Transformers and Feeder

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Jlewis7457

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I have "step up step down" Installation for a single family residence that consist of a reverse fed 480v/240v/1Phase/ 37.5kva transformer, conductors are
(2) x #2/0 THHN Copper protected by a 200amp OPD with a #4 copper EGC. The feeder circuit between transformers is (2) x 1000' of 650mcm XHHW-2 AL and (1) x #2 AL equipment grounding conductor. The step down transformer 480v/120v 240v / 1 phase / 37.5kva at the house. ( Transformers are identical ACME transformers with multi-taps for fine tune adjustments, model # TP530193S

Questions:

1) confirm the size of the feeder EGC
2) will the neutral at the load side transformer jumper with the EGC
3) Is the service at the house considered 150amps
4) Can the load side of the step down transformer land on a 2 pole 200amp OPD in the first Sub-panel

Thank you in advance for your expertise and advice, JL
 
The #2/0 on the primary of the step up are too small as #2/0 is only rated for 175 amps @ 75° C but the EGC is good. Is there an OCPD on the secondary of the step-up transformer?

Welcome to the Forum. :)
 
Thanks Rob,

I don't anticipate an OCPD between transformers, the first OCPD on the secondary of the step down would be with-in 25' and I'm hoping to land on a 200 amp 2-pole Breaker inside the exterior of the first sub-panel mounted on the SFR.
 
To provide proper Art 450 protection for your 37.5 transformer the 200 amp breaker is allowable but 2/0 is not allowable on the 200 amp OCP device.
The load side transformer should comply with 250.30
It's foolish of me to question Infinity's answer but if you are speaking of the grounidng conductor between the two transformers, I feel the #2 is undersized based on the 650 phase conductors.
 
Hi Augie, cool walker!

the 650 mcm phase conductors are sized to accommodate voltage drop, my confusion is how to then size the EGC ?

Regarding the Neutral bonding jumper... 250.30 would say to do it at the secondary, there would be no parallel path created in my installation?
 
Jlewis7457 said:
Thanks Rob,

I don't anticipate an OCPD between transformers, the first OCPD on the secondary of the step down would be with-in 25' and I'm hoping to land on a 200 amp 2-pole Breaker inside the exterior of the first sub-panel mounted on the SFR.
Click to expand...
I think I'll go with 250mcm XHHW-2 then for the primary. 205 amps 75C

I've been using a lot of the XHHW-2 lately for cost and weight considerations
 
Jlewis7457 said:
Hi Augie, cool walker!

the 650 mcm phase conductors are sized to accommodate voltage drop, my confusion is how to then size the EGC ?

Regarding the Neutral bonding jumper... 250.30 would say to do it at the secondary, there would be no parallel path created in my installation?
Click to expand...
250.66 is showing 4/0 AL for the EGC if run with 600mcm or larger ungrounded?
 
Jlewis7457 said:
250.66 is showing 4/0 AL for the EGC if run with 600mcm or larger ungrounded?
Click to expand...
250.66 is for GEC's not EGC's. EGC's are under T250.122. The question is what is the conductor run between the 480 secondary and the 480 primary? It is a supply side bonding jumper? If so then that's T250.102(C)(1). Also 650 kcmil is an odd size at least around here no one will stock that. It's either 600 or 750 kcmil.
 
JL:

If this is just in the design phase, I would STRONGLY recommend against this setup. Unless you have something unusual or extenuating circumstances, 1,000 ft is not nearly enough length to justify a step up step down. You will be looking at $350 a year in no load losses with this setup. Your transformers are likely too large, and the load you are using for your voltage drop calcs is likely too high.

If you must go with a step up step down, I would HIGHLY recommend going up to medium voltage. Set up as an MGN you would need one #2 concentric neutral cable, about $2.50 per foot. Don't be scared of the medium voltage work, all you have to do is be able to make up a load break elbow, it is not complicated. You just need a semicon stripper and a BG crimper. Feel free to PM me if you want more info.

As far as your original plan, if you were to make one of the 480 lines of grounded conductor you would not need to run an egc with the two line conductors.
 
infinity said:
250.66 is for GEC's not EGC's. EGC's are under T250.122. The question is what is the conductor run between the 480 secondary and the 480 primary? It is a supply side bonding jumper? If so then that's T250.102(C)(1). Also 650 kcmil is an odd size at least around here no one will stock that. It's either 600 or 750 kcmil.
Click to expand...
Yes I I thought I was looking at the Equipment Grounding Conductor table... my mistake.

250.122 (B) is an interesting note " EGC's shall be permitted to be sized by a qualified person to provide an effective ground fault current path " and "sized proportionately to the increase in circular mil of the ungrounded conductors... I don't think I've ever seen a note like this before, weird
I think a 4/0 awg AL Equipment grounding conductor between transformers that is run in the same conduit with the 600kcmil phase conductors should make sense?
 
