700' to cabin?

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  #1  
Old 05-09-05, 12:30 PM
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700' to cabin?

We are building a small cabin on the back side of a hunting property and are trying to run electricity to it about 700' from there the utility ended their service installation. As I read the chart if we can get by with 50 amp service at the cabin we can run three direct burial wires 3/0 size. This will give us 50A 240 service. The chart seems to be saying for 240 volt service, 50 amp, 12000 watts, we can us 3/0 wire up to 725'. Can I get an opinion as to whether I'm reading the chart correctly? Also does it make a diffeerence if the wire is copper or aluminum? The fellow at the electrical supply said aluminum is used almost exclusively for service entrance because copper is so high. Thanks for any help/opinions I can get. Denny
 
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  #2  
Old 05-09-05, 02:11 PM
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The first thing that you need to determine is what you want to run in this cabin. If you are just talking about a small cube fridge and a radio, then you don't need 50A. If you need to run an HVAC system, then 50A may be much too small. Describe how big this cabin is, and what you want to be able to run.

Next you need to determine what sort of power quality you can accept. The wires that you will be using to feed the cabin will act as _resistors_, dropping the voltage supplied to the cabin. But the issue is that this voltage drop will _change_ with load. If you have something like a fridge with a motor, _every_ time that motor starts, the voltage will drop. This voltage drop will do things like make lights flicker, and can make motors harder to start. This voltage drop can be reduced by using more expensive, thicker wire...so you need to decide what sort of voltage drop you would accept.

Copper has lower resistance than aluminium, so for the same size wire copper will have less voltage drop. But you could simply use thicker wire if you choose aluminium. Copper is often considered more reliable, and terminations for aluminium must be made much more carefully.

-Jon
 
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Old 05-09-05, 02:22 PM
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Have you priced 700 feet (times four) of 3/0? Then price a generator.
 
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Old 05-09-05, 02:24 PM
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I think the minimum service to a structure can be 100A. It seems that this would be the main panel not a sub off of an existing service for the main building, right?
Since you have a long run, your conductors will be sized sufficiently to account for a technically sized 100A service.
 
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Old 05-09-05, 03:33 PM
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I doubt there are local ammendements allowing less than a 100 amp service.
It is not beneficial IMO to try and "make it work" by oversizing wire to a very small service. Try to terminate 3/0 in a 100 amp main breaker panel. I bet it can't be done without special terminations. This is just one factor in this type of scenario.

This should be done the right way if you insist on an electrical service. Go from the POCO point with primary, to a transformer placed a logical distance from the cabin.
I am NOT suggesting you do this yourself also. This is most definitely NOT a DIY job.

I'll have to agree with seriously considering a generator if this will just be an occassional use cabin.
 
  #6  
Old 05-09-05, 04:30 PM
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Small 14 X 18 hunting cabin

This is just a small 14 X 18 hunting cabin. It seems to me that a 50A service should be fine to run a small refrigerator (300W max), a 5000 BTU AC (maybe 2000W max), and a small hot water heater. Not running all at the same time. 50A X 120V should allow for about 12000 watts of power - a little less considering voltage drop.
This cabin may be occupied 5-6 days per month during nice weather. The cabin is in a location where the power co only inspects to the meter base which is out by the road.
We're not trying to wire this as a house with all the amenities, but just a little cabin with a few amenities.
Thanks for your opinions.
 
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Old 05-09-05, 05:31 PM
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Since this is the case, maybe you can get a local override to the 100 amp minimum service. I'd check with your local inspector/POCO.
 
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Old 05-10-05, 05:44 AM
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denn78961,

Speedy kinda hit it on the mark a few posts ago on this topic. Even if the local POCO will not allow a variance to a lesser than 100A service and the code inspectors go for it...provided you are inspecting it I guess....

Getting 3/0 to terminate in a 100A panel is going to be dang near impossible, I guess you can change lugs and so on but it also to me it would depend alot on how often you are going to use this cabin.

As job says a Generator may be easier as you can bid on one of those at any online auction as you are looking at about...$ 2.85-3.85 per foot on the wire .

