No primary protection required?

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Isaiah

Senior Member
Location
Baton Rouge
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Electrical Inspector
Contractor wants to put two, single phase 3kVA transformers in line to boost the voltage up then bring it down at the load (120/480V to 480/120V) due to V-drop at 1700+ feet from source. Cable between transformers is 2#6 AWG w/#8 Gnd.There is no primary protection on the second transformer. This seems to be a violation of450.3(B). Contractor says "No" since fault current between the two Xfmrs is not sufficient to cause damage. I don’t see any exceptions that would allow this. Comments?
 
The secondary of the second transformer would be grounded but the wiring between the first transformer and the second transformer would usually be balanced, as I understand such uses. Thus no extra protection would be needed there. Or at least that is what I have been understanding , though I have never done such a set up.
 
Adamjamma said:
The secondary of the second transformer would be grounded but the wiring between the first transformer and the second transformer would usually be balanced, as I understand such uses. Thus no extra protection would be needed there. Or at least that is what I have been understanding , though I have never done such a set up.
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If a ground fault occurred between the two xfmrs how would it be cleared if there is no overcurrent device? The primary device, i.e. CB on the first transformer would not see the fault.
 
If both are two wire in / two wire out, I think one OCPD in the supply can protect everything. Had the load end been 120/240 on secondary then it would have needed protection on it's secondary side.
 
kwired said:
If both are two wire in / two wire out, I think one OCPD in the supply can protect everything. Had the load end been 120/240 on secondary then it would have needed protection on it's secondary side.
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I do not know of any exceptions to omit primary protection on the second tranny.

Second tranny also needs a disconnecting means, 450.14.
 
Isaiah said:
If a ground fault occurred between the two xfmrs how would it be cleared if there is no overcurrent device? The primary device, i.e. CB on the first transformer would not see the fault.
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For this install I believe that 240.4(F) allows the primary protection for the first tranny to protect the primary and secondary of the tranny and secondary conductors also.
 
jumper said:
For this install I believe that 240.4(F) allows the primary protection for the first tranny to protect the primary and secondary of the tranny and secondary conductors also.
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I'm more concerned about the primary windings of the second tranny.
 
jumper said:
Well, when the electrician installs the required disco and OCPD, yout concerns will be gone.:)

Besides that, I am not 100% sure about the first tranny OCPD protecting the conductors so landing them in an OCPD is a good idea.
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Here's the problem: cables between transformers on the 480V side have already been installed. Contractor doesn't want to spend the $ to fix it, so he's using his imagination to avoid the rework.
 
Isaiah said:
Here's the problem: cables between transformers on the 480V side have already been installed. Contractor doesn't want to spend the $ to fix it, so he's using his imagination to avoid the rework.
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Contractor really has no choice. Tranny has to have primary OCPD and disconnecting means. Fused switch or breaker can accomplish both.
 
jumper said:
No problem. Trust me, this site is totally pro, if I am wrong someone will call me on it.

Let’s wait a few minutes and see if anyone does, but I am pretty sure I am correct.
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Here's the other part of the issue:
Contractor bought a step-down transformer to utilize as a step-up ('flipped' it 180 degrees - secondary is now the primary @ 120V and primary is now secondary @ 480V). This is not considered a good practice since secondary windings are not normally suited to withstand inrush current as is the primary; plus 450.11(4) requires nameplate to identify primary and secondary voltages.
The 480V side is also not grounded - 250.21(B) requires grounding detectors for ungrounded AC systems - another cost adder.
To do this right, I believe he needs to replace the step-down xfmr with a true step-up, 120-480V and corner ground one side.
Thoughts?
 
Isaiah said:
....To do this right, I believe he needs to replace the step-down xfmr with a true step-up, 120-480V and corner ground one side.
Thoughts?
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It's no problem to reverse feed a transformer. 450.11(B) makes this clear. Besides, it's only been recently that the code had anything to say about it at all and I don't think the manufacturers did anything different to their transformers other than add a note to the instructions that says, "It's ok if you want to backfeed this".
 
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