Running service to cabin gets electricians buzzing

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Old 05-07-16, 12:03 AM
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Running service to cabin gets electricians buzzing

I am running 800 feet of 100 amp entrance wire (buried) from a drop down off a transformer into a cabin on the property. I know that the amps will drop but I only need about 60 to 70 amps to run items in the cabin (lights, small refrigerator, etc.). Any concerns or foreseeable problems? It is aluminum wire and I do have an electrician that will hook everything up for me. Thanks for any comments, suggestions!
 
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Old 05-07-16, 12:08 AM
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No such thing as "100 amp". Wire size is specified by gauge.
 
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Old 05-07-16, 04:31 AM
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Locally, service wire is often referred to by it's intended amp size. My house has a 200 amp service while my shop only has 60 amp ...... I don't remember the wire sizes but the electrical supply house does Wouldn't there be a significant voltage drop at 800'?

btw - welcome to the forums!
 
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Old 05-07-16, 06:49 AM
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Amps do not drop over long distances voltage does. Amps will actually go up when the voltage goes down.

Since copper would be very expensive at 800' I would recommend going aluminum direct burial wire. With a 70 amp load you would need 3/0 aluminum for your phase conductors (hots) to keep the voltage drop to less then 5%. You could do 3 - #3/0 USE and 1 - #6 USE for the ground if you are coming off an overcurrent device (breaker/fuse) where the run originates. If you just want to buy something off the shelf at the big box store you could get some 4/0-4/0-2/0-4 URD.
 

Last edited by Tolyn Ironhand; 05-07-16 at 08:04 AM.
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Old 05-07-16, 07:51 AM
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You could do 3 - #3/0 SEU and 1 - #6 SEU for the ground
If he is burying the wire he can't use SEU. Plus SEU is not an individual wire type, it's an assembled cable.
 
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Old 05-07-16, 08:04 AM
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Sorry, that was a typo. I meant USE. Fixed above.

Need more
 
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Old 05-07-16, 08:11 AM
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I would see if the power company would set poles and a transformer closer to the cabin.
 
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Old 05-08-16, 01:03 AM
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Thank you but you all must be lawyers....

....the information is accurate, but not worth a darn...still didn't answer my concerns, will it work? Thanks!
 
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Old 05-08-16, 01:54 AM
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still didn't answer my concerns, will it work?
We can't answer because you haven't told us what gauge wire, or the type of wire. As stated before "100 amp wire" is meaningless. Proper wire size for 100 amps varies with distance and acceptable voltage drop. What gauge wire do you have?
from a drop down off a transformer
Do you have a fused disconnect at the pole?
 
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Old 05-08-16, 07:45 AM
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A lot of things can be made to work. That doesn't mean they are safe or efficient or even the best way to solve the issue.

You need to supply the information that was requested.
 
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Old 05-08-16, 08:10 AM
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The wire gauge is most important as everyone else has said.

Assuming 2ga alumimum wire (which is what is typically used as 100A service entry cable), at 800' and 70A, you would see about 11.5% voltage drop at the cabin. That's a lot, instead of 120v, you will see about 92v, which isn't all that usable.
 
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Old 05-08-16, 10:13 AM
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That's a lot, instead of 120v, you will see about 92v, which isn't all that usable.
You can also damage equipment, like refrigerators.
 
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Old 05-08-16, 11:29 AM
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Motors, especially compressor motors will be the worse off in situations like these. Including the refer, and any air conditioners. Fans, ovens, lights pretty much don't care. If you will be bothered by lights dimming when the refer comes on, then upsize the wire.
 
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Old 05-08-16, 05:05 PM
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Well, I am leaning things, thank you!

The 100 amp entrance service wire in the US (commonly referred to as mobile home service wire?) is typically 2/00. I don't know what that means but there are 3 strands with each having 7 alum wires running through each strand. I have a 100 amp disconnect "drop-down" from the electric company transformer, which already has power to it (I can plug in a 30 amp camper plug and/or 110 service for construction tools, saws, drills, etc.). I have an 100 amp circuit box/breaker box in the cabin that I want to hook up to. I hope this helps. Thank you all for your time
 
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Old 05-08-16, 05:54 PM
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If you take 2/O aluminum conductors to run the 800' from the utility pole transformer to the cabin, for all intents and purposes you have a 60 amp service allowing a 5% voltage drop.

Note that the actual performance of the service will vary depending on how evenly the A leg and the B leg are loaded and under some conditions the voltage drop can be more than 5% in this example. The best performance is had with the two legs balanced.

Because nobody can predict when you will be drawing what amount of power, it is instead customary to spec out the service as being so many amperes (say, 60) at (usually in the U.S.) 240 volts and then take the acceptable voltage drop (say, 5%) and then compute the wire size needed for the distance.



.
 
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Old 05-08-16, 06:14 PM
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You are also supposed to do a "load analysis" to determine how many amperes (or watts) you should supply to the cabin. The NEC has one set of rules for this. This takes iinto account the square footage of living space, appliances installed in the building, heating, air conditioning, etc.

There is no problem hooking up the panel with the 100 amp top breaker to the 2/O cable feed. This cable won't overheat with 100 amps. While the result of voltage drop is heat given off by the applicable wire or conductor, the heat is distributed over the entire length of the wires so in this case no one spot will get hot enough to start a fire.
 

Last edited by AllanJ; 05-08-16 at 06:37 PM.
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Old 05-08-16, 06:35 PM
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or all intents and purposes you have a 60 amp service allowing a 5% voltage drop.
For clarity: Your service size is based on the over-current protection device protecting the conductors feeding the service.

You could have a 100 amp service fed with #4 copper THHN wire that is 800' long. If your load is 1 amp, it will work just fine. However, as your current draw goes up, so does the voltage drop. As the current continues to climb the voltage will drop to a point to where equipment starts to struggle to keep running, or fails to start. That is when things start to get damaged or destroyed.

The NEC recommends to keep voltage drop to less than 5%. As AllenJ mentioned with 2/0 Aluminum you would need to try to keep the load to no more then 60 amps or bad things may start to happen.
 
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Old 05-08-16, 06:51 PM
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Oops, you said you have a 100 amp disconnect on the pole below the transformer. You won't be able to use service entrance cable because you need a ground wire as a 4'th conductor. I came uip with #2 aluminum to go with 2/O circuit conductors.

(Keep neutrals and "grounds" separate in the cabin's panel.)
 
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Old 05-08-16, 06:54 PM
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#4 aluminum is OK with 2/0 aluminum conductors based on table 250.66.
 
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Old 05-09-16, 01:03 PM
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wow, you guys know you stuff

Thanks for all the info, it has really helped to make some decisions. I will not bother you anymore but will let you know of the outcome of what we do and how it works out. The electric company wanted $13,000 to place it in and I'm trying to save a little money by DIY (they wanted me to do everything anyway, they just supply the wire and hook it up!). Thanks for all your help!
 
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Old 05-09-16, 05:14 PM
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#4 aluminum is OK with 2/0 aluminum conductors based on table 250.66.
If you are talking about equipment ground conductors, 250.66 is the wrong table. Equipment ground conductors are sized from 250.122. The size is the same, just wrong table. 250.66 is for grounding electrode conductors.
 
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