No primary protection required?

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jumper said:
Me like academic questions.

Where in the heck is Walter?....!!!
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I agree with Derek. From a plain reading of 450.3, I don't see that an OCPD on the primary of the first transformer would meet the requirements of primary protection for the second transformer.
 
question? What happens when the primary transformer is shut down by its OCPD? Does power flow to the secondary transformer? Especially when doing a step up transformer system, is it easier or harder to find disconnects and fuses for the higher voltage or the lower voltage?

My own view is that is I shut the power at the primary side of the first transformer the circuit is now shut down. When the poco runs its transformers, they do fuse the various legs and shut them down, but not on the secondary or 110 side and only so they can replace an individual transformer without affecting the other three or four hundred on their lines.
But, when using a single transformer up.. to a single transformer down, I do not see the need for the extra fuses although I will agree a disconnect is nice on occasion, so one can change transformers easier sometimes....

However, putting fuses before and after all parts of each transformer seems like overkill to me.
 
Adamjamma said:
question? What happens when the primary transformer is shut down by its OCPD? Does power flow to the secondary transformer? Especially when doing a step up transformer system, is it easier or harder to find disconnects and fuses for the higher voltage or the lower voltage?

My own view is that is I shut the power at the primary side of the first transformer the circuit is now shut down. When the poco runs its transformers, they do fuse the various legs and shut them down, but not on the secondary or 110 side and only so they can replace an individual transformer without affecting the other three or four hundred on their lines.
But, when using a single transformer up.. to a single transformer down, I do not see the need for the extra fuses although I will agree a disconnect is nice on occasion, so one can change transformers easier sometimes....

However, putting fuses before and after all parts of each transformer seems like overkill to me.
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Adam, every transformer needs primary protection and a disco. Whether you agree or not is not relevant.

Secondary and secondary conductors need protection also, but design plays a part on how that is accomplished.
 
electrofelon said:
If you had some sort of "feeder extension" like from subfeed lugs out of a panelboard, or a "tap" that was actually a "splitting splice" not a tap, you wouldnt require another OCPD would you? IMO the only argument than could be made for not allowing the discussed situation is deduction from extension of the fact that they DO give permission for conductor protection and panelboard protection in those respective articles, but they do not in 450. IMO those articles should remove that permission as the NEC is supposed to be permissive - just let the electrical theory determine when you can do it. I dont see the NEC delving into electrical theory for other stuff like why we bond both sides of a metal raceway containing an EGC, parallel conductor grouping, etc. Maybe just put in an informational note for when you can protect through a transformer since it is a bit on the not so intuitive side.
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jumper said:
You get David Lucini or similar to agree and I will step down. Else, I am sticking to my guns till proven else wise.
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david luchini said:
I agree with Derek. From a plain reading of 450.3, I don't see that an OCPD on the primary of the first transformer would meet the requirements of primary protection for the second transformer.
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Code may or may not require separate protection on the second transformer (I did not double check what is written), but from theory perspective and remember we were talking two wire to two wire transformers here, there is no reason why one device can't protect both. It is multiwire secondaries that kick in the need for separate secondary protection. Need for second disconnect could depend on many additional circumstances though.
 
kwired said:
Code may or may not require separate protection on the second transformer (I did not double check what is written), but from theory perspective and remember we were talking two wire to two wire transformers here, there is no reason why one device can't protect both. It is multiwire secondaries that kick in the need for separate secondary protection. Need for second disconnect could depend on many additional circumstances though.
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Theoretically, maybe. Code wise, nope.

The second tranny needs its own primary OCPD and disconnecting means.

I see no exceptions for this per NEC.
 
jumper said:
Theoretically, maybe. Code wise, nope.

The second tranny needs its own primary OCPD and disconnecting means.

I see no exceptions for this per NEC.
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Agreed - there are no listed exceptions to use calculations, theory, assumptions etc to suffice for NEC 450.3(B) and 450.14.
 
I may go along, though a little reluctantly, with the need for a disconnect on second transformer. This only for a two wire secondary/three phase three wire application though.

240.21(C)(1) says the conductors on a two wire secondary can be protected by the primary device - if you have done this, adding additional secondary protection (at same primary to secondary ratio and same trip curve type) doesn't increase or decrease protection level at all, it will be a race to see which one will trip first when there is overcurrent conditions.
 
kwired said:
I may go along, though a little reluctantly, with the need for a disconnect on second transformer. This only for a two wire secondary/three phase three wire application though.

240.21(C)(1) says the conductors on a two wire secondary can be protected by the primary device - if you have done this, adding additional secondary protection (at same primary to secondary ratio and same trip curve type) doesn't increase or decrease protection level at all, it will be a race to see which one will trip first when there is overcurrent conditions.
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Makes all the sense in the world - except the code doesnt allow it. Might be a great suggestion for a future code cycle.

Sent from my LM-X212(G) using Tapatalk
 
Interesting, Canada has the same disconnecting requirement, but allow it to be unfused.

https://www.technicalsafetybc.ca/al...-installation-step-step-down-single-phase-low

typical-transformer-installation.gif
 
I know when doing radio work you can create more than one transformer from one input, but, would it be possible, theoretically at least, to make a transformer supply 110/240 USA as above, with 240 single phase UK, from one 600 volt source as above? Or, would it be too imbalanced and so be better always to do as two separate transformers, or a 110 to 240 step transformer? I was told to simply buy a step up transformer like we use as step down here to make 110.. but it is actually a 55/110 in truth... at least that is what my tests showed...
 
The only difference between those diagrams and my idea is the grounded center tap on the first transformer's secondary. Does that make any difference to the rest of the set-ups? Seems like halving the voltage to earth makes it a tad safer.
 
LarryFine said:
The only difference between those diagrams and my idea is the grounded center tap on the first transformer's secondary. Does that make any difference to the rest of the set-ups? Seems like halving the voltage to earth makes it a tad safer.
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Both yours and texie’s are legit. The Canadian one would not fly here.

Design choice. I could either way depending on the job.
 
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