25kva 480v transformer fuse

Status
Not open for further replies.
H

hoyerr

Member
Location
Warrensburg Missouri
I have a 25KVA transformer fed from 2 legs of a 480V 3 phase line.
My primary is fused with Low Peak 50A J fuses and I have secondary fuse protection. I believe this to be wrong. I believe at 125% it should be a time delay 70A fuse (say FRS-R-70) or at 250% 130A fuse. Am I missing something?View attachment Document1.pdf
 
Last edited:
There is no requirement for what the primary fuse should be, however, there are requirements for what it cannot be.

The best procedure is to determine the inrush of the transformer and compare it to the performance of the fuse. Make sure your fuse does not exceed the size limits in 450.3.
 
its not wrong unless the code says so. 50A primary fuses on a 25kVA 480V xfmr seems kind of low to me. maybe they were trying to save on wire.

however in this case it won't matter much since there is a dead short shown wired across the secondary.
 
Last edited:
hoyerr said:
I have a 25KVA transformer fed from 2 legs of a 480V 3 phase line.
My primary is fused with Low Peak 50A J fuses and I have secondary fuse protection. I believe this to be wrong. I believe at 125% it should be a time delay 70A fuse (say FRS-R-70) or at 250% 130A fuse. Am I missing something?View attachment 8491
Click to expand...

Whay is the secondary? Are you providing secondary protection? Are we to "assume" that the transformer is a Iph 480-120/240 25kva transformer with a 1ph3w secondary?
And where do you propose to locate the primary fuse?
 
petersonra said:
however in this case it won't matter much since there is a dead short shown wired across the secondary.
Click to expand...

I noticed that too...not sure what they're trying to show there.

Also, assuming that this a premises wiring system, the grounding location is in violation of 250.26(2).
 
You really need to know the inrush of the transformer.

The time delay of the type LPJ50A would allow it to handle a 25kVA with an 8X inrush current.
 
hoyerr said:
See the attachment.
Click to expand...

I also noted that the line labeled 240VAC circled A is unfused. Being as it is not a grounded conductor it probably needs a fuse. Of course the dead short shown across the secondary suggests that the circuit is probably not actually wired the way the drawing shows it anyway since it would trip the primary fuse as soon as it was powered up.
 
hoyerr said:
See the attachment.
Click to expand...

I had tried down load the attachment with my NEXUS7 but it failed to download and I can't see why. I'll look at it with my PC:)
Also, since I was and application engineer I do have some data that I put together regarding inrush. As a rule of thumb as the kva of a transformer is increased the % inrush of the FLA decreases. The inrush also varies at a given KVA depended upon the transformer efficiency, 'k' factor, and temperature rise.
 
templdl said:
I had tried down load the attachment with my NEXUS7 but it failed to download and I can't see why. I'll look at it with my PC:)
Also, since I was and application engineer I do have some data that I put together regarding inrush. As a rule of thumb as the kva of a transformer is increased the % inrush of the FLA decreases. The inrush also varies at a given KVA depended upon the transformer efficiency, 'k' factor, and temperature rise.
Click to expand...

I finally downloaded the schematic. In my opinion you do need secondary protection however I find that the secondary connections to the transformer very strange.
I find that should you connect X2 and X3 which is OK and the connect XI to X2/X3 it is a direct short circuit of the X1-X2 winding as well as the connection between X4 to X2/X3 we which is a direct short of the X3-X4 winding.
It is of my opinion that the connection between X-1 and X2/X3 be removed as well as the connection between X4 to X3/X3 removed also.
Then the connection between X2-X3 becomes the neutral and the point which is grounded.
The X1 is fused and X4 is fused. The voltage between X1 and X2/X3 is 120v and X4 and X2/X3 is 120v. Between X1 and X4 is 240v.
The the secondary fused are based upon 104FLA. x 125% or 130a. Because you are providing secondary protection you are allowed up to 250% of the primary 52FLA.
I see that the primary fusing is 50A and the secondary fuses 25a. Why? Am I missing something? Have I misunderstood the purpose of the secondary connection?
 
Status
Not open for further replies.
Top