Electrical question.

[andrew3d]

I have utility installed 200 amp electrical service at the county road and my meter at the county road as well. I put a travel trailer
in to stay in while I build my cabin.

Originally,  I was going to build the cabin near the electrical meter, but now I have found a spot with a much nicer view.  This
is approximately 1500 feet (towards the back of my property) away from the road. 

I don't want the electric utility to string wires all the way back there as I prefer the wiring crossing my land
to belong to  me and be underground.

Do I have any options to make this work without the electric company putting in another transformer and more
wire/poles?

Any suggestions?

[Willy]

1500 ft is a LONG WAYS for the low voltage side of the poles transformer. If you were just going to run a travel trailer maybe but a home no way. The size of the wire needed to feed the 200 amp service would be way higher than the wire for the high voltage run. They do have and will run undergound high voltage wire to a transformer that sits on the ground. If you had your service fed underground it will be easy to pull it and put in near the new transformer. If it was fed overhead you will have to change the meter socket out and a few other things to make it work unless you used a combination meter socket/panel used for mobile homes. Depending on the Power Company you may have to pay for the run and the transformer. Hopefully the poles are on your side of the road to save some money not crossing the road. This will cost big bucks if you have to. If you do pay for it the wire will be yours along with the transformer. Since you have to dig a ditch you might as well do it right. Talk to your power company and see what they want or charge to do it. 1500 ft is a big voltage drop proplem to correct. It will take 2000 MCM alumimin wire to work for 180 amps and 750 MCM is about 1 1/4" accross!! You could not even connect it to lugs in a 200 amp meter socket and the conduit would probley be 6 inch. Mark

[MountainDon]

Mark, I'm curious....  just what is the max voltage drop (%) you try for in a 120 VAC circuit? The Ideal?

[Willy]

Mark, I'm curious....  just what is the max voltage drop (%) you try for in a 120 VAC circuit? The Ideal?

3 percent volt drop is a good percent to shoot for. 3.6 volts on a 120 volt circuit leaves 116.4 volts at the outlet. Now you lose more on the cord pluged in a little at each plug conection ect so you try to keep the volts as high as you can. Once you start putting a load on the circuit the volts will drop more so it is figured using length/volts/amps/wire size to come up with the answer. I used to do it long hand but many years ago I bought a $150.00 electricians calculator that has all the formulas in it. All I need to do now is plug in the factors and it gives me the answers, pipe size, volt drop %, wire size ect. Mark

[glenn kangiser]

Careful, Mark.  You'll make Don lust after one of those calculators.

[Willy]

Careful, Mark.  You'll make Don lust after one of those calculators.

I use it all the time for just math but the extra stuff comes in handy. It is like a little computer. I am sure they sell them cheaper now since I bought mine in 1992 16 years ago! Wow it still works, I guess they made electronic stuff better back then?? Mark
There a little cheaper now. This is the same one I have except older.
http://www.spectrum-research.com/V2/electrical-calculator.asp

[MountainDon]

  tool Mark. That would be very handy.

I was wondering about what a master electrician would say about the VD%. I have a solar PV array planned for the cabin and want to keep the drop to 2% or less from the panels to cabin. There are too many trees in the cabin vicinity (southwards) and I will have to place the panels at least 300 feet away to the south. 

I have a computer program that does some of the same I imagine. I D/L'd it a few years ago from here...
http://www.edreference.com/
It has a 30 day time limit, but I accidentally found a way to make it work forever.  It's the older 2005 version... the new 2008 version is smarter and won't fall for my tricks. 

For the following inputs,
72 VDC
7.36 amps max
copper
75 degrees C rating
325 feet, one way
#4 AWG
the program comes up with.... VD = 2.05%

[wingam00]

MountainDon,

I was wondering about what a master electrician would say about the VD%.

I would think that you would want to talk to a doctor about VD and not a master electrician. 
I know dumb joke but I was having my coffee and it has not kicked in yet to to get the brain going. 

Mark

[glenn kangiser]

How could I have missed that? 

Sorry guys -- not reading close enough.  Good catch, Mark.

