2 pad mount transformers different KVA’s ?

[IntheWoods]

Hello All,
our residential building is approximately 1100 feet from a grid power source, which is a single phase pad, mount transformer of 7620 V primary.
So far the most economical hook up to the grid is for the power company to connect to a meter/disconnect switch (120/240) adjacent to their pad mount transformer. Then we install our own single phase pad mount transformer (#1) and step it back up to 7620v or 4800v run 1100ft single conductor 1/0 AWG URD 15kv cable to another single phase pad mount transformer (#2) adjacent to our residential building then step back down to 120/240v .
my first question is on the the transformers can #1 and #2 be different KVA ratings as long as primary and secondary Voltage and all other parameters are the same?
thanks for the help

 

[IntheWoods]

When I say the ” most economical” that may not be entirely correct as the cost of this is still a moving target. Transformer lead times and prices are scary.

 

[Elect117]

What is the size of the existing utility pad mounted transformer and what size are you trying to install? What is the panel size? and is this 120/240V 1PH or 3PH?

You will have engineering concerns if the customer transformer is nominally larger due to inrush current.

 

[LarryFine]

So far the most economical hook up to the grid is for the power company to connect to a meter/disconnect switch (120/240) adjacent to their pad mount transformer. Then we install our own single phase pad mount transformer (#1) and step it back up to 7620v or 4800v run 1100ft single conductor 1/0 AWG URD 15kv cable to another single phase pad mount transformer (#2) adjacent to our residential building then step back down to 120/240v .

That's nuts! You should try to get the primary run to transformer #2 and omit transformer #1 entirely.

You're already planning on running the HV line. Why pay to continuously heat up transformer #1?

You didn't mention the load current, but I think the step down just for the metering is just plain silly.

 

[jim dungar]

Yes the transformers can be any size you want as long as they can handle the load.
Large feeding small
Large feeding large
Small feeding large small feeding small

 

[electrofelon]

IMO 1100 feet is far too short to do a step up step down, unless maybe you have a really big energy hog home. For example parallel AL 350's gets you under 3% drop @ 80amps. 350 URD is about $4.25/foot and (coincidentally) I just got a quote for 15kv #2 CN and it's $3.68. do the math and that gives you a transformer budget of around 5k for two, good luck with that. Also with transformers dont forget you will be tied to the wonderful losses and poor voltage regulation forever

 

[electrofelon]

Oh as to the specific question, no problem with two different sizes for a step up step down and I actually did just that several years ago. System was 7.2KV MGN and I had one 25KVA padmount and the other was a 15KVA. The reason for the different sizes was I got these on the surplus/used market and its what was available in DOE 2016 units. System works great. This was twice your distance.

 

[electrofelon]

A few more things I wanted to mention: I admit that in the past I have told people to not mess around with 480 or 600 volt step up step down schemes and have been adamant that if you're going to mess with transformers you might as well go up to MV. However due to the doubling of the cost of MV cable and the shortage and increased cost of padmount transformers in the past 4 years, there is a sweet spot where a 600V transmission may be the most economical. #2 CN 15KV cable was $1.85 3-4 years ago, and you could pick up used/surplus DOE 2016 padmounts readily and quickly. Although I am still adamant that in your case its not worth a step up step down period, but just to look at some numbers, to compare to the MV option: again say a 240 load of 80 amps, with 600v transformers you would be at 1/0 AL conductors, which looks like is .52/ft right now so $1.04/foot for you wire, and 25KV 120/240-600 transformers are readily available and I see them online for $2300 each delivered. Thats pretty cheap.

 

[Jpflex]

Hello All,
our residential building is approximately 1100 feet from a grid power source, which is a single phase pad, mount transformer of 7620 V primary.
So far the most economical hook up to the grid is for the power company to connect to a meter/disconnect switch (120/240) adjacent to their pad mount transformer. Then we install our own single phase pad mount transformer (#1) and step it back up to 7620v or 4800v run 1100ft single conductor 1/0 AWG URD 15kv cable to another single phase pad mount transformer (#2) adjacent to our residential building then step back down to 120/240v .
my first question is on the the transformers can #1 and #2 be different KVA ratings as long as primary and secondary Voltage and all other parameters are the same?
thanks for the help

I’m not sure how a utility can hook up a meter/disconnect switch for 120/240 adjacent to their pad mount transformer when their primary is 7,620 volts what will their secondary be (not 120 or 240 v)?

As stated earlier what do you mean by step up voltage back up to 7,620 volts with another 1 transformer when this is already the boltage and the transformers are close / adjacent.

What’s the secondary voltage from utility transformer or KVA to figure out voltage drop for 1000 feet plus?

 

[electrofelon]

I’m not sure how a utility can hook up a meter/disconnect switch for 120/240 adjacent to their pad mount transformer when their primary is 7,620 volts what will their secondary be (not 120 or 240 v)?

As stated earlier what do you mean by step up voltage back up to 7,620 volts with another 1 transformer when this is already the boltage and the transformers are close / adjacent.

What’s the secondary voltage from utility transformer or KVA to figure out voltage drop for 1000 feet plus?

He means the utility would provide a standard 120/240 service and then after the meter and disconnect there would be a pair of customer owned transformers that would bump it back up to MV for the distance. Most utilities around here will only provide a 120/240 single phase service for rural residential property.

