Sizing a transformer

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Ukiah ca
I'm trying to figure out how to calculate what size transformers I need and the size of the underground cable to be installed. Here's the scenario.
I have a customer installed pole. 120/240 with a 200 amp meter main. I need to install a step up transformer at one end of the run and a step down at the other end of a 600' run to feed the customers distribution panel to eliminate any voltage drop. How do I calculate the size of the transformer and the wire size I need for the job. It's been awhile since I've done this and I can't remember how to calculate the required equipment. Doh.


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augie47

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It all falls back to kw (kva). You need to decide if you want the transformers to be capable of the full 200 amp or simply supply the connected load.
From a practical standpoint 240v @ 200 amps would be a 48 kva load.
Taking standard size transformers into account, IMO, under normal load conditions a 37.5 kva should be more than adequate (emphasizing IMO) but "eliminating voltage drop" (from your OP) is not going to happen. Reducing voltage drop to an acceptable level is very possible.
 
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Ukiah ca
Thanks a lot that's what I figured out to with the 48 kva. So the 37.5 kva would work because the house won't be using 200 amps continuous. Now can you tell me how to pick the size of the wire.


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kingpb

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If you need the full 200A, then you need a 50KVA transformer. But, stepping up and stepping down only reduces the cable size required. However, you can utilize the transformer taps to improve the voltage, especially since you would have taps on both ends; could actually make it slightly above 240V.

Cable depends on what your stepping up too. 480?
 
Location
Ukiah ca
I'm stepping it up to probably 37.5 kva like suggested above to get it the 600 feet then back down to 240/120 I'm just want to know how to calculate the high voltage section with underground high voltage cable and how many are required. I'm just double checking. I'm having conflicting answers from another contractor and the supply house. He wants to believe the supply house. So I want to be sure I get the right answer. So it's 240/120 stepped up to let's just say 48 kva 600' then back down to 240/120. I just need to know the cable size for the high voltage at 600 feet at 48kva


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augie47

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You are mixing kv and kva it seems.
Once you go above 600v you open a whole different can of worms.
Stepping up from 240 to 600 to use a smaller conductor is to me a viable option.
Going above 600v introduces many requirements that I would think would be cost prohibitive for a 200 MP load at 600 ft.
 

GoldDigger

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I'm stepping it up to probably 37.5 kva like suggested above to get it the 600 feet then back down to 240/120 I'm just want to know how to calculate the high voltage section with underground high voltage cable and how many are required. I'm just double checking. I'm having conflicting answers from another contractor and the supply house. He wants to believe the supply house. So I want to be sure I get the right answer. So it's 240/120 stepped up to let's just say 48 kva 600' then back down to 240/120. I just need to know the cable size for the high voltage at 600 feet at 48kva


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Your answer looks a bit confusing.
37.5 kva is a power handling capacity of the transformer, not a voltage, so "stepping it up to probably 37.5kva" does not make much sense. What voltage do you want to step it up to? Keeping in mind that anything above 600V will require a lot more complicated (read expensive) wiring and additional safety concerns.

If you have a particular load current at 240V, then you will have half that current on a 480V feeder between two transformers. Size the wire by the calculated load current and then make it bigger if you need to for voltage drop. Remember that using transformer taps can increase the voltage across the board but cannot help you with the voltage swing between minimum and maximum load.
 
Location
Ukiah ca
I appreciate the help sorry for being so confusing. That's why I'm here guys trying to make sense of this too. I haven't worked on transformers that much, not like in this type of application. So the contractor I'm working for had me put together 3 bids 1 using copper, 2 al., 3 two transformers. Ok the copper and aluminum I can oversize for the voltage drop sure. But what would the transformers be for. Can someone explain it to me. How could it be beneficial to boost to high voltage then back down. He wants it to be like pge lines 10kv or so for 600' to try to save money. I have never done something like this. I can install anything but I need to have a better understanding. Can anyone explain what he is trying to do. Cause he can't explain it either.


