Long distance electrical service to barn

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  #1  
Old 04-27-05, 05:42 AM
beartoothranch
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Long distance electrical service to barn

I have recently built a barn 1200 feet from my house. I need to get electrical service to it. The utility company wants $7800 to run a seperate service to the barn. I would like 100 amp service out at the barn. What are the dificulties in setting a 25 kva transformer at my house, stepping the 240 up to 480 or higher, running direct bury #2 or 0-2 wire to the barn, setting another 25 kva transformer and stepping back down to 240. The transformer at the barn could have adjustable taps to dial the voltage to just about 240. The barn will be a full workshop. Thanks
 
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  #2  
Old 04-27-05, 05:57 AM
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You would need 500 KCMIL at 240v/single phase to get the voltage drop down to below 3%. Stepping up to 480 is actually a great ideal. Also, good selection of a 25 kVA xfmr. It is good for 104 amps at 240/single phase.

The 480 secondary runs at 52 amps. My information shows that to run 52 amps 1200 feet, assuming an 80% power factor, in non-magnetic conduit or direct-buried cable, would require 1/0 conductors.

Note that direct-buried cable must be at 18" below grade, and run where not normally exposed to vehicular traffic. Conduit and deeper depths are required for other specific situations. Otherwise, I think the hardest part of this project is not the electrical, but the trenching!

I can't think of any other difficulties doing this.

Hope that helps. Good luck.

Juice
 
  #3  
Old 04-27-05, 06:03 AM
beartoothranch
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Trenching is easy with a trencher from the rental company. The hardest part is paying for it. Does anyone have any sources for 25 kva transformers that are used but still have life left in them? Maybe extras in the back room? What does it do to my available amperage and required wire size if I used 15 kva transformers instead. One went on e-bay this morning for $175 plus $100 shipping?
Thanks
 
  #4  
Old 04-27-05, 06:12 AM
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For a 25 kVA transformer like you are describing, my Square D catalogue shows a (list) price (which nobody ever pays!) of $2,366. Ouch!!!

A 15 kVA transformer will give you 62 amps on the primary, and 31 amps on the 480v secondary. If you can live with a 60 amp panel, which is usually more than adequate for an average home workshop, you've got your wire down to #1 AWG copper, a huge savings. Also, $275 ain't too shabby for a 15 kVA transformer that Square D lists at $1,818. (No wonder shipping is $100, my catalogue shows a weight of 200 pounds!) Sounds like a plan to me.

Juice
 
  #5  
Old 04-27-05, 10:44 AM
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Just a note.
480 volts / .707 = 679 peak voltage.
I don't know what an electrician would use.
But I think you will need to use wire rated for 800 volts or more.
What's the price of the hi voltage wire ?
 
  #6  
Old 04-27-05, 08:19 PM
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Originally Posted by GWIZ
Just a note.
480 volts / .707 = 679 peak voltage.
I don't know what an electrician would use.
But I think you will need to use wire rated for 800 volts or more.
What's the price of the hi voltage wire ?
What makes you think that the voltage rating on the wire is not the RMS average rating? You can use wire rated for 300 volts on 240 volt circuits. And 240 volts AC peaks at a little over 339.4 volts.
 
  #7  
Old 04-28-05, 01:48 AM
beartoothranch
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I thank all who responded for the information. Now on to the next detail... Buying used transformers. What is the life of transformers? How do they go out (Just stop working, firey explosion, or something in between (as one of these will be going in my basement and the other in my barn))? what causes early failure. Advice on buying used transformers in general. Please help again.
Thanks.
 
  #8  
Old 04-28-05, 03:14 AM
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Transformers fail because heat causes the insulation to degrade, so that they short out. Keep them clean, dry, and well ventilated, and they will last a good long time.

IMHO using transformers as you describe is not an ideal design.

You are trying to avoid voltage drop in a long underground run by stepping up the voltage to reduce the current. So far, so good. By doubling the voltage, you have half the current and double the allowed voltage drop, which means that you can use wire that is 1/4 the cross section to get the same power loss in the wire. This is exactly the sort of technique used for long distance power transmission.

However you must also consider the impedance of the transformers themselves. This is a measure of the voltage drop expected _in the transformer_ without regard for the losses in the wire. This voltage drop is caused by a combination of reactive impedance and internal resistance. Impedance is usually stated in terms of percentage voltage drop at full load.