Now it is possible if you can get the PACO to run the lines to the shed as speedy said it is not a DIY job....then you could install a 100A panel and supply a hot gutter and use a " kearney" split bolt and so on but again way more than a DIY project in my opinion.
 
  #9  
Old 05-10-05, 07:20 AM
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I'd strongly suggest that you go out and get a copy of the book 'Wiring Simplified', as well as at least a couple of other books, and read them. There are so many details required for proper installation of a service, that there is no way we could tell you everything that you need to know, nor correctly guess at the details that you are not asking about because you don't know how to ask. The best that we can do is provide some input to help direct your early stage planning, eg. deciding between a generator and installed lines.

I only just got a chance to check the numbers. I don't know what table you are looking at, but you are not reading it correctly. For a distance of 700 feet, and a supply voltage of 240V, and a load of 50A, using #1 Cu will let you just squeak by with a 3.5% voltage drop. The 3/0 conductors that you were suggesting would be more than twice as thick, and certainly not needed to provide the small loads that you are running.

The bit about 100A minimum that others are mentioning is only partially applicable here. The relevant article of code is 230.79 This section states the required minimum rating of installed service entrance equipment. It provides for several different occupancies and situations, and specifically requires a minimum of 100A 240V service for a residence.

It doesn't say anything about the actual current that the power system could reasonably provide, nor does it say anything about voltage drop. The actual conductors going down the side of your house would need to be 100A conductors, and the service panel would need to be rated for a minimum of 100A.

If this cabin is considered a dwelling, then the minimum rated capacity of the service entrance equipment and conductors is 100A. If it is considered something other than a dwelling, then the minimum is 60A. Finally, if it is something other than a dwelling, and only _2_ general purpose branch circuits are served, then you can install 30A service, and for a single circuit, 15A service.

But just because the minimum capacity _must_ be 100A (assuming residence), this does not say that you have to design the system with the expectation that you will be regularly using the full 100A capacity. It simply means that you must make sure that all of the conductors and equipment must be able to safely carry 100A. Everything else is just _design_ and _preference_.

If you actually had a load of 100A, then you would really want to do what Speedy suggested, and get the power company to install a primary near the cabin. It is simply dumb to run 100A at low voltage for a long distance. But your load is much lower than this, and I bet your usage could tolerate the lights flickering a bit every time the fridge kicks on.

Not having looked at any of the other details, from your description it seems that the most you will possibly need is 20A at 240V, and that is presuming that the air conditioner and the water heater are running full steam at the same, 3000W for the heater, 1000W for the air conditioner (a 5000 BTU AC should take about 700W). Your assessment of 50A at 120V is probably more than sufficient, though of course this would be wired at 240V to minimize voltage drop.

That being the case, I'd suggest that using the bare minimum #4 Cu conductors required for a 100A service would be quite sufficient. With #4 conductors, you can draw up to 20A at 240V with a 3% voltage drop. 3% is generally considered a good design point to aim for in a feeder. It is _not_ a requirement, but much more than this and you are looking at serious flicker and motor start problems. For the loads that you describe, #4 conductors would be safe and a reasonable design.

Now, if you should later decide to install electric heat or an electric range, the #4 conductors would be quite limiting and annoying (imagine all the lights all turning noticeably dim every time the heating element cycles on). And if you install as suggested, then you will see some flickering. Should you decide to do some work on the building, every time you start a circ-saw the lights will flicker noticeably. But design is about balancing perfection with costs and reality.

Take the time to find an AC and water heater that both use 240V. This might be difficult since you are looking for low capacity devices, and usually only larger units use 240V. But this is essential, since for the same power and the same wires, doubling the supply voltage reduces the percentage voltage drop by a factor of 4.

Finally, after you price out the #4 wire (or the #2 wire, if you want to go a touch larger to get less voltage drop), you may find that the generator is still the best option.

-Jon
 
  #10  
Old 05-10-05, 09:16 AM
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Smile

Great discussion and thanks for all the help. I am pretty familiar with wiring having built 4 or 5 new houses and wired and plumbed them all. But I am not an electrician and when it gets into something a little unusual I like to get others opinions or advice. Plus it has been awhile since my last big wiring project. So thanks a lot it is really appreciated.

