Primary Fuse Sizing for 13.8 kv transformers

[metallica5]

I have one 13.8kv Delta pri x 4.16kv Wye Secondary 4000kva transformer and the second is 13.8kv Delta pri x 480/277 Wye secondary 2/2.6MVA.
Transformers are individually fed from an S&C metal clad switch gear. I am looking to size the fuse needed in the S&C gear feeding the primary side of each transformer.
Thank You

 

[david luchini]

See Table 450.3(A) for transformer protection, and 240.101(A) for feeder protection.

 

[metallica5]

Thanks Dave, I was looking for the correct calculation to determine the fuse size.

 

[Smart $]

I have one 13.8kv Delta pri x 4.16kv Wye Secondary 4000kva transformer and the second is 13.8kv Delta pri x 480/277 Wye secondary 2/2.6MVA.
Transformers are individually fed from an S&C metal clad switch gear. I am looking to size the fuse needed in the S&C gear feeding the primary side of each transformer.
Thank You

Not enough detail. See Table 450.3(A).

Don't forget the table is only for transformer protection, as a percentage of transformer nameplate ratings. It does not include conductor protection.

 

[metallica5]

per your comment, what other details are needed for the calculation?
Thanks

 

[mayanees]

here's a good start

here's a good start

S&C Selection Guide for Transformer Primary fuses in MV & HV Utility and Industrial Substations
http://www.sandc.com/edocs_pdfs/edoc_025854.pdf

This document is a excellent reference to be used in conjunction with 450.3.

.. and these docs used to be enough to size transformer prinary fusing.

BUT, now arc flash has added a new wrinkle to the sizing exercise, as the xfmr primary fuse closely governs the Hazard Risk Category of the secondary bus, such that a lower fuse size may yield more desirable results, while still meeting NEC requirements. But that takes a lot more info for determination of incident energy levels, like primary source contribution, as well as the characterisitces of the ocpd powering the transformer.

John M

 

[metallica5]

Thanks for passing the S&C info off. Looks like the tables they have in that doc are for 34.5kv and above. Here is what I came up with for calculations based on the 2005 NEC (this project is going off 2005).

13.8 kv delta x 4.16kv wye 4,000kva transformer with 6.14% Imp. Primary Fuse sizing
1. 4,000,000/(13,800x1.732) = 4,000,000/29,301.6 = 167.35 amps
2. 167.35 amps x 300% (430.3A Supervised location) = 502.05 amps
3. 2005 NEC Art. 450, Table 450-3(A) under supervised location at impedance of 6% or more, tells me to go to 300% for fuse sizing however it stops short of telling me to go to next higher size fuse or next lower size? Since we are talking 2.05 amps, I will revert to 500 amp fuse?

 

[mayanees]

oops! I missed that the doc was 34.5 - 138 kV. I'll look around for another applicable doc.
I'm not sure about the 2005 NEC vs 2011, but you can round up to the next standard rating. However, that's 300% MAXIMUM, so I'd stick to the 500.

One big issue though is still - what is the ocpd that's powering the transformer? If it's fed from something less than the 500-amp fuse you're proposing, it may not coordinate. Find out the device and I'll plot it out for you.

John M

 

[metallica5]

Hi John,
Don't sweat looking for another doc. for the 13.8 kv range, I will use the one you sent for the 115kv job I'm doing right now so it's not wasted after all. However, on the other job, the situation is new added transformer is inside the building and fed from an S&C outdoor metal clad 13.8kv switchgear. This gear has two open bays for future loads, the future is today so we are feeding the new transformer from one of the open fusible bays. I was originally looking for the fuse size calculation for the S&C switch that feeds the transformer . I will have to look further to the fusing on the transformer itself per your comment.
Thank You

 

[mayanees]

Metallica5,
The fuse in your outdoor gear IS the fuse for the transformer, by the way you described it. If you also have a fused switch for the transformer, the feeder from the outdoor gear AND the fused switch could have the same fuses, since coordination wouldn't be critical for the 2 series fuses. But then you should look ahead of the outdoor gear to see what's ahead of the transformer fuse to verfiy or maintain coordination.
PM me if you want me to plot anything out for you. You're actually real close to me if you're in DC.
John M