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GoldDigger

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I appreciate the help sorry for being so confusing. That's why I'm here guys trying to make sense of this too. I haven't worked on transformers that much, not like in this type of application. So the contractor I'm working for had me put together 3 bids 1 using copper, 2 al., 3 two transformers. Ok the copper and aluminum I can oversize for the voltage drop sure. But what would the transformers be for. Can someone explain it to me. How could it be beneficial to boost to high voltage then back down. He wants it to be like pge lines 10kv or so for 600' to try to save money. I have never done something like this. I can install anything but I need to have a better understanding. Can anyone explain what he is trying to do. Cause he can't explain it either.


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Unless you have both knowledge and experience dealing with MV/HV (over either 600 or 1000V depending on your code cycle) do not even consider bidding a 10kV option. JMO.
Two transformers with 480, 640, or even 1000V (if you are on 2014NEC already) in between is still a possibly cost effective option.
 

ActionDave

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I appreciate the help sorry for being so confusing. That's why I'm here guys trying to make sense of this too. I haven't worked on transformers that much, not like in this type of application. So the contractor I'm working for had me put together 3 bids 1 using copper, 2 al., 3 two transformers. Ok the copper and aluminum I can oversize for the voltage drop sure.
What is the application? What is the load calc? Even assuming you need all 200A and not just a 200A circuit six hundred feet at 240V is doable without any heroics.

But what would the transformers be for. Can someone explain it to me. How could it be beneficial to boost to high voltage then back down.
Less voltage drop with smaller conductors. The reality of saving money after you purchase transformers and associated disconnects may not come to pass.

He wants it to be like pge lines 10kv or so for 600' to try to save money. I have never done something like this. I can install anything but I need to have a better understanding. Can anyone explain what he is trying to do. Cause he can't explain it either.
Sounds like he is misinformed or talking in metaphors. If he is serious then he is whacko.
 
Location
Ukiah ca
I say throw copper in conduit drop it in the trench and be done with it. He's definitely over thinking a six hundred foot run for sure.


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winnie

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Springfield, MA, USA
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Electric motor research
I would suggest, in addition to the 3 quotes that you are providing, that you contact (or suggest to the GC to contact) about extending distribution voltage underground to a pad mount transformer near the home. In many areas the customer provides things like trenching, conduit, a pad for the transformer, etc. These are all built to the POCO specs.

The benefit is that the 600 foot run is all done at high voltage, greatly reducing voltage drop. Also any continuing energy lost in the 600 foot run is _before_ the meter, rather than having the customer pay for it. The POCO is in charge of the high voltage portion of things, so you don't have to worry about installation issues that you are not familiar with.

Some other points:

The reason to even consider the transformer arrangement is that it permits the use of smaller wire to get an acceptable voltage drop. This is the exact reason that the power company distributes at higher voltage and then uses transformers to step down to the usage voltage near the loads.

Consider delivering 24KW at 240V and 100A to a load. If you had some way to operate at 480V, then you would only need 50A to deliver the same total power. Half the current means half the voltage drop in a given conductor, but it gets better: the voltage drop in volts is halved, but in % it is quartered.

In your case, with your 240V supply and minimum sized service conductors over 600 feet, you are looking at perhaps a 10% voltage drop if the full 200A is used. If you could somehow use 480V to deliver the same power, then with the same conductors you would only have a 2.5% voltage drop. The higher the voltage, the better things look in this simple view.

Unfortunately you have a few complications.

1) What current should be used for the voltage drop calculations? A residence with a 200A panel just doesn't draw 200A on a regular basis. But if there are some large motors (well pump, hvac, etc) they could draw much more than 200A for very short periods of time.

2) If you use two transformers for step up and step down, in addition to the voltage drop in the wires you will have voltage drop in the transformers themselves.

3) If you use the transformers then they are running 24x7 consuming power because transformers always have magnetizing losses. Even if the transformer approach is the least expensive up front, it will cost in terms of long term power consumption.

4) As others have noted, you are limited in the voltage that you can reasonably use.

-Jon
 

Knightryder12

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Clearwater, FL - USA
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Sr. Electrical Designer/Project Manager
I'm starting to realize this after two weeks of working for the guy ActionDave.


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After doing a quick VD calc and utilizing the full 200a load, here are the wire sizes I came up with:

2 sets of 350mcm al = 2.53% VD
1 set of 500mcm al = 3.5% VD
1 set of 350mcm cu = 3.1% VD
1 set of 500mcm cu = 2.16% VD

All to me are acceptable VD allowances.
 
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