Not to bore you but this is the plan at the moment. Called the power company and no one inspects the installation so it doesn't have to be 100% up to code. Went into our local electrical supply house (not lowes, etc.) and they recommended 4/0 aluminum (saying copper was just to expensive), direct burial and their price was $2.01 per foot for all three wires. Since we are going to run the 4/0 wire anyway, we will put in a 100A service entrance box & 100A main, with the understanding that the more juice we use at one time the more voltage drop would occur. But I doubt we would ever go over 50A at any one time.

I haven't actually checked but I'm reasonable sure that 4/0 aluminum will not go in the 100A main breaker so I don't see a problem in trimming off wires at the end until it will fit into the breaker opening. The reason for using such big wire is to minimize voltage drop, so trimming off a few strands at the end wouldn't matter.

We have decided the generator is just not an option as we don't want to hear it running or scaring away the deer. Plus we would have to pack it up every trip or someone would steal it. Someone said I need to buy a "wiring simplified book". I have it and several others as well (ha).

Again thanks for all the help and patience. Denny
 
  #11  
Old 05-10-05, 09:26 AM
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Winnie

I am looking in the wiring simplified book now and the chart I am looking at is for "one-way distances for 2% drop at 240 volts single phase". It shows 50A supplies 12000 watts, and for a distance of 725 it takes 3/0 wire... and I assume that is copper. Am I reading/interpreting this wrong? Thanks, Denny
 
  #12  
Old 05-10-05, 09:35 AM
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Don't do a half way job that will fail, or worse, get someone injured.

Just because it will not be inspected does not mean that you can ignore code. Code is not god's law handed down from on high, infallable in all regards. Instead code is the distillation of many person-years of experience, with electricians working to balance what is reasonable to install with what fails. Sometimes you can reasonably make an argument that a particular feature of the code is just wrong headed, but good workmanship is always the right way to go, and the code will point you in the correct direction.

Based upon what you describe, the 4/0 aluminium will be more than adequate. You suggest that you won't go over 50A...my bet is that you won't go over 20A. I suggested that you could use #4 copper; #2 aluminium has essentially the same characteristics. 4/0 aluminium is almost 4 times thicker, meaning 1/4 the voltage drop. With 4/0 aluminium you will be able to use the full 100A with high but acceptable voltage drop.

But using the thicker wire introduces additional requirements, and I strongly suggest that you follow them.

You should make sure that any conduit and junction boxes that you use are suitably sized for these conductors. You have to bend the conductors and properly connect them, and there are code rules for how big the boxes should be. Follow these rules for an easy install that doesn't risk cutting the installation on edges on and the like.

In no case should you cut strands off the conductor to fit in the breaker. Use a short pigtail of thinner wire, with a proper splicing connector, or use a suitable adapter. Trimming strands means that you are not evenly and correctly connecting the wire, and one thing that is _critical_ with aluminium conductors is that they must be terminated properly.

Make sure that all the terminations (breakers, lugs, splices, etc) are rated for aluminium conductors, use 'no-al-ox' and follow the directions, and then use a torque wrench to tighten all the lugs to the torque specifications.

-Jon
 
  #13  
Old 05-10-05, 09:49 AM
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On the table, the key point is '2% voltage drop'. You are reading the table itself correctly, but missing the assumptions that the table makes. If you can comfortably accept a 4% voltage drop (twice what the table is based on) than you can use higher resistance (thinner) wire.

The table is also assuming the full 50A, but if your loads are connected as 240V loads, then you'll only be using about 20A; so if you do your calculations at 20 or 25A, the wire gets thinner still. But if you stick with the 50A assumption and the 2% voltage drop assumption, then the table is correct.

Note: it turns out that you won't be able to get a 5000 BTU 240V AC for the American market, since the NEC requires that loads less than 1440 VA (including this air conditioner) be 120V devices.

-Jon
 
  #14  
Old 05-10-05, 09:59 AM
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Jon

What makes you think I'm going to do a half way job that will fail? Where is the half-way and where will it fail?

To cut off a few strands (if necessary) should be no different than using a connector to step down to a smaller size 100A rated wire. If you use a connector that clamps the two wires together the power flow would be no different than what I am suggesting. I have bent many SE wires to get the in the box and I know they can be a real bear, especially the last one.

