Step-Up Transformer

[jimsomers10]

Recently I came across a question and I was stumped by it so I figured maybe someone else might know the answer. First I will give a little background information. I am dealing with a 3 phase 15KVA step-up transformer 208V:460V. The load on the secondary is 4300 VA. The transformer is rated for 41.69A on the primary side so with 125% protection it is 52.11A. The primary is being supplied by a 3 phase 60A breaker and the panel has plenty of room and will not be overloaded. The equipment that is being feed from the secondary has 15A fuses on each phase internally. According to Table 450.3 in NEC 2008, I do not need secondary protection for the TRANSFORMER, however according to article 240.4 F there needs to be overcurrent protection on the secondary for the conductors. Here is where my question comes in. Is article 240.4 F covered since there are internal fuses in the equipment or do I need to connect a fuse disconnect on the secondary conductors as well, and if so what size would you recommend. Thanks for all your help!

 

[augie47]

The "devil may be in the details". Sounds like you have 450.3 covered.
As as as 240.4(F) you may or may not be covered. If it's a delta-delta arrangement you may be o.k. depending on the wire sizes and ratio.
However, the particulars of the equipment may be such that a disconnect is required (is it a motor, etc).
It also sounds like you may have a problem with 240.40

More details are needed.

 

[texie]

The "devil may be in the details". Sounds like you have 450.3 covered.
As as as 240.4(F) you may or may not be covered. If it's a delta-delta arrangement you may be o.k. depending on the wire sizes and ratio.
However, the particulars of the equipment may be such that a disconnect is required (is it a motor, etc).
It also sounds like you may have a problem with 240.40

More details are needed.

I would agree. Also, I might add, the OP may have issues with 240.21(C). I would also wonder if the secondary is grounded or ungrounded and the details of same.