Transformers for Voltage Drop

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dereckbc

dereckbc

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Hey guys I know some of you have done this and can help me out here.

I have a friend with a cabin (man-cave) he built about 1500 feet from his house. POCO wants $20K to install service. He asked me to design a off-grid solar system, but quickly found out the POCO was a good deal when I gave him the price.

Ok so here is the question what transformer voltage and models do you use to step up the voltage and back down? Looks like all he needs 25 to 30 amps at 240/120.
 
For only 30 amps of load I think it would be more economical to skip the transformers and run larger conductors.
 
Well curt you might be right, but 1500 feet is quite a distance so I am not too sure if it is less expensive to run over sized feeders, or install transformers and smaller conductors. I do industrial stuff so this one is a bit out of my area.
 
it sounds more like equipment likely to be found in industrial work than in your average cabin to me. Still think larger conductors may be best way to go, if you step voltage up beyond 600 volt conductors and terminations probably get expensive really fast
 
dereckbc said:
Ok so here is the question what transformer voltage and models do you use to step up the voltage and back down? Looks like all he needs 25 to 30 amps at 240/120.
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Why not see if you can get the POCO to set, say, a 480v transformer (and meter, I guess?) instead of a 120/240v one, since you won't need a neutral anyway, and you'll only need to supply a 480-120/240v one. You'll likely need a 100a service at the house, by the way.
 
LarryFine said:
Why not see if you can get the POCO to set, say, a 480v transformer (and meter, I guess?) instead of a 120/240v one, since you won't need a neutral anyway, and you'll only need to supply a 480-120/240v one. You'll likely need a 100a service at the house, by the way.
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Assuming or a 240 to 120/240 transformer at the cabin 3/0 will give you a 2.5% drop with a 30 amp load at that distance. If you size the cable based on 30 amps at 120, you would need 350kcmil for a 2.5% drop.
If you use a 480 to 240 transformers, you could use a #4 for the same voltage drop.

I am not sure you can buy the transformers for the difference in the wire cost. If this is a job that he will pay some one to do, I expect that the larger wire would be cheaper as a lot of the cost would be the 1500' trench.
 
You might consider running 2 separate feeders, which is arguably permitted for different characteristics. Put things that can tolerate voltage drop on one feeder (heaters, for example) and put stuff that should have tighter voltage drop on the other.

-Jon
 
don_resqcapt19 said:
I am not sure you can buy the transformers for the difference in the wire cost.
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I am not sure either, that is why I am asking here. I have never done this before.
don_resqcapt19 said:
If this is a job that he will pay some one to do, I expect that the larger wire would be cheaper as a lot of the cost would be the 1500' trench.
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He will be hiring someone, I am just trying to find the least expensive route. Beginning to think the POCO $20K route might be the best deal.
 
winnie said:
You might consider running 2 separate feeders, which is arguably permitted for different characteristics. Put things that can tolerate voltage drop on one feeder (heaters, for example) and put stuff that should have tighter voltage drop on the other.
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Largest load is a 1-ton window shaker unit. The rest is just lights, dorm size fridge, TV, and a coffee pot.

Like I said earlier we ran the numbers for solar off-grid, but the AC unit really sent price to the stars. We also looked at 24 VDC AC and fridges made for off-grid solar, but the steep price of these units did not offset the savings in solar PV modules and batteries. Affraid he is just going to have to bite the bullet and either pay the POCO or the local EC big bucks.
 
Would a buck and boost transformer work

Buck-Boost Transformers

Buck-Boost transformers are small, single phase, dry type distribution transformers designed and shipped as insulating/isolating transformers. They have a dual voltage primary and a dual voltage secondary. These transformers can be connected for a wide range of voltage combinations. The most common use is to buck (lower) or boost (raise) the supply voltage a small amount, usually 5 to 27%. Buck-Boost transformers are in compliance with NEC Article 210-9, Exception 1 when field connected as an autotransformer.

The major advantages of buck-boost transformers are their low cost, compact size and light weight. They are also more efficient and cost less than equivalent isolation transformers. When connected as an autotransformer, they can handle loads up to 20 times the nameplate rating. A buck-boost transformer is the ideal solution for changing line voltage by small amounts.

Buck-Boost Transformers
 
Hmm. I'd look more closely at the load side of the equation first.

For example, there is a vast range of performance difference in window air conditioners, and you might be able to go with a more expensive AC that is more efficient. The big voltage drop issue is the drop when the AC motor kicks in; you might be able to use an inverter based AC that runs continuously. You might be able to get an AC that runs at 240V, which effectively reduces the voltage drop issue for that component.

If you use a 240V AC and a 240-120V single phase transformer for the 120V loads, then all your load would be 240V, and you might be able to get away with a 4 or 6 ga feeder.

-Jon
 
dereckbc said:
...what transformer voltage and models do you use to step up the voltage and back down? Looks like all he needs 25 to 30 amps at 240/120.
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Go with 600V. Transformers will be 240X600, 7.5kva. SQD catalog shows a 7SF4 at $1100 list. You will need a pair.

