Transformers and Feeder

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electrofelon said:
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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I'm curious ,
what is the approximate unit price for the MV system transformers suitable for my application? I remember you said about 2.50 per foot on the conductor.
JL
 
Jlewis7457 said:
I'm curious ,
what is the approximate unit price for the MV system transformers suitable for my application? I remember you said about 2.50 per foot on the conductor.
JL
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Prices are of course wacky right now.....but some numbers from the past: Usually I get them from the surplus or refurbished market such as ebay or sunbelt/solomon or maddox. In 2019 I got a quote for new 15 KVA pads and they were $2500. This was built to order by Eaton. Last years job were 25's which were new surplus DOE 2016 and were about $1600 each delivered. I did get a quote for 25KVA new prior to finding those and it was $4500. Also last year I got a price on a new surplus 25 KVA for $2500. You have to be careful of the refurbished ones because these refurbishers remove the original label so you cant tell the date it was made. Personally I would not get anything that is not DOE2016 or newer and would not buy anything I cant verify.
 
JL,. I would highly recommend skipping the transformers entirely for this project. Step up step down is a horrible solution and should only be used if there is no other option. 1000 feet is not that far. When you take into account the losses, the poor voltage regulation, transformer/disconnect/labor costs,. You should just put that money into larger wire. Remember, each transformer will have a few percent of voltage drop, so you can roll that into your figures for wire without. If you use a REALISTIC current figure for your VD calc, I would probably run two sets of 350 urd for this (no transformers).

The PV system is nice to make up for losses, but you will have quite a lot of voltage swing because instead of now just considering say 60 amps of current in one direction, you effectively double that (for current going the "other way") which doubles your voltage swings. I have to tap my transformer down to 110/220 to keep my voltage within parameters at full PV output.

IMO step up step down shouldn't be considered until about 1600 feet.
 
electrofelon said:
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.
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Or at least look into if POCO can supply you with 480 volt single phase service and only have the one transformer at load end of the run.

Some may not do that at all, here we have many agricultural 480 volt single phase services or even open delta 480 volt systems so the transformers and metering needed is something they do have on hand at least.
 
infinity said:
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. :)
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If supplying a dwelling and using 83% rule (which section depends which code edition applies) 2/0 copper should comply.
 
electrofelon said:
JL,. I would highly recommend skipping the transformers entirely for this project. Step up step down is a horrible solution and should only be used if there is no other option. 1000 feet is not that far. When you take into account the losses, the poor voltage regulation, transformer/disconnect/labor costs,. You should just put that money into larger wire. Remember, each transformer will have a few percent of voltage drop, so you can roll that into your figures for wire without. If you use a REALISTIC current figure for your VD calc, I would probably run two sets of 350 urd for this (no transformers).

The PV system is nice to make up for losses, but you will have quite a lot of voltage swing because instead of now just considering say 60 amps of current in one direction, you effectively double that (for current going the "other way") which doubles your voltage swings. I have to tap my transformer down to 110/220 to keep my voltage within parameters at full PV output.

IMO step up step down shouldn't be considered until about 1600 feet.
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Really depends on actual loading, as well as how much tolerance you can handle for temporary voltage drop say when a compressor motor starts. If you can soft start said compressor you can probably get away with even smaller conductor though.
 
electrofelon said:
JL,. I would highly recommend skipping the transformers entirely for this project. Step up step down is a horrible solution and should only be used if there is no other option. 1000 feet is not that far. When you take into account the losses, the poor voltage regulation, transformer/disconnect/labor costs,. You should just put that money into larger wire. Remember, each transformer will have a few percent of voltage drop, so you can roll that into your figures for wire without. If you use a REALISTIC current figure for your VD calc, I would probably run two sets of 350 urd for this (no transformers).

The PV system is nice to make up for losses, but you will have quite a lot of voltage swing because instead of now just considering say 60 amps of current in one direction, you effectively double that (for current going the "other way") which doubles your voltage swings. I have to tap my transformer down to 110/220 to keep my voltage within parameters at full PV output.

IMO step up step down shouldn't be considered until about 1600 feet.
Click to expand...
Good morning,
such great input on this forum! more variables below...

1) The second roll of 3" HDPE is 3500.00 and close to double the conductor cost to run Parallel conductors.

2) The solar load is not flowing through the feeder circuit we are discussing, solar load will land on the meter main distribution buss ahead of the first transformer.

3) The AHJ wants 150 amps available at the sfr . 37.5kva / 240=156.25amps x 125% for the OCPD =200amp OCPD


vd=2x21.2x1100x150=6996000/600000=11.66v 240v x .03=7.2v

how about 1 run of 1000kcmil ?

vd=2x21.2x1100x150=699600/1000000=9.328v this is pretty close to 3% vd for feeders

I'm checking chapter 9 table 1 for conduit fill
 
kwired said:
Or at least look into if POCO can supply you with 480 volt single phase service and only have the one transformer at load end of the run.

Some may not do that at all, here we have many agricultural 480 volt single phase services or even open delta 480 volt systems so the transformers and metering needed is something they do have on hand at least.
Click to expand...
Yes, that would be nice of PGE to do that but, PGE would never supply anything other than 120/240 to a sfr
 
electrofelon said:
Prices are of course wacky right now.....but some numbers from the past: Usually I get them from the surplus or refurbished market such as ebay or sunbelt/solomon or maddox. In 2019 I got a quote for new 15 KVA pads and they were $2500. This was built to order by Eaton. Last years job were 25's which were new surplus DOE 2016 and were about $1600 each delivered. I did get a quote for 25KVA new prior to finding those and it was $4500. Also last year I got a price on a new surplus 25 KVA for $2500. You have to be careful of the refurbished ones because these refurbishers remove the original label so you cant tell the date it was made. Personally I would not get anything that is not DOE2016 or newer and would not buy anything I cant verify.
Click to expand...
What are the considerations regarding underground conduit buried depth when the conduit has a medium voltage 7000v circuit in it? The conduit will run down the center of the driveway. The depth of conduit as currently proposed requires 24" to top of conduit in a driveway.
 
