Xformer size for step up step down to dwelling

Hv&Lv

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Engineer/Technician
I should have been


Personal opinion...
I think it’s a shame the POCO won’t allow you to run the MV cable to the top of the pole.
I understand their convoluted thinking though...
Something goes wrong below their transformer, it’s on you to troubleshoot and fix and nothing is higher than 600 volts...
 
I think it’s a shame the POCO won’t allow you to run the MV cable to the top of the pole.
I understand their convoluted thinking though...
Something goes wrong below their transformer, it’s on you to troubleshoot and fix and nothing is higher than 600 volts...
As I said earlier in the thread, the other POCO does allow it. It's a customer owned underground primary that the customer installs, customer provides and installs transformer pad and runs primary 10 feet up the pole, POCO provides transformer and terminates each end. Seems like a win win for everyone. It's really too bad national grid doesn't allow the same thing.
 

topgone

Senior Member
As I said earlier in the thread, the other POCO does allow it. It's a customer owned underground primary that the customer installs, customer provides and installs transformer pad and runs primary 10 feet up the pole, POCO provides transformer and terminates each end. Seems like a win win for everyone. It's really too bad national grid doesn't allow the same thing.
Correct! Whatever you do on your side is none of the POCO's business. Their involvement ends at the metering point.
 

GoldDigger

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Retired PV System Designer
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Also; utilities can "get away with" over stressing their transformers a LOT more than we can because THEY own it and have to maintain / replace it, plus NEC rules don't apply to them. So I'd be careful about assuming that because they will use a 15kVA transformer for a 100A service that you can get away with that.
Also, when a transformer fails they can just grab a replacement from their yard and install it immediately. With a truck with a crane to place it.
 
I think I'm going with a 15 KVA. Based on my quick-and-dirty calcs, it seems like it's only a couple volts difference of voltage drop at what would likely be the full loading for this place. Also, of the two best choices I have available right now, the 15kva is a DOE 2016 while the 25kva is a DOE 2010.

FWIW, I have a step up step down at my house with 15KVAs. I have a large PV system, 16.5KW DC or about 60 amps AC. It works fine. in my case due to the high continuous loading, if I could do it over I probably would have gone with 25's, but it does seem fine.
 

Hv&Lv

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Load factor is a large part of our transformer sizing.
It’s calculated monthly and on a spreadsheet.
Everything’s easier with a spreadsheet...

I get a monthly transformer overload report for all transformers.
I only care about the red ones. ADABE1B4-D4F8-433D-95D7-E2F514CE1406.jpg

the really high ones are incorrect transformer sizes.
Got to fix that...
 
Last edited:
Load factor is a large part of our transformer sizing.
It’s calculated monthly and on a spreadsheet.
Everything’s easier with a spreadsheet...

I get a monthly transformer overload report for all transformers.
I only care about the red ones. View attachment 2556858

the really high ones are incorrect transformer sizes.
Got to fix that...

Yeah this being a dwelling, I am not at all worried about overloading/damaging/ shortening the life of the transformer by "under sizing it". Even a 15 KVA is 62 amps, not going to happen for longer than a couple minutes for a million years at this place. the whole point of this thread is just about the power quality / voltage regulation side of things and if that is worth considering, even at the expense of higher no load losses.

How does your company size residential transformers? Do they just provide a default 25kva unless it's something unusual? Some utilities will default to even a 10 KVA for a dwelling.
 

Hv&Lv

Senior Member
Location
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Occupation
Engineer/Technician
Yeah this being a dwelling, I am not at all worried about overloading/damaging/ shortening the life of the transformer by "under sizing it". Even a 15 KVA is 62 amps, not going to happen for longer than a couple minutes for a million years at this place. the whole point of this thread is just about the power quality / voltage regulation side of things and if that is worth considering, even at the expense of higher no load losses.

How does your company size residential transformers? Do they just provide a default 25kva unless it's something unusual? Some utilities will default to even a 10 KVA for a dwelling.
Generally a 15.
Big houses and pad mounts are 25
 

MD Automation

Senior Member
Location
Maryland
Occupation
Engineer
Generally a 15.
Big houses and pad mounts are 25
Last May, I started one of my first threads here when my (25 kVA) transformer decided to kick the bucket. It started an interesting discussion about sizing, this particular pot serves only my house - at the time the service was established in 1985 the house was a small (~900 SF) modular home. So for the next few weeks I naturally looked up a lot at pole mounted transformers to eyeball sizes. I did see a few 15s, but not many. It seems that my local POCO (BGE) puts in a lot of 25s, even for single home use. I assume it varies with utilities across the country.

I would think my house would work just fine powered with a 15. This is the new shiny 25. I don't pay to magnetize it, so I guess I don't care.

1623521951554.png
 

Hv&Lv

Senior Member
Location
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Occupation
Engineer/Technician
Last May, I started one of my first threads here when my (25 kVA) transformer decided to kick the bucket. It started an interesting discussion about sizing, this particular pot serves only my house - at the time the service was established in 1985 the house was a small (~900 SF) modular home. So for the next few weeks I naturally looked up a lot at pole mounted transformers to eyeball sizes. I did see a few 15s, but not many. It seems that my local POCO (BGE) puts in a lot of 25s, even for single home use. I assume it varies with utilities across the country.

I would think my house would work just fine powered with a 15. This is the new shiny 25. I don't pay to magnetize it, so I guess I don't care.

View attachment 2556859
Actually you do.
System losses are paid by the customer in rates and service charges.
 
My monthly service charge is $17. I figure probably a third of that is no load loss (at retail rate anyway).

Then I have 300 watts of customer-owned transformers (3 total, but two are older energy hogs) that I pay for, good thing I have a big PV system with covers that
 

Hv&Lv

Senior Member
Location
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Occupation
Engineer/Technician
My monthly service charge is $17. I figure probably a third of that is no load loss (at retail rate anyway).

Then I have 300 watts of customer-owned transformers (3 total, but two are older energy hogs) that I pay for, good thing I have a big PV system with covers that
That $17 is supposed to be for plant expenses.
It never covers all of them, they are artificially kept low to make the customer feel good. The true numbers for service charges should be around $50-$65.
the way it is now the service charge is low and the difference is made up in rates for kWH sales
We raised ours to $26 and kept the rates from raising too dramatically
 
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