E-rated fuses and Table 450.3(A)

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chuckd83

Member
I'm looking at a single phase 50 kVA, 13.8/0.24 kV transformer that is protected by a 10E fuse. The primary FLA is 3.6 A. This is a station service transformer and the fuse is its only protection.

When I plot the fuse and transformer on a TCC, it becomes obvious that the transformer damage curve is not fully protected. This is because by NEMA standard, it takes up to 5 minutes for an E-rated fuse to operate for currents 200-240% of its rating. i.e. the 10E fuse will never operate for currents less than 20 A.

Given this, I'm inclined to replace it with a 5E or 7E fuse. I'm also curious if the current installation meets the requirements of NEC Table 450.3(A) which stipulates the maximum fuse rating must be 300% FLA (10.8 A in my case). The fuse rating is 10E, or 20 - 24 A.
 
Utilities do not follow the NEC so their transformer fuse sizing does not need to protect the transformer from damage. Their fuse are to protect their grid from failed transformers.
 
jim dungar said:
Utilities do not follow the NEC so their transformer fuse sizing does not need to protect the transformer from damage. Their fuse are to protect their grid from failed transformers.
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This is not a utility transformer. It's a station service transformer for an industrial installation.
 
I typically choose MV E Rated fuses at the next standard size above the FLA because of the high long time protection as you mentioned.

From a code standpoint, I cannot imagine an AHJ would know much about the long time characteristics of a 10E rated fuse and how it isn't really protecting at 10A
 
ron said:
I typically choose MV E Rated fuses at the next standard size above the FLA because of the high long time protection as you mentioned.

From a code standpoint, I cannot imagine an AHJ would know much about the long time characteristics of a 10E rated fuse and how it isn't really protecting at 10A
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That's my plan and what I've always done. So you'd go with a 5E? My only concern would be it's characteristic curve is below the assumed transformer inrush of 8x FLA at 0.1s.

My code question is more to do with its intention. A 10E fuse will not operate for current below 20A and the transformer FLA is 3.6A. That seems to violate the code's intention.
 
chuckd83 said:
My code question is more to do with its intention. A 10E fuse will not operate for current below 20A and the transformer FLA is 3.6A. That seems to violate the code's intention.
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Who recommended the 10E fuse?
Most literature about MV seems to assume a utility system, not one subject to the NEC.
The NEC allows oversizing of short circuit primary side protection if adequate overload protection has been provided on the secondary.
 
Is secondary protection provided? and what is its rated? If 200A then it would be good. If secondary are outside then 25' rule won't be applied. Ensure the secondary conductors is rated more than 200A (round up is prohibited) if multiple CB/Fuse on secondary are connected, then sum of them can't exceed X-former rating (~260A (125%)). Notes Protection conductors of primary and secondary are also required to meet 240.
 
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