 

[IntheWoods]

I’m not sure how a utility can hook up a meter/disconnect switch for 120/240 adjacent to their pad mount transformer when their primary is 7,620 volts what will their secondary be (not 120 or 240 v)?

As stated earlier what do you mean by step up voltage back up to 7,620 volts with another 1 transformer when this is already the boltage and the transformers are close / adjacent.

What’s the secondary voltage from utility transformer or KVA to figure out voltage drop for 1000 feet plus?

As Electrofelon stated the utility company would provide 120/240 off there transformer , then following there directions we would bump up with transformer to MV (for 1100ft) then back down at residents for standard 200 amp service.
What we are trying to avoid is the having the utility company lay the cable as they will require an access road able to support their trucks along the buried cable. That would be very expensive. It will be huge difference between a moderate size excavator trenching through there and the utility company requiring a road.

 

[IntheWoods]

What is the size of the existing utility pad mounted transformer and what size are you trying to install? What is the panel size? and is this 120/240V 1PH or 3PH?

You will have engineering concerns if the customer transformer is nominally larger due to inrush current.

The panel is 200 amp
it’s 120/240 single phase
I am not sure of the KVA size of their transformer as it has no exterior label

 

[IntheWoods]

IMO 1100 feet is far too short to do a step up step down, unless maybe you have a really big energy hog home. For example parallel AL 350's gets you under 3% drop @ 80amps. 350 URD is about $4.25/foot and (coincidentally) I just got a quote for 15kv #2 CN and it's $3.68. do the math and that gives you a transformer budget of around 5k for two, good luck with that. Also with transformers dont forget you will be tied to the wonderful losses and poor voltage regulation forever

It has been suggested to us before to run parallel conductors of 350 kcmil aluminum. Would there be enough power for a 200 amp service and keep the voltage drop within parameters?

 

[LarryFine]

It has been suggested to us before to run parallel conductors of 350 kcmil aluminum. Would there be enough power for a 200 amp service and keep the voltage drop within parameters?

Depends on the actual load at any given moment.

The current at 240v will be (7620/240) 31.75 times as much as it would be at 7620v.

For reference, 200a at 240v would be only (200/31.75) 6.3a.

 

[ruxton.stanislaw]

Would the power co. provide a 480 V or even 2,400 V service on the disconnect side? That could make your transformer on the utility side more economical or not necessary.

 

[winnie]

1) Forum rules do not permit providing installation assistance to non professionals. If you are a non professional asking for guidance in selecting something that will be installed by a professional, that usually gets allowed by the moderators. If you are a non professional asking for DIY assistance, the thread will be shut down.

2) Before you have the discussion of the most economical installation, you need to define _what_ will be installed. For electrical, this means doing a load calculation. It doesn't matter if you have a 200A service. What matters is what you will realistically be powering with that service.

3) If you are in the planning stages for a new home, learning how to do a load calculation is a great way to understand the implications of different appliance choices. I encourage DIY load calculations.

Jon

 

[Knightryder12]

It has been suggested to us before to run parallel conductors of 350 kcmil aluminum. Would there be enough power for a 200 amp service and keep the voltage drop within parameters?

By my calculations, with a 200 amp service, at 120/240V single phase, the max load with 2 sets of 350 AL would be about 80 amps of load to keep it under 2% VD. If your load is more, than you will need a minimum of 2 sets of 500 AL for a 120 amp load. Anything more than 120 amps of load you would need 3 sets of wires to keep it under 2%VD.

 

[electrofelon]

It has been suggested to us before to run parallel conductors of 350 kcmil aluminum. Would there be enough power for a 200 amp service and keep the voltage drop within parameters?

absolutely. There are three big flaws people make when deciding on these step up step down schemes. First, as Kightryder said, you need to use an actual realistic current as the basis for you load calculation. It is highly unlikely actual load will ever exceed 60-80 amps for a house - it doesnt matter if its a 200 amp service. Even f you go with parallel 500's, you are still at only a 9k premium over the MV Cable which you could think of as your transformer budget.

The second thing is people ignore the voltage drop of the transformers, so they end up doing a completely unfair comparison. I have a step up step down at my house and trust me, it is not a very stiff supply despite the MV run having essentially no loss.

The third issue is losses. Not even considering the load losses, the no load losses of two 25 KV transformers will be about 200 watts with DOE 2016 units. If you get some older recon units from the 90's, they can easily be 2.5 times that. But even the DOE units are $260/yr at .15/KWH.

DONT DO IT

 

[IntheWoods]

1) Forum rules do not permit providing installation assistance to non professionals. If you are a non professional asking for guidance in selecting something that will be installed by a professional, that usually gets allowed by the moderators. If you are a non professional asking for DIY assistance, the thread will be shut down.

2) Before you have the discussion of the most economical installation, you need to define _what_ will be installed. For electrical, this means doing a load calculation. It doesn't matter if you have a 200A service. What matters is what you will realistically be powering with that service.

3) If you are in the planning stages for a new home, learning how to do a load calculation is a great way to understand the implications of different appliance choices. I encourage DIY load calculations.

Jon

Ok
thank you for the info