Xformer size for step up step down to dwelling

I live in a rural area and due to high utility installation costs to extend primary, I frequently run into situations where taking service at 240 and stepping up to MV and then back down is the only option (typically this becomes cost-effective around 1500 ft). 240 is the only service voltage available to a dwelling, they will not provide MV and they will not provide 480 or 600 or anything else just in case anybody asks.

So I do these occasionally, and I'm always a bit torn on the transformer size. 25kva seems to be sort of the standard, just because the one other guy in the area that does this uses that size, and that is the size the utility provides service with for a typical residential project.

The big concern in my mind is no load losses, since the customer is of course paying for them, so there is an incentive to keep the transformers as small as possible. 2010DOE pad mounts, based on measurements I have taken, have a no load loss of around .25%, DOE2016 a little less. So this is about $85 per year for two 15KVA and $145 per year for two 25KVA.

Capital cost of course comes into play with a 15 KVA being theoretically cheaper, but these things are usually acquired on eBay or the surplus/refurbished market so it's a crap shoot as to availability, so let's just ignore the capital cost.

On the other hand, it seems there is better power quality with larger transformers, less voltage sag under load. For example a 25 KVA at full load will have more voltage drop than a 50 KVA at half load. So this is the part I struggle with: weighing The increased losses versus The increased voltage stability and voltage regulation, which is not insignificant now that you have three total transformers basically in series before it gets to the house.

So what size would you choose? What about if it was your house? What about if it for was for a client would you have a different answer? And to preempt what seems to be the glaring missing piece of information, what the load is.......this is in the Northeast, where there isn't really much air conditioning maybe a window unit or a small mini split. It's cold enough that nobody really uses significant electric heat other than maybe 2,3, or 4 KW total, electric dryer, electric range maybe, nothing like on demand electric water heaters, just a 4500 watt tank, maybe a hobby workshop with a 3 horsepower table saw tops. Basically your kind of typical place where my rule of thumb would be the load will only ever get to about 60 amps and even that would only be quite brief. Don't consider NEC load calcs. Okay have at it.......
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Seems simple to me:

You must size everything to supply the peak load. If you're sure the peak will be 60a, then use a 100a service and 25Kva units.

If you're installing a 200a service (why 200a ?), wouldn't you be required to provide for that throughout the system at 50Kva?
 

Xptpcrewx

Senior Member
Location
Las Vegas, Nevada, USA
Occupation
Power System Engineer
I live in a rural area and due to high utility installation costs to extend primary, I frequently run into situations where taking service at 240 and stepping up to MV and then back down is the only option (typically this becomes cost-effective around 1500 ft). 240 is the only service voltage available to a dwelling, they will not provide MV and they will not provide 480 or 600 or anything else just in case anybody asks.

So I do these occasionally, and I'm always a bit torn on the transformer size. 25kva seems to be sort of the standard, just because the one other guy in the area that does this uses that size, and that is the size the utility provides service with for a typical residential project.

The big concern in my mind is no load losses, since the customer is of course paying for them, so there is an incentive to keep the transformers as small as possible. 2010DOE pad mounts, based on measurements I have taken, have a no load loss of around .25%, DOE2016 a little less. So this is about $85 per year for two 15KVA and $145 per year for two 25KVA.

Capital cost of course comes into play with a 15 KVA being theoretically cheaper, but these things are usually acquired on eBay or the surplus/refurbished market so it's a crap shoot as to availability, so let's just ignore the capital cost.

On the other hand, it seems there is better power quality with larger transformers, less voltage sag under load. For example a 25 KVA at full load will have more voltage drop than a 50 KVA at half load. So this is the part I struggle with: weighing The increased losses versus The increased voltage stability and voltage regulation, which is not insignificant now that you have three total transformers basically in series before it gets to the house.

So what size would you choose? What about if it was your house? What about if it for was for a client would you have a different answer? And to preempt what seems to be the glaring missing piece of information, what the load is.......this is in the Northeast, where there isn't really much air conditioning maybe a window unit or a small mini split. It's cold enough that nobody really uses significant electric heat other than maybe 2,3, or 4 KW total, electric dryer, electric range maybe, nothing like on demand electric water heaters, just a 4500 watt tank, maybe a hobby workshop with a 3 horsepower table saw tops. Basically your kind of typical place where my rule of thumb would be the load will only ever get to about 60 amps and even that would only be quite brief. Don't consider NEC load calcs. Okay have at it.......
Sounds like you are asking us to solve a design problem on a message board with little to no information. Without running numbers and doing an evaluation between alternatives, I don’t think you will get anything useful other than personal opinions.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
How would you handle the MV disconnect and circuit protection issues? Seems to me that would be expensive and huge...

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.
 
How would you handle the MV disconnect and circuit protection issues? Seems to me that would be expensive and huge...

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.
There is no MV equipment. The transformerw have Integral bayonet fusing

Yes to be blunt these are done "utility style".

A 25 KVA for a single dwelling as I described, just seems like Overkill. Why does my utility provide these as standard? Is my power quality / voltage regulation theory sound?
 