[Willy]

tool Mark. That would be very handy.

I was wondering about what a master electrician would say about the VD%. I have a solar PV array planned for the cabin and want to keep the drop to 2% or less from the panels to cabin. There are too many trees in the cabin vicinity (southwards) and I will have to place the panels at least 300 feet away to the south. 

I have a computer program that does some of the same I imagine. I D/L'd it a few years ago from here...
http://www.edreference.com/
It has a 30 day time limit, but I accidentally found a way to make it work forever.  It's the older 2005 version... the new 2008 version is smarter and won't fall for my tricks. 

For the following inputs,
72 VDC
7.36 amps max
copper
75 degrees C rating
325 feet, one way
#4 AWG
the program comes up with.... VD = 2.05%

My calculater does not work DC Circuits but that looks right to me. Problem with DC is the BIG Voltage drops compaired to AC and very large size of wire needed to prevent it. If that was a AC Circuit #12 wire would work fine. To bad you could not concert it to AC at the panels then send it to the cabin. You would save a lot of money on the #4 Copper Wire. Be sure to let the doctor check out that VD just to be safe. Mark

[MountainDon]

...Too bad you could not convert it to AC at the panels then send it to the cabin. You would save a lot of money on the #4 Copper Wire... .Mark

Yep, but I've thought through that and came up with other problems...

If the batteries were located downhill, along with the Inverter, the cables from downhill to the cabin would have to support up to 20 amps. Even if I used 15 amps as the most likely maximum draw at one time [microwave of 1100-1200 watts (9-10 amps) plus lights & other small loads] I'd still need 4 gauge wire.  120VAC at 15 amps, 4 ga. = 2.52% VD voltage drop. And that wouldn't allow for any real use of a small table saw or whatever.

And that's not even thinking about access to the inverter with all the snow that could accumulate, and so on.

Inverting the DC power from the PV panels to AC and transmitting that AC up to the cabin looks good on paper. 525 watts from the panels at 120 VAC = 4.4 amps. Cable size for that would be 10 gauge (2.9& drop) or 8 gauge (1.9% drop). But then we'd have to convert to DC to charge the batteries, and back again for AC needs that exceed the PV output. Way too much equipment, and inherent power losses with all the changing.

If anyone has a better idea than DC from panels to cabin/batteries, I'd love to hear them. Remeber the bugaboo in this situation is the distance from the best, the only solar collection site; 300 to 330 feet estimated. The cabin has changed position so many times I'm no longer up to date on that.

andrew3d.... I don't mean to hijack your question... but this seemed to fit like a glove. 

[davidj]

For DC, can't you use ohms law, V=IR?  I.e. the voltage drop is the current multiplied by the resistance.   And the resistance is the resistance per foot of the wire gauge used multiplied by the length in feet (including out and back distances).

I.e. voltage drop in volts = resistance per foot * total feet * current

The resistance per foot of copper wire is (according to a google search):

AWG ohms/foot
4      0.00029
8      0.00074
10     0.0012
12     0.0019
14     0.0030

(apparently the resistance/foot doubles for each 3 steps of wire gauge, so the voltage drop will do the same)

So with e.g. 200A and 4 gauge wire, you lose about 200 * .00029= .06V/foot (for DC).  You'll be lucky to get across a big room, let alone 1500ft.

Using the same calculations gives me a VD = 7.36 x 325 x 2 x x.0003 = 1.43V = 1.99% of 72V for mountain Don's DC setup (remembering to do the distance x 2 for out and back).  That's closer enough to the calculator, given that resistance/foot of big wires is probably quite sensitive to temperature and exactly what table you look in.

There's a voltage drop calculator at http://www.nooutage.com/vdrop.htm that seems reasonable if you are using standard voltages.

For a/c you need to start using impedance rather than resistance, and impedance is sensitive to more factors including frequency,adjacent conductors and insulation.  But it looks like that, for smaller wires, impedance @ 60Hz and resistance are about the same.  So the above calculations will work approximately for that too.

All that being said, I do software for a living.  Don't blow up any friends or solar panels based on this.