Also, if the loads are reasonably balanced, as they should be, then the amps on one phase should be nearly equal to the amps on the other phase, and the amps on the neutral should be nearly zero.
 

72.5kv

Senior Member
The KVA rating are usually clearly written on the front of the transformer.

what ever the number is, multiply is by 1000 and devide by 240

(number*1000)/240
 

mdshunk

Senior Member
Location
Right here.
Pole top transformers usually have a sticker on them on the side, about 3" x 5", that will normally say 5, 10, 15 or 20. This is the KVA rating of the pot.

If you're speculating that the transformer that serves your home is too small, then welcome to the world of residential distribution. They work to a whole other set of rules. Plus, their transformers are designed that they run way overloaded if need be.
 

hoodf

Member
transformer

transformer

The name plate on it is 75 KVA
I work for the gov't and we are in charge of the distrution system here
My question is its a single phase and on a phase we are reading 197 amps and B phase 207 amp is the transformer undersize. We have replace this transformer 3 times in the past 3 weeks or so

Frank
 

72.5kv

Senior Member
If your wondering why the transformer is rated so small compared to your service size, most utilites will size a residential transformer at 30% of the service size.

ie. 200amp service, 15KVA at the pole

while interning with a distribution engineer, he would spec. for the SE cables to sized according, but the transformer was sized based on expected demand.
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Thank you for clarifying the question. My initial reaction is that the transformer is not undersized. Indeed it is loaded only to about 65% of its capacity. However, there are other facts that you did not yet give, and they might easily change my answer.

First, what is the reading on the neutral? If loads are nearly balanced, then it should be close to zero. If that is the case, then the total load on the transformer is about 49 KVA (figured as an average current of 205 amps times the voltage of 240).

On the other hand, if there is a significant neutral current, then you are dealing with a significant harmonics load. That could well be the cause of overloading the transformer.

Secondly, what is the nature of the failures you have been experiencing? Do they failed transformers show signs of overheating? Is the insulation system damaged?

Finally, what is the scheme for primary and secondary transformer protection?
 
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