Figure 5% voltage drop. Give up 2% for the house wiring and the transformers. 3% on 600V is 18 volts. 30A at 240V is 12A at 600V. 18/12 = 1.5 ohms for 3000' gives .5ohms/1000". #6 will do it for the 600V link. You won't need a secondary disconnect at the sending end, but you will likely want one at the receiving end 600V.

Material list
2 ea transformers $2200
3000' #6 DB
1 ea 2 pole 600V disconnect Nema 3R $300

Compare with upsizing conductors:

Don has already done the calc, I'll just do a check using CF rule number 5C: feet/Nominal volts = number of wire sizes to increase

1500'/200V = 7.5 sizes. So starting with #10 for 30A,
8
6
4
2
1/0
2/0
4/0
250kcmil

Pretty close to what Don said.

This is an economics problem. Call your local supplier and get prices for the conductors and equipment.

cf

ps: This may not be a DIY project. But it is DIY engineering. So keep in mind, my post may well be worth what you paid for it:)

pps: Either way, if it was me, I'd be out there with a out there with a rental ditchwitch for a couple of weekends.
 
Another Option:
I've got a remote access cabin. 1/2 hour by river boat, or 20 minutes by snow machine. As the crow flys, power is more than 2 miles away.

I put in a 5kw gen with a big muffler and gen house about 50 feet away from the main cabin. Small 24V battery bank (200AH) and a 3000W, 120V inverter The inverter and batts are enough to run the lights and music at night so I don't have to listen to even a muted gen if I don't want to.

Including the gen house, which doubles as storage, I've got about $5k in it. The batteries were surplus and that helped.

cf
 
russ said:
Would a buck and boost transformer work ...
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Not very well. Here is an example:
Let's take Jon's #4 and add a buck-boost.

Vd on a #4 for 3000 feet at 30A is .5 x 3 x 30 = 45 volts. So we set the BB or 22 volts boost.

With full load, 30A, receiving end voltage is 217V. With two light bulb on or just the TV, say 2A, the voltage drop is .5 x 3 x 2 = 3V, so receiving end voltage is 240 + 23 - 3 = 260V

Pretty poor regulation, 217V full loat to 260V light load

cf
 
CF thanks for doing the math.

If my calculations are right 3000 feet of 6 AWG will run around $1.20 per foot so 1.2 x 3000 = $3600.
Transformer = $2200
Disco = $300
Total material + 10% fudge = $6710.

Just off the top of my head 3000 feet of 250 MCM runs about $7 per foot??? If statement is true cost $21K

Sounds like transformers is the way to go but depends on trench and labor cost in which case the POCO $20K may be the best deal.
 
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Cold Fusion said:
Go with 600V. Transformers will be 240X600, 7.5kva. SQD catalog shows a 7SF4 at $1100 list. You will need a pair.
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Again, if the POCO does not already have a transformer at the source end, why not see if the one they provide can have a secondary that eliminates the need for you to provide another one there.

Why step the primary down to 240v just to step it up again? Maybe they can provide and meter 480 or 600v and eliminate one transformer for you to buy. Why does this not make sense?
 
LarryFine said:
Again, if the POCO does not already have a transformer at the source end, why not see if the one they provide can have a secondary that eliminates the need for you to provide another one there.

Why step the primary down to 240v just to step it up again? Maybe they can provide and meter 480 or 600v and eliminate one transformer for you to buy. Why does this not make sense?
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Never said it did not make sense, I just have not thought about it yet. This is a rural coop and the existing service runs along the dirt road he lives on in front of his house. There is a lateral tap line from the distribution line ran over head to a pole with his existing transformer. From looking at the transformer can appears to be about a 17 to 20 KVA for his house. To do what you suggest will require and additional transformer at 600 volts, and run approx 1600 feet to the cabin located behind his house. It is an option, but I have not looked into it yet.

That is why I am here looking for options. :) As CF stated this is a economical problem, not so much technical. At this point I am not sure it would be any less expensive to have the POCO set another transformer or not vs the customer provide his own sourced from his existing service. I have not pursued that option yet. All I know for sure the POCO quoted $20K to extend the over head lateral back to the cabin and drop a 100 amp service to it. At first the POCO option seemed high, but after waying some numbers may not be a bad deal after all. :confused:
 
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Larry, That would be my idea. If POCO is setting a transformer anyway, see if they will give you the higher voltage and send that to your step-down transformer at the cabin.
Even at 480, you're looking at a #4 (load guesstimated)
 
LarryFine said:
Ah. I didn't know if there was one already there.
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Yeah being a rural coop really complicates things. Just no way for then to ever make a profit running a small service 1600 feet with no other potential customer load. They would be lucky to see 4 or 5 Kwh per day use. Hard to justify any investment on a $1 a day or less return.
 
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