JL:
electrofelon said:
If this is just in the design phase, I would STRONGLY recommend against this setup. Unless you have something unusual or extenuating circumstances, 1,000 ft is not nearly enough length to justify a step up step down. You will be looking at $350 a year in no load losses with this setup. Your transformers are likely too large, and the load you are using for your voltage drop calcs is likely too high.

If you must go with a step up step down, I would HIGHLY recommend going up to medium voltage. Set up as an MGN you would need one #2 concentric neutral cable, about $2.50 per foot. Don't be scared of the medium voltage work, all you have to do is be able to make up a load break elbow, it is not complicated. You just need a semicon stripper and a BG crimper. Feel free to PM me if you want more info.

As far as your original plan, if you were to make one of the 480 lines of grounded conductor you would not need to run an egc with the two line conductors.
Click to expand...
Thank you for the reply!

your correct about the no load losses, about 23.00 per month here. The site will have solar to offset this and the transformers are on site, so it's in motion. I'm intrigued and fascinated with the various options that have been suggested regarding this installation. I was discussing the install with a PGE field rep. and he was talking about a 7000 volt system and grounding grid at the secondary? I've worked on everything from hospitals to military bases and we always used "transformer to transformer" installations for long distance feeders. I'm curious if there are any engineering drawings (single line) that show your ideas listed above? I simply have never heard of such installations? JL
 
Jlewis7457 said:
I think a 4/0 awg AL Equipment grounding conductor between transformers that is run in the same conduit with the 600kcmil phase conductors should make sense?
Click to expand...
Not as much sense as the single-ungrounded-conductor method E-felon mentioned.
 
Jlewis7457 said:
JL:

Thank you for the reply!

your correct about the no load losses, about 23.00 per month here. The site will have solar to offset this and the transformers are on site, so it's in motion. I'm intrigued and fascinated with the various options that have been suggested regarding this installation. I was discussing the install with a PGE field rep. and he was talking about a 7000 volt system and grounding grid at the secondary? I've worked on everything from hospitals to military bases and we always used "transformer to transformer" installations for long distance feeders. I'm curious if there are any engineering drawings (single line) that show your ideas listed above? I simply have never heard of such installations? JL
Click to expand...
JL,

Yeah the MV system is vastly superior. There is really no reason whatsoever not to do it. The transformers have fusing built in so no extra equipment to procure or hook up (unfortunately for your installation you will need an OCPD on the primary of the step down to meet 450.3(B)). I did one last summer that was 7620 - that is probably the most common MV distribution voltage in the country. I have step up step down at my place and it is 2400. There isnt much to it, really not worth even seeing a one line. You would use an MGN system per NEC 250.184(C) with a single CN cable. The outer is you grounded conductor which lands on the tank of the transformer, and the center goes to an elbow connector which connects the the high side bushing. Same on both units. Ill attach a few pictures. and here is a link to a video on making up an elbow connector. You NEED the semicon stripper and the BG crimper, but you can get by without the other tools he has. You do need to use care and careful craftsmanship making these up, but watch the video, take you time, and do a practice run or two and you will be fine.

 

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Great videos! Thank you,
I see now what you are talking about. The variable that is the deal breaker in my application using the MV system is that the utility primary is overhead across the street and PGE would charge about 30,000.00 minimum to set a primary pole and 8000.00 to connect to the MV system. Remember we are talking about California and PGE...

Thank you for the education regarding the MV system, I really appreciate it ,JL
electrofelon said:
JL,

Yeah the MV system is vastly superior. There is really no reason whatsoever not to do it. The transformers have fusing built in so no extra equipment to procure or hook up (unfortunately for your installation you will need an OCPD on the primary of the step down to meet 450.3(B)). I did one last summer that was 7620 - that is probably the most common MV distribution voltage in the country. I have step up step down at my place and it is 2400. There isnt much to it, really not worth even seeing a one line. You would use an MGN system per NEC 250.184(C) with a single CN cable. The outer is you grounded conductor which lands on the tank of the transformer, and the center goes to an elbow connector which connects the the high side bushing. Same on both units. Ill attach a few pictures. and here is a link to a video on making up an elbow connector. You NEED the semicon stripper and the BG crimper, but you can get by without the other tools he has. You do need to use care and careful craftsmanship making these up, but watch the video, take you time, and do a practice run or two and you will be fine.

Click to expand...
 
Jlewis7457 said:
Great videos! Thank you,
I see now what you are talking about. The variable that is the deal breaker in my application using the MV system is that the utility primary is overhead across the street and PGE would charge about 30,000.00 minimum to set a primary pole and 8000.00 to connect to the MV system. Remember we are talking about California and PGE...

Thank you for the education regarding the MV system, I really appreciate it ,JL
Click to expand...

Well you would still take service at 120/240.... its a step up/ step down just like you are doing but at some 15kv class voltage instead of 480.
 
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