Xptpcrewx

Senior Member
Location
Las Vegas, Nevada, USA
Occupation
Power System Engineer
I should have been

NEC does not apply. Personal opinions are great.
Not sure why the NEC wouldn’t apply as this is going to drive certain requirements that could make or break your installation as well as limit your possible alternatives. Personally, MV in residential applications is a no go for me and seems like a liability due to the fact that residential contractors have an electrical license within the scope of low voltage applications only. Don’t quote me on this but I feel there may be building code restrictions at the local jurisdiction level for allowable system voltage.

You need to fully flesh out your alternatives from a technical standpoint and then do a cost analysis between your practical alternatives.
 
Not sure why the NEC wouldn’t apply as this is going to drive certain requirements that could make or break your installation as well as limit your possible alternatives. Personally, MV in residential applications is a no go for me and seems like a liability due to the fact that residential contractors have an electrical license within the scope of low voltage applications only. Don’t quote me on this but I feel there may be building code restrictions at the local jurisdiction level for allowable system voltage.

You need to fully flesh out your alternatives from a technical standpoint and then do a cost analysis between your practical alternatives.
Things are a lot different here in upstate New York then you may be used to.

NYSEG lets customers (you don't even have to be an electrician - there isn't licensing in most areas here) install customer-owned 15kv primary and they will hook it up. This is a customer-owned line. But my question involves another utilities territory where the customer cannot run the primary, and POCO charges big bux for them to do it hence the step up step down scheme.
 
Anyway, guys if we can just focus on the original question and how you would approach sizing. I suspect if doing this for a customer you would tend to oversize, just like how most heating systems are oversized, contractors want to cover their butts.....maybe for yourself you would go smaller to cut down the no load losses?
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
I would think your voltage regulation issue might loom larger in the long run that the operating costs. He is going to call you back out on voltage drop issues, not because of the size of his power bill... it also covers the possible load increases, i.e. the coming days of electric cars for everyone.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
There is this thing called time value of money that you need to look at. If you spend an extra $5,000 to save $100 a year it's probably not worth it. If you spend an extra $100 a year and save $5,000 upfront it probably is worth it. You have to do these calculations. Usually if you figure somewhere between 7 and 10-year payback, that's probably a pretty good bet where the break even is.

I don't know that the code allows you to have medium voltage equipment in a residence. But for the sake of argument let's say it does. You are going to need some kind of disconnecting means for the two medium voltage transformers. Then you will need the medium voltage transformers. And the medium voltage cable that runs between them. None of this stuff is inexpensive compared to low voltage equipment.

Chances are unless you do a fair amount of medium voltage work you will not get a very good deal on the equipment because you won't know who to buy it from and they are going to know you are a relative amateur at such things and you will pay a lot more than you need to. That's almost a given. You might be better off talking to whoever the contractor is for the local utility that puts in the services for them and seeing if they'd be willing to do it for you. They almost certainly have access to parts at much better pricing than you'll ever see.

1500 ft is a long way to run a trench. It would probably be a lot more cost effective to run poles.

I think you just have to price up your various options and see what it comes out to.

I know for some reason you want to make this a medium voltage system but I'd be willing to bet if you got a couple of 600 to 240 volt transformers They would be a lot cheaper than the same va rating of two medium voltage transformers. You would need larger wire to run between the two transformers but it would be common cabling instead of medium voltage cabling which is generally considerably less money on the whole even if it's a little bit bigger.
 
I would think your voltage regulation issue might loom larger in the long run that the operating costs. He is going to call you back out on voltage drop issues, not because of the size of his power bill... it also covers the possible load increases, i.e. the coming days of electric cars for everyone.
Thank you. Great reply. the possible future electric car thing as a good point, then you are getting into say a 50 amp continuous load, which is still well within a 15 KVA, but possibly if you're charging in the evening when you're showering in cooking might get up there a little bit. On the other hand this is a very rural area, electric cars are probably a bit further off than cities and suburban areas would be.
 
I don't know that the code allows you to have medium voltage equipment in a residence. But for the sake of argument let's say it does. You are going to need some kind of disconnecting means for the two medium voltage transformers. Then you will need the medium voltage transformers. And the medium voltage cable that runs between them. None of this stuff is inexpensive compared to low voltage equipment.

Chances are unless you do a fair amount of medium voltage work you will not get a very good deal on the equipment because you won't know who to buy it from and they are going to know you are a relative amateur at such things and you will pay a lot more than you need to. That's almost a given. You might be better off talking to whoever the contractor is for the local utility that puts in the services for them and seeing if they'd be willing to do it for you. They almost certainly have access to parts at much better pricing than you'll ever see.

1500 ft is a long way to run a trench. It would probably be a lot more cost effective to run poles.

I think you just have to price up your various options and see what it comes out to.

I know for some reason you want to make this a medium voltage system but I'd be willing to bet if you got a couple of 600 to 240 volt transformers They would be a lot cheaper than the same va rating of two medium voltage transformers. You would need larger wire to run between the two transformers but it would be common cabling instead of medium voltage cabling which is generally considerably less money on the whole even if it's a little bit bigger.