The transformers that you will likely be buying will have an impedance of between 3% and 5%, and you will put two of them in series. This means that you will see a voltage drop (from zero load to full load) of between 6% and 10% before you even get to the drop in the wire. If you use transformers, you will have to expect a minimum of 10% voltage drop under full load once you add the copper loss.

On top of this, transformers always use a little bit of power, simply to provide for the magnetic field losses. This power is consumed whenever the transformer is on, even if the load is zero, and is probably about 100W per transformer. Running this transformer pair for a year means about 1800 KWH in power consumption just to have the transformers on.

I'd suggest that you rethink your design in terms of allowable voltage drop and amp capacity. Do you _really_ need 100A of capacity with only 3% voltage drop? If so, then you are looking at $5K-$10K of wire no matter how you slice it.

The first thing that you need to do is a load calculation. What do you need to run the tools that you want to run?

The next thing that you need to do is determine what sort of voltage drop you can tolerate. You may be much better off going with tools that can tolerate a large voltage drop, rather than spending lots of money on wire to minimize voltage drop. For example, some motors are rated for single phase use on both 208V and 240V systems. These would certainly tolerate _significant_ voltage drop on a 240V system. If all of your large tools have such motors, then you could reasonably design for 10%-15% voltage drop at 100A. For some systems, you will need to carefully select motors and controls to tolerate the voltage drop, but with some design forethought 15% voltage drop is reasonable.

In general, there are two problems with voltage drop: lights flickering every time you start a motor load, and reduced motor life because the voltage is too low when the motor starts. By selecting motors that can tolerate the low voltage, you take care of the motor life problem. To take care of the lights flickering, you might consider running _two_ feeders to the barn: a 100A 240V feeder that will be used for 240V loads that can tolerate significant voltage drop, and a 20A 240V feeder that is for lighting and small loads (computers, radio, etc) only. I think that you could get away with 2 #2 Cu conductors for the 100A feeder 'hot' connections, 4 #4 Cu conductors for the 20A feeder (2 hot, 1 neutral, and one EGC that is shared with the 100A feeder.

A couple of points that I am not sure about:

1) I don't know if the 100A panel would be required to have a neutral, since it should be supplying only 240V loads. If it is required to have a neutral, I believe that you could _share_ this neutral with the 20A feeder (see 215.4 Feeders with common neutral). In this case I would stick with the #4 Cu for the neutral, since there will actually be no neutral loads on the 100A panel.

2) If you have a common neutral as above, I don't know if you would be permitted to reground the system as permitted in 250.32(B)(2) for subpanels in detached structures. If you do this, then you won't have to run an equipment ground conductor, and you will have better localized ground bonding, however you will be restricted from having _any_ metallic systems (pipes, fences, phone wires) connecting the two buildings. In general I recommend running an EGC (ground wire), however for such a long _outdoors_ run regrounding the system and not running a ground wire may in fact be a better option.

-Jon
 
  #9  
Old 04-28-05, 03:51 AM
beartoothranch
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I thank you for all of the information. As I already have the power tools, selecting tools and motors based on their ability to run at reduced voltages is not practical. I do not anticipate using 100 amps ever. The largest consumer of power is a stick welder ( Approximate draw is 46 amps if I remember correctly). Running that at the same time as 10 florescent lights, a blower motor for the furnace, a 5 hp dust collector and having a 5 hp air compressor kick on is all I ever anticipate. I think that I could get by with the 15 kva transformers and smaller wire ("60 amp system"), however, I only want to do this once. I have found triplex wire for $1.50 a foot (1/0, 1/0, 2) and I have a good deal on two identical 25 KVA transformers. I am just learning as I go along, so please bare with the questions. My wife (who also lives in my house) is concerned about the idea of using used transformers, and does not want to wake up dead one night because a transformer exploded down in the basement. If the used transformers last a few years before one or both need replacing, I can handle that, I may even be able to locate a couple of new quality transformers in the interum. For now however, I have a barn full of tools and equipment that can not be run (not wthout firing up the generator and pluging each tool in one at a time, (no welder or dust collector)), in addition, as I stated before, the barn is a long distance from the house, and thus would like to install a mercury light and alarm system for security. I guess I am trying to figure out if this will work, is practical, and cost effective relative to the quote from the power company. This will be a hobby workshop, used occasionally in the evenings and on weekends, not a full time production shop.

Thank you
 
  #10  
Old 04-28-05, 04:14 AM
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Correct me if I'm wrong, but isn't there a code violation with having a voltage-to-ground of greater than 120V in a home?