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.

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This MGN system ( 250.184(C) is really interesting! 1300' so I'd ground it at each end only, not a big deal at all. I'm wondering about the overcurrent protection.

does 450.3 state that the transformers working together are considered polyphase and are a unit? Can the built-in primary protection at the first transformer satisfy 450.3(A) ?
 
Jlewis7457 said:
What are the considerations regarding underground conduit buried depth when the conduit has a medium voltage 7000v circuit in it? The conduit will run down the center of the driveway. The depth of conduit as currently proposed requires 24" to top of conduit in a driveway.
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It wouldnt be any different, see 300.50. We have lots of places here that have an UG MV POCO feed, often down the driveway. The one POCO Direct buries it in sand (well you do, they just lay the cable in and charge you like $20/foot :mad: ). The other one lets customers supply and run their own MV primary, and they provide the transformer and make the connections. When I run those I put it in 2" PVC, IIRC they want it 36".
 
Jlewis7457 said:
This MGN system ( 250.184(C) is really interesting! 1300' so I'd ground it at each end only, not a big deal at all. I'm wondering about the overcurrent protection.

does 450.3 state that the transformers working together are considered polyphase and are a unit? Can the built-in primary protection at the first transformer satisfy 450.3(A) ?
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The step up transformer would have the primary (240V) protected by the service OCPD (or other OCPD). Padmounts have fusing built in so that would satisfy the secondary protection of the step up, and the primary protection for he step down.
 
Jlewis7457 said:
Good morning,
such great input on this forum! more variables below...

1) The second roll of 3" HDPE is 3500.00 and close to double the conductor cost to run Parallel conductors.

2) The solar load is not flowing through the feeder circuit we are discussing, solar load will land on the meter main distribution buss ahead of the first transformer.

3) The AHJ wants 150 amps available at the sfr . 37.5kva / 240=156.25amps x 125% for the OCPD =200amp OCPD


vd=2x21.2x1100x150=6996000/600000=11.66v 240v x .03=7.2v

how about 1 run of 1000kcmil ?

vd=2x21.2x1100x150=699600/1000000=9.328v this is pretty close to 3% vd for feeders

I'm checking chapter 9 table 1 for conduit fill
Click to expand...
Basically the way I do these is like the POCO would, typically a 25KVA transformer. We all know the NEC calcs are highly excessive and way outside of reality, so if you have a picky AHJ that wants NEC sized equipment, this can result in much higher costs unfortunately. That said, the NEC deosnt address transformer sizing, so you can "undersize" your transformers to a more reasonable 25KVA size with less cost and less losses, and have the protection values in 450.3 probably still meet the NEC calculated Loads.

AS far as Voltage drop, Most houses will not see over 60-70 amps, even if all electric, so that is the figure I use to calculate wire size. So using that, I only get 2 sets of 250 AL or 1 set of 750 AL for under 3% at 70 amps. Staying under 500KCMIL is much more efficient because skin effect starts to come into play. Remember you transformers will have voltage drop also, so be "fair" in your analysis and if you are figuring 3% for the VD for the wire with a step up step down, that is probably the same as say 6% without the transformers. I just hate to see money wasted on excessively large wire.
 
electrofelon said:
Basically the way I do these is like the POCO would, typically a 25KVA transformer. We all know the NEC calcs are highly excessive and way outside of reality, so if you have a picky AHJ that wants NEC sized equipment, this can result in much higher costs unfortunately. That said, the NEC deosnt address transformer sizing, so you can "undersize" your transformers to a more reasonable 25KVA size with less cost and less losses, and have the protection values in 450.3 probably still meet the NEC calculated Loads.

AS far as Voltage drop, Most houses will not see over 60-70 amps, even if all electric, so that is the figure I use to calculate wire size. So using that, I only get 2 sets of 250 AL or 1 set of 750 AL for under 3% at 70 amps. Staying under 500KCMIL is much more efficient because skin effect starts to come into play. Remember you transformers will have voltage drop also, so be "fair" in your analysis and if you are figuring 3% for the VD for the wire with a step up step down, that is probably the same as say 6% without the transformers. I just hate to see money wasted on excessively large wire.
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Absolutely, you are thinking real world applications and I try to go that route too! I'm in contact with the senior building inspector in charge of my project . I have additional data and options to present (thanks to this great and informative forum!) to him and we'll see what shakes out. I'd like to make sure I get clearance from him first so all on the same page with no expensive surprises .

You guys are great,
Best, JL
 
Jlewis7457 said:
Absolutely, you are thinking real world applications and I try to go that route too! I'm in contact with the senior building inspector in charge of my project . I have additional data and options to present (thanks to this great and informative forum!) to him and we'll see what shakes out. I'd like to make sure I get clearance from him first so all on the same page with no expensive surprises .

You guys are great,
Best, JL
Click to expand...
If it were me (and granted making some assumptions not knowing a lot about the project, I would go two sets of 250 or 350 URD (triplexed USE with 2 sizes reduced neutral) in 4" SDR35 sewer pipe for protection, no transformers. I would make the long run service conductors to avoid the EGC. Sounds like you have a future PV system at the beginning, but you can use 230.40 exception #3 to feed that and keep it a service all the way to the house.
 
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