120/208v side

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Alwayslearningelec

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Hello all. Need to run 1200A riser up 700'( found out load). Using (6) sets #600 AL MC Cable. The source and load is 120/208v but going to step it up then back down with xfrmr.
How do i calculate the 208v side conductors/sets and xfrmr size for step up and step down? Thanks.
 
horsegoer said:
How do i calculate the 208v side conductors/sets and xfrmr size for step up and step down? Thanks.
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For the source and load at 208v, the normal numbers will apply.

For the high-voltage section, the current ratio will be the inverse of the voltage ratio.
 
LarryFine said:
For the source and load at 208v, the normal numbers will apply.

For the high-voltage section, the current ratio will be the inverse of the voltage ratio.
Click to expand...
(Ok, (4) sets 500 AL for 120/208v side will be good for 1200A. A little lost on xfrmr sizing for this.
 
VA remains the same.

Voltage and current is what changes.

240 to 480 is simpler because the ratio is 2:1 where 208 to 240 is 2.31:1
 
horsegoer said:
The source and load is 120/208v but going to step it up then back down with xfrmr.
Click to expand...
I know this is a bit of a tangent, but does it make sense to see if you can get a 480Y/277V source? Is there any future expansion planned where a 480V system would be beneficial? Would it make sense to get another 208Y/120V service to run the 700' as utility primary?
 
horsegoer said:
Hello all. Need to run 1200A riser up 700'( found out load). Using (6) sets #600 AL MC Cable. The source and load is 120/208v but going to step it up then back down with xfrmr.
How do i calculate the 208v side conductors/sets and xfrmr size for step up and step down? Thanks.
Click to expand...

I am not sure you will save anything in the end by only stepping up to 480. You are looking at probably over $30k in transformers here. If you went to medium voltage you would presumably spend less on transformers because these things are a dime a dozen used at the KVA you are looking at, and you wire cost would be only about $7.50 per foot.

Since you seem determined to not go to MV, there is another option (which is still technically MV) and that is to have a pair of 208 to 2000 volt transformers made for you and use aluminum 2 KV PV/RHH-2 wire. You can easily get that in larger sizes to handle the 125 amps, it's used in large PV systems to serve combiners. The terminations for this would just be standard mechanical connectors, perhaps that is more in your comfort zone.

If you are really going to stick with the low voltage transformer idea, you might as well go to the full 600V, get as much of your money's worth as you can.
 
oldsparky52 said:
I imagine you would use a Delta/Wye for the step-down transformer, but what would be the prefered configuration for the step-up transformer, and why?
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On the other hand, if you went delta:delta for the step up you would have one of those configurations where primary OCPD could protect the secondary.

Would it be possible to build this step up...step down system without any OCPD or switching for the higher voltage portion?

-Jon
 
600 AL MC vertical riser 700 ft.
Would not even think about without getting some engineering involved. Support issues, etc.
I think there are better options such as riser buss duct or something designed for that use.
 
winnie said:
On the other hand, if you went delta:delta for the step up you would have one of those configurations where primary OCPD could protect the secondary.

Would it be possible to build this step up...step down system without any OCPD or switching for the higher voltage portion?

-Jon
Click to expand...
I was thinking the same thing. For medium voltage , typical oil-filled pad mounts have built infusing so that would take care of your transformer and conductor protection (another advantage of medium voltage). For low-voltage you may be able to go Delta Delta and Skip any equipment between the two transformers. the only sticky thing, is there is no specific allowance to protect the primary of the second transformer (450.3(B)) with the device protecting the first transformer. I guess one could argue by extension of 240.21(C)(1) and 408.36(B) exception that they should be allowed but again there is no specific allowance for ignoring 450.3(B) for the primary of the second transformer.
 
I think the 2kv approach has very strong merits, perhaps the best of all worlds here, with the only disadvantage being having to get custom transformers built. However I wouldn't worry too much about this as transformers are custom-built all the time. Practically all pad mounts are built to order I believe. A few years ago I inquired about ordering some padmount transformers from my supplier and although those were a standard voltage, they were built to order and I believe I could have specified any voltage I want. 2 KV PV wire is incredibly affordable and has no shielding so there are no "special" terminations to make. It is also available in aluminum for cost effectiveness. Pretty much any electrician could wire this up without noticing any difference from 600V class equipment.

Mr Gunn from over on the PV formum (who hasn't been on in a while I believe) did this 2KV trick on a step up step down job for a PV system. And this is the method I use at my house too.
 
augie47 said:
600 AL MC vertical riser 700 ft.
Would not even think about without getting some engineering involved. Support issues, etc.
I think there are better options such as riser buss duct or something designed for that use.
Click to expand...
300’ horz and 400’ vert
 
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