Would it be practical to install the transformer in an shed or outbuilding that's near the house and feed 240V to the shed before the step-up to 480V? Although your transformer should be safe if installed properly and breakered, it may alleviate your wife's concern of having it in the house.
 
  #11  
Old 04-28-05, 04:24 AM
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Oh, learning as you go along. If that is the case, then I suggest that you take the utilities offer; it will be about half what this setup ends up costing you after you get it wrong and have to pull it out and start over.

Electrical installations are all about the details, and there are _many_ of them. The last thing that you want to do is to start buying stuff now and then plan as you go along. Get several books on electrical wiring, read them, and then carefully _plan_ what you are going to do. Then get in touch with a nearby electrical inspection department. I'm guessing that you are out of town and inspections are not required. If so, find a town where inspections are available and pay to go over your plans with one of their inspectors. You don't want to mess up on the details.

On to your question about the safety of the transformers:

If you get 'dry type' transformers, then you won't have to worry about blowing them up. They might short out and trip their supply breaker, and they might make a bunch of smoke if the breaker doesn't trip, but they won't blow up. Install them in a well ventilated area and don't store anything near them. Additionally, it would pay to have an easy to use disconnect; since this shop will only be used on off hours, it will pay bigtime to simply turn off the transformers when not in use. The transformer will require some sort of disconnect anyway (possibly just the breaker in the main panel), so it will pay to make this an easy to use convenient disconnect.

Be sure to carefully follow the rules about grounding separately derived systems and overcurrent protection for the transformers. Depending upon how they are designed and connected, you may need OCPD on both the primary and secondary side of each transformer.

But please read what I said about transformer impedance. I believe that if you step up the voltage to 480V, run the 1200 feet, and then step down to 240V, that the over-all voltage drop (counting the transformer impedance) will be essentially the same as if you simply ran the 240V in the exact same wire. It would take a detailed calculation to confirm this, but I believe that the transformers will be a waste of money; poorer electrical performance with additional losses for more money.

I can understand already having a collection of tools that you want to run. Go to each of them and see what the voltage ratings are. See how easy it would be to change out the motors, or if the motors are rated for low voltage already. I bet the stick welder has a 208V rating that is safe.

Is that wire aluminium or copper. If aluminium, then it isn't such a great deal. Is the wire rated for underground use? What is the insulation rating? As triplex wire, you won't have the option of having a separate ground and neutral. This is one area where the transformers help, because you are creating a 'separately derived system' with additional grounding options. See, many issues, and if you go out and but $2K of wire only to find out that it is the wrong stuff, then the $8K to the power company starts looking better and better.

-Jon
 
  #12  
Old 04-28-05, 04:50 AM
beartoothranch
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Lightbulb

Now that you have thrown an wrench into the gears of my wonderfully concieved plan, I am beginning to wonder if this is such a good idea. One suggestion I recieved at the beginning of this design was to just go with a generator and fire that up when I go to the barn. As I will have heat at the barn, this could be propane powered and run off the bulk tank. The downside (and ultimatly the reason I never explored this further) was that I wouldn't have power for a security light and an alarm system. Your idea of two feeders gave me a new thought .... what if I ran an underground 20 amp line to power a couple of lights, the blower for the furnace, and an alarm system, then when I am using tools, fire up a generator. I don't know if this will be inconvient, and a pain in the rear down the road, but would work. Off the top of your heads (or from your calculations), what size wire would I need for a small system like this.
I am not an electrical engineer, but I play one on TV... Actually I am an environmental engineer who plays with electricity for fun. I have wired a couple of houses in the past though, even passed inspections.

Thank You

Graham
 

Last edited by beartoothranch; 04-28-05 at 05:32 AM.
  #13  
Old 04-28-05, 05:57 AM
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How about you install a main panel in your barn as if you were going to have the power company drop a service there. Instead, just install a generator transfer switch and use that to power the main panel box in your barn. That way you won't need to switch plugs all of the time, and you will always have the option of a full service if you want to spend the cash later.

The small circuit for lights and security sounds like a good idea too.
 
  #14  
Old 04-28-05, 06:08 AM
beartoothranch
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That is exactly the current thought process, in thinking about this over some food (food for thought), I guess I need to know what size wire I would have to run from the house to the barn. This would be for one 20 amp circuit ( as mentioned before, a couple of lights and the fan for the furnace (Modine 75,000 btu ceiling hung propane fired) and an alarm system). The distance is about 1200 feet.