By installing 4/0 aluminum we are minimizing voltage drop, and allowing for possible increased usage at the cabin if ever needed. I suppose we could install a 200A breaker box that would accept the 4/0 aluminum wire... again with the knowledge that the more power we use the more voltage drop we would have. If we never go over 50 or 60A I don't guess it matters if we have a 100A or 200A box.

Even though you doubt we would ever use over 20A ( and you might be right) we don't want to limit our capacity to that. I'd rather have more capacity than needed ie less voltage drop.
 
  #15  
Old 05-10-05, 10:30 AM
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A/c

We were planning on using a single 120V 5000 btu A/C. Thanks.
 
  #16  
Old 05-10-05, 10:31 AM
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Originally Posted by denn78961
What makes you think I'm going to do a half way job that will fail? Where is the half-way and where will it fail?
I think ignoring code because it won't be inspected would be considered a half-way job by most of the regular posters to this forum.
 
  #17  
Old 05-10-05, 10:54 AM
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I agree that trimming strands off to make the wire thinner seems like it 'should' be no different than using a short length of thin wire and then making a splice....however:

1) Trimming strands is not an approved method, and if used in an inspected job, the inspection will _fail_.

2) There is quite a bit more to connectors than simply 'squeeze the wire tight', especially for aluminium conductors. When the wire conducts electricity, it will heat up and expand. The connector has to expand and contract to correctly accommodate the wire. This is why you have to correctly torque the screws. Trimming wire down to fit is almost certainly doing something that the connector has not been tested to deal with.

3) As you mention, bending these thick wires can be a bear. Trimming strands makes for a weak point in the wire, right at the connector. It is entirely possible that additional strands will break at this point.

4) Also, this thick wire will now be connected right to the breaker, with no strain relief and a pretty direct coupling. Net result, any forces in this really thick wire will be applied to the breaker, possibly exceeding the mechanical limits of the breaker.

Bottom line: doing this will probably work, but now you are doing the job of UL, testing an electrical component for proper functionality.

You are going to be spending time and money ($1400 for the wire alone!) to do a good job; why skimp on this small portion of the job.

Everything else that I know of seems reasonable. As I said before: if you want to spend the money on additional performance, go for it. The 4/0 aluminium should be fine to well in excess of 50A.

-Jon
 
  #18  
Old 05-10-05, 11:07 AM
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Thanks. We will go ahead and make the connection to a smaller 100A wire in a junction box before going into the service entrance box as you recommend or use a 200A box. Thanks again for your help. Denny
 
  #19  
Old 05-10-05, 11:30 AM
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Originally Posted by winnie
The bit about 100A minimum that others are mentioning is only partially applicable here. The relevant article of code is 230.79 This section states the required minimum rating of installed service entrance equipment. It provides for several different occupancies and situations, and specifically requires a minimum of 100A 240V service for a residence.
Doesn't sound like this is a residence, to me. The code refers to a one-family dwelling. I think this does fall under the definition of a dwelling, but not under a one-family dwelling (more like a zero-family dwelling). IMHO, the "all others" applies here, so the requirement would be 60 amps. The code omits a definition that covers this rare scenario.


Originally Posted by winnie
If this cabin is considered a dwelling, then the minimum rated capacity of the service entrance equipment and conductors is 100A. If it is considered something other than a dwelling, then the minimum is 60A. Finally, if it is something other than a dwelling, and only _2_ general purpose branch circuits are served, then you can install 30A service, and for a single circuit, 15A service.
The code lists these options in the opposite order that you just gave. It also says "or" as a choice, so it seems one could choose the 15A or 30A method under those specific conditions even if it is a one-family dwelling unit (which I don't think is).


Originally Posted by winnie
If you actually had a load of 100A, then you would really want to do what Speedy suggested, and get the power company to install a primary near the cabin. It is simply dumb to run 100A at low voltage for a long distance. But your load is much lower than this, and I bet your usage could tolerate the lights flickering a bit every time the fridge kicks on.
For the purpose I might have a cabin in the woods, I sure wouldn't want some primary distribution lines coming anywhere near there. And the POCO charges out the wazoo to run primary underground at distances like that.