Nope, the MV is definitely the way to go. Right now I am doing an 1800' step up step down, underground, 7200V. #2 AL Concentric neutral primary is a commodity item and even stocked at some of the supply houses here. You set it up as an MGN and you only need one cable at $2.50/foot (was $1.90 pre pandemic prices). Pad transformers are readily available (just got a quote for 2016 DOE surplus units, $1600, 15kva or 25 kva) and they have integral fusing on the primary sides so no other MV equipment is needed other than a few load break elbows. Theres nothing to it. All you need is a BG crimper for the load breaks. BTW these are all padmount transformers outside.

I cant imagine poles are cheaper due to how specialized it is. Any redneck with a mini ex can dig a trench. Plus most people dont want to see the poles. The other guy in the area that does these step up step downs does have pole setting equipment, Ill ask him for some ballpark numnbers for pole runs next time I see him at the supply house.
 
Anyone want to take a stab at the voltage regulation difference for two 15KVA vs two 25KVA? I know we would need to know PF and X/R ratio. Because this is residential, I think it is fair to say near unity PF since any big loads are going to be resistive.....perhaps the case of starting say a 3 or 5 HP motor would be different and worth considering? I have never seen X/R for small padmounts. For load we could say a worse case of 62 amps (FLA for 15 KVA) is reasonable in most situations.

An online calculator (guessing at X/R) shows .89% VD for each transformer (1.8% total) with .95PF, for a 15KVA with 2%Z at 100% loading. Now running the same curent on a 25 KVA which is 60% loading and the other parameters the same gives .53% VD each or 1.06% total. So If am am doing this correctly with reasonable assumptions, a difference of a few volts. fairly insignificant IMO.
 

Xptpcrewx

Senior Member
Location
Las Vegas, Nevada, USA
Occupation
Power System Engineer
Anyone want to take a stab at the voltage regulation difference for two 15KVA vs two 25KVA? I know we would need to know PF and X/R ratio. Because this is residential, I think it is fair to say near unity PF since any big loads are going to be resistive.....perhaps the case of starting say a 3 or 5 HP motor would be different and worth considering? I have never seen X/R for small padmounts. For load we could say a worse case of 62 amps (FLA for 15 KVA) is reasonable in most situations.

An online calculator (guessing at X/R) shows .89% VD for each transformer (1.8% total) with .95PF, for a 15KVA with 2%Z at 100% loading. Now running the same curent on a 25 KVA which is 60% loading and the other parameters the same gives .53% VD each or 1.06% total. So If am am doing this correctly with reasonable assumptions, a difference of a few volts. fairly insignificant IMO.
If you want to be safe then assume a load power factor equal to the cable power factor. Also, you don’t want to guess with the X/R or %Z for the transformer. Temperature correct all resistance values. Also, given this a remote service, assume the lowest service voltage possible. Don’t focus on %VD. Calculate the voltage at the receiving end of the line.
 
If you want to be safe then assume a load power factor equal to the cable power factor. Also, you don’t want to guess with the X/R or %Z for the transformer. Temperature correct all resistance values. Also, given this a remote service, assume the lowest service voltage possible. Don’t focus on %VD. Calculate the voltage at the receiving end of the line.
What would you think the PF would be at the first transformer vs the second transformer with a 1800' CN run?
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
Anyway, guys if we can just focus on the original question and how you would approach sizing. I suspect if doing this for a customer you would tend to oversize, just like how most heating systems are oversized, contractors want to cover their butts.....maybe for yourself you would go smaller to cut down the no load losses?
I think your numbers are off.
No load losses on a 25kVA PMT are around 60 watts.
Most specs say maximum no load losses are 80 watts
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
My biggest objection was a disconnecting means that an average ho could operate. It just occurred to me that it is 240 volts coming off the pole so a normal fused switch or cb is adequate.

I probably would not go for mv transformers of unknown provenance, but as long as a customer understands what he is getting, I am not too offended.

Personally I would run larger wire than needed so down the road I could upsize the transformers if needed.
 
My biggest objection was a disconnecting means that an average ho could operate. It just occurred to me that it is 240 volts coming off the pole so a normal fused switch or cb is adequate.

I probably would not go for mv transformers of unknown provenance, but as long as a customer understands what he is getting, I am not too offended.

Personally I would run larger wire than needed so down the road I could upsize the transformers if needed.
The company I got the quote of $1600 from says the unit "is new surplus but will have to go thru our reman process" whatever that means.

The CN cable is #2 AL and is the smallest size available and has lots of headroom at 7200V so im covered there.
 
I think your numbers are off.
No load losses on a 25kVA PMT are around 60 watts.
Most specs say maximum no load losses are 80 watts
That sounds about right. I have two 15KVA DOE2010 units here that come in at about 40 watts. DOE2016 is only about a .3% increased requirement in efficiency, but that is load+no load so not sure how much just no load typically goes down. I have not had the opportunity to measure one. Losses increase a lot with age. I have two other units that dont have a date but I would guess early 90's or late 80's and those are 120 watts!
 
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