Thanks.

Graham
 
  #15  
Old 04-29-05, 03:33 AM
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Depending upon the actual loads you want to run, and how much light flicker and voltage drop you are comfortable with, you could use anything between #2 and #8. My best guess is that you should go with a #6 and deal with a certain amount of light flicker.

With #8 you get to run about 8A before you hit 5% drop, which is enough for about 2KW of lights. The problem is the furnace fan; fan loads are just fine starting with lots of voltage drop, but the lights will flicker considerably each time the fan kicks in. Make sure that you use a 240V fan to make best use of the power available at the end of the long run.

With #4 you get to 12A with a 3% drop, and your flicker will be reduced but not eliminated.

Since you will be doing the work for trenching anyway, you might want to consider putting in conduit which you could use in the future to pull the fat 'full service' wires. However conduit costs money, and thick conduit costs more, and if you are just running #6 or #8 you might also consider running direct bury wire; costs much less initially, but in the future you will have to redo the trenching work.

-Jon
 
  #16  
Old 04-29-05, 03:39 AM
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This has been a quite interesting thread. I thank those of you who have contributed your knowledge.
 
  #17  
Old 04-29-05, 07:40 PM
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Originally Posted by ibpooks
Correct me if I'm wrong, but isn't there a code violation with having a voltage-to-ground of greater than 120V in a home?
NEC 210.6 details various limitations on voltage. 210.6(A) limits the voltage, in dwellings and the like, to 120 volts for terminals of luminaires and anything at or under 1440 VA (approximately watts). Note that 210.6(A) says "between conductors", so it's not addressing just line to ground, but even line to line. Yet 240 volt appliances, both hard wired and plug connected are semi-common. Examples include larger window air conditioners (plug) and water heaters (hard wired).

210.6(B) through 210.6(D) have other limitations, particularly involving luminaires, but no mention of dwellings is found here (so aside from 210.6(A) these rules do not vary between commercial and residential). 210.6(E) would seem to be the real limit at 600 volts.
 
  #18  
Old 04-30-05, 06:47 AM
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Originally Posted by winnie
.....you might consider running _two_ feeders to the barn: a 100A 240V feeder that will be used for 240V loads that can tolerate significant voltage drop, and a 20A 240V feeder that is for lighting and small loads.....
230.2 would preclude running these two feeders to the structure. 230.2(D) would allow two feeders if the 100A was 240V only and the 20A was 120V only.


Originally Posted by Skapare
NEC 210.6.............

210.6 does not apply as it only deals with Branch-Circuit Voltage Limitations.

Transformers are not utilization equipment therefore they are fed via feeders.


NEC Definitions
  • Feeder. All circuit conductors between the service equipment, the source of a separately derived system, or other power supply source and the final branch-circuit overcurrent device.
  • Branch Circuit. The circuit conductors between the final overcurrent device protecting the circuit and the outlet(s).
  • Outlet. A point on the wiring system at which current is taken to supply utilization equipment.
  • Utilization Equipment. Equipment that utilizes electric energy for electronic, electromechanical, chemical, heating, lighting, or similar purposes.
 
  #19  
Old 04-30-05, 07:04 AM
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Originally Posted by Bolted Fault
210.6 does not apply as it only deals with Branch-Circuit Voltage Limitations.

Transformers are not utilization equipment therefore they are fed via feeders.
So even if 210.6 did restrict the voltage to no more than 120 volts L-N anywhere in a home (which it does not by my interpretation), it would not preclude getting service fed by as much as 600 volts into a transformer that steps it down to the utilization voltage.

Now what if I got a power feed at say 480 volts (single phase), but ran it to a panel with a few branches, and each branch then fed a transformer that stepped the voltage down? Would the all be called a feed? What if one or more of those transformers step the voltage down to 12 volts for lighting (and are not part of the luminaire). it would seem to me that the limitations in 210.6 would not apply at the transformer, and article 411 can still be met with the appropriate transformer type (under 30 volts, ungrounded secondary, maximum of 25 amps secondary, maximum feed of 20 amps).

I've found that many water heaters, for example, have thermostat switches rated for up to 600 volts, and replacement elements are available for other voltages up to 600.
 