OTOH, I might be more inclined to use a generator. But if I did run utility power, I'd want it underground, and not over 600 volts.


Originally Posted by winnie
Take the time to find an AC and water heater that both use 240V. This might be difficult since you are looking for low capacity devices, and usually only larger units use 240V. But this is essential, since for the same power and the same wires, doubling the supply voltage reduces the percentage voltage drop by a factor of 4.
Lower power appliances using 240 volts would be rare due to 210.6(A)(2). Not that I consider that small bit of code important enough to follow, but it will be reflected in the market.


Originally Posted by winnie
Finally, after you price out the #4 wire (or the #2 wire, if you want to go a touch larger to get less voltage drop), you may find that the generator is still the best option.
Thats more like what I am thinking. But there is the issue of hauling fuel to the location. Getting NG service is likely to be more expensive than even electricity (except maybe underground primary).

But I'm looking at having a full residence/dwelling (e.g. 200A service) at a significant distance off the beaten path. And I still don't want primary run out there. I'll figure out what I will do when that time comes based on how clueful the people at the power company are.
 
  #20  
Old 05-10-05, 11:41 AM
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Winnie is right. He gave several reasons why you shouldn't trim the strands off a wire, not to mention that it makes the job look like it was done by a jackleg.

Here is an alternate way of doing the project. Install a 200 service pole at the point of service. From your main panel, run a subfeeder to the cabin. Run the feeder in 2" PVC conduit, using (3) #4 Cu THHN conductors and (1) #8 Ground. Install a 50A breaker to feed the circuit. On the cabin end, install a 100A subpanel to terminate your circuit and feed your load.

If, on the slight chance that you ever needed to upgrade your service, you could remove the #4 conductors and pull something larger. With the voltage drop consideration and size of the conduit, you could probably never get more than a 125A or 150A service, unless you installed larger PVC. However, I can't see any reason why you would ever have need for more than a 50A service at a small hunting cabin. If you decide you want a stove and heat, you can always use propane. Your 50A service will handle lights, refrigerator, water heater, and window air conditioner.

If you can't install your circuit with less than 360 degrees of bends in the conduit, you will need to set a junction box in the run.
 
  #21  
Old 05-10-05, 02:40 PM
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Originally Posted by Skapare
I sure wouldn't want some primary distribution lines coming anywhere near there. And the POCO charges out the wazoo to run primary underground at distances like that......
But if I did run utility power, I'd want it underground, and not over 600 volts.
Why, may I ask? What is the problem with underground primary with a transformer +/-200' from the cabin??


A single primary is not much different in price than 4/0AL URD. So why not run primary to near the house and go with #2 URD from there?

In my area 200 amps is the smallest underground service allowed. We would run 4/0 to the meter and #2 to the panel.
Also we run the underground from the POCO primary pole/pad. The POCO does not do it.
 
  #22  
Old 05-10-05, 09:16 PM
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Originally Posted by Speedy Petey
Why, may I ask? What is the problem with underground primary with a transformer +/-200' from the cabin??
I'm not the original poster. Maybe he doesn't mind a pad transformer 200 feet away. I wouldn't want it. But I'm also not talking about a cabin; I'm talking about a full house way off the beaten path. I don't want utility maintained wiring running on my land. So I will take power somewhere near where they have their lines, and run it underground from there at a voltage I can deal with (e.g. not more than 600).


Originally Posted by Speedy Petey
A single primary is not much different in price than 4/0AL URD. So why not run primary to near the house and go with #2 URD from there?
Either way, if the utility installs it, it costs a lot. And then you still don't actually own it.


Originally Posted by Speedy Petey
In my area 200 amps is the smallest underground service allowed. We would run 4/0 to the meter and #2 to the panel.
Also we run the underground from the POCO primary pole/pad. The POCO does not do it.
For my house, 200 amps sounds right. But I could take it overhead from the power company to their meter, then run it underground to the house. It's just that at 240 volts, that's gonna be some major voltage drop. 480 volts will give 0.25 times the drop. 600 volts will give 0.16 times the drop. The big issue is what the POCO would be willing to provide. Most don't have either of these in their tariffs. Maybe I'd have to take 240 and kick it back up to 600 on my own.
 
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