transformer cal. check please

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paul hodge

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15kva
1 Phase
Primary voltage 480
Secondary Voltage 120/240


My calculations are:
Primary Breaker=40a
Primary Conductor=#8
Secondary Breaker=80a
Secondary Conductor=#4
Bond Jumper & Electrode=#8
 
You do not necessarily need a breaker on the secondary side for transformer protection. You will need to protect the secondary conductors at some poit usually 10 feet out.

I think your numbers look fine is this a test question? or what is the load on the secondary??
 
paul hodge said:
15kva
1 Phase
Primary voltage 480
Secondary Voltage 120/240


My calculations are:
Primary Breaker=40a
Primary Conductor=#8
Secondary Breaker=80a
Secondary Conductor=#4
Bond Jumper & Electrode=#8
Click to expand...

The professional response: Without the transformer and cable damage curves it is not possible to ascertain if the protection is selected properly.

Rule of thumb: If you followed the NEC, most likely it meets the minimum level of acceptability.
 
kingpb said:
The professional response: Without the transformer and cable damage curves it is not possible to ascertain if the protection is selected properly.

Rule of thumb: If you followed the NEC, most likely it meets the minimum level of acceptability.
Click to expand...

In my opinion, that might be overthinking it a little for a 15KVA. But that's just a difference of opinion.

If this is a test, should you be using the 60 deg. ampacity? If so you might check your #4.

Also, it might be possible to use #10 on the primary if you count the length as part of the tap distance. 240.21(B)(3).

Steve
 
steve66 said:
In my opinion, that might be overthinking it a little for a 15KVA. But that's just a difference of opinion.

If this is a test, should you be using the 60 deg. ampacity? If so you might check your #4.

Also, it might be possible to use #10 on the primary if you count the length as part of the tap distance. 240.21(B)(3).

Steve
Click to expand...

The following illustrates why it is important to understand the impact of damage curves when it comes to selecting protective devices. As shown, the selection of a primary breaker @ 250% does not adequately protect the transformer or cable, even though it would be allowed by 450.3(B).

View attachment 139

The correct primary protection should be 40A breaker, however #10 would not be appropriate as shown below:
View attachment 140
 
So take the very small chance that a fault occurs on the output of one particular transformer, and combine it with the small chance that the fault just happens to be the right magnitude to damage something, and what have you lost? Some small wire or maybe even a small transformer. In most cases, not a big loss.

And after all that work, what is the result? A 40A breaker and #8 on the primary which is what we had to start with by following the NEC anyway.

Steve
 
steve66 said:
So take the very small chance that a fault occurs on the output of one particular transformer, and combine it with the small chance that the fault just happens to be the right magnitude to damage something, and what have you lost? Some small wire or maybe even a small transformer. In most cases, not a big loss.

And after all that work, what is the result? A 40A breaker and #8 on the primary which is what we had to start with by following the NEC anyway.

Steve
Click to expand...

As far as the amount of risk involved or taking chances, that you'll have to take up with your Chief Engineer. But I prefer, as well as our clients, to not have any unncessary loss of equipment or downtime due to a failure, especially since it can lead to personal injury or loss of property. It's also my stamp, and I design conservatively.

As far as after all that work, it didn't take but a few minutes to create those graphs. My point in doing them was to help everyone interested to understand that the NEC is not some black magic voodoo book. Behind each reference there is solid engineering that was done in order to establish those rules.
 
King,
#10 should be capable of 50A x 10 = 500A for 5 secs, without damage
Click to expand...
That sounds high. I was taught that the maximum withstand for an insulated copper conductor was 1 amp for 5 seconds for each 42.25 circular mil of the conductor. That would only be 240 amps for the #10. That is based on the conductor temperature not exceeding 150?C.
Don
 
Don,

That may be a good rule of thumb for 150 deg C, and a good conservative approach, but as far as I know, cable short circuit damage curves are always presented based on 250 deg C.
 
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