  #20  
Old 04-30-05, 09:46 AM
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Skap,

I didn't mean to imply that I disagree with your interpretation of 210.6, only that it wouldn't apply to this scenario. I should have said "These transformers are not utilization equipment and feed a panelboard therefore they are fed via feeders."


Originally Posted by Skapare
Now what if I got a power feed at say 480 volts (single phase), but ran it to a panel with a few branches, and each branch then fed a transformer that stepped the voltage down? Would the all be called a feed?
It all depends on where the final overcurrent device protecting the circuit is located (supplementary OCPD's excluded). If the transformers directly fed utilization equipment, the transformer would be fed by a branch circuit, but the transformer would have to be fairly close to the utilization equipment per 240.21(C). The outlet is at the utilization equipment.

The OP would certainly need an OCPD on the secondary conductors.

Originally Posted by Skapare
What if one or more of those transformers step the voltage down to 12 volts for lighting (and are not part of the luminaire). it would seem to me that the limitations in 210.6 would not apply at the transformer, and article 411 can still be met with the appropriate transformer type (under 30 volts, ungrounded secondary, maximum of 25 amps secondary, maximum feed of 20 amps).
210.6 would apply since there is no secondary overcurrent protection with these transformers as allowed by 240.21(C)(1).
 
  #21  
Old 04-30-05, 05:38 PM
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Originally Posted by Bolted Fault
I didn't mean to imply that I disagree with your interpretation of 210.6, only that it wouldn't apply to this scenario.
No problem with that. My original point was that 210.6 does have a limit, but if it were interpreted as a limit applying to the line-to-neutral (grounded conductor) voltage, it would prohibit lots of things we have now since it does say "between conductors" which would be a line-to-line voltage. So, sure, it doesn't apply in this case. But it is the place where the voltage is limited, effectively countering the statement made elsewhere that L-N cannot be more than 120 volts in a home.

I see no problem with the original idea of going with 480 volts for the transformer driven feed (aside from the cost) to the barn in question. In fact I'm planning to build a remote ham shack, the distance to be determined depending on what location I end up buying. But I have already considered running the feed to it with as much as 600 volts if the distance is long.

Originally Posted by Bolted Fault
It all depends on where the final overcurrent device protecting the circuit is located (supplementary OCPD's excluded). If the transformers directly fed utilization equipment, the transformer would be fed by a branch circuit, but the transformer would have to be fairly close to the utilization equipment per 240.21(C). The outlet is at the utilization equipment.

The OP would certainly need an OCPD on the secondary conductors.
You can protect a secondary circuit with OCP on the primary if certain conditions are met. One of them is the secondary be a single 2-wire circuit unless either side of a 3-wire circuit is rated individually for the full load. I don't remember where that rule was, but I understood it's reasoning from an engineering perspective. Transformers make it rather easy to overload things without being properly protected if the protection is on the primary only.

411.2 seems to want 25 amp protection on the secondary of a low voltage lighting circuit. If the primary is limited such that the secondary won't exceed 25 amps, I think that may comply. But even if this were 120 volts to 24 volts, that's just 5 amp OCP on the primary, so the branch circuit feeding it can't serve that purpose (since it would need to be no less than 15 amps). I think supplementary OCP at the transformer is fine (at least for 411.2), but probably not practical since it would limit the total capacity of all secondaries to 25 amps, whereas with separate secondaries each could be 25 amps. 411.2 does not seem to prohibit multiple secondaries, though 411.6 limits the circuit supplying this power to 20 amps.

The reason I had this in mind was I was looking into using 240 volts (instead of 120) to supply power to various LV track lighting systems. Since 210.6 refers to the terminals of luminaires and lampholders (limiting these to no more than 120 volts in dwellings), I don't see that as preventing the use of more than 120 volts to a transformer that then supplies low voltage power to the light terminals (hence the light terminals only have 12 or 24 volts). I just have to comply with rules like 411.5(A) and 411.5(B) to be able to use 411.5(C).

One reason I want to use 240 volts instead of 120 volts to power these lights is so that voltage drops caused by other loads on 120 volt circuits have less effect.

Originally Posted by Bolted Fault
210.6 would apply since there is no secondary overcurrent protection with these transformers as allowed by 240.21(C)(1).
Secondary OCP can be provided. It would need to be if it's a big multi-secondary transformer supplying multiple strings of LV lights.
 
  #22  
Old 05-01-05, 11:56 PM
beartoothranch
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I would really like to think everyone who has posted a reply. The information you have given me has been really useful.
Thanks Again

Graham
 
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