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Old 03-29-2010, 11:14 AM   #1
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1,200 ft -underground wire size


Hi All,
Am wiring 1,200 ft to the ranch entrance.

The wire will be in conduit and is to run two low amp devices (gate opener and a Fiber Optic transmitter). The total of the current draw is less than one amp (essentially trickle chargers) at 110 volts . . . the source will be 125 volts.

To be safe, I am assuming we will need 2 amps of service at 110 volts at the delivery end.

The run is perfectly straight . . . no turns, but due to the distance there will be several splices (my rolls are 500 ft)

What kind of wire do we need to run? THHN? Solid? Stranded?

Burial depth? 18"?

What gauge do we need? My calcs show 10 gauge works?

Note: Electrician is firm that we need 6 gauge for this application, believing it is mandatory that we supply 15 amps at the delivery end . . . If 6 gauge is a regulatory must, then the the economics favor a solar solution . . . But, 6 gauge really does not make design sense, since we only need a couple amps (solar is equally challenging due to foliage) . . . Have a hunch that is a little bit of "over-design" is going on here.

Note2: 1,200 ft of wire is expensive . . . and 6 gauge is real expensive

Do advise!

Yours,

Larry

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Old 03-29-2010, 11:23 AM   #2
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1,200 ft -underground wire size


The voltage drop for 120 volts @ 2 amps for 1200' calculates to # 6 Cu .

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Old 03-29-2010, 11:40 AM   #3
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1,200 ft -underground wire size


Hi Brric,

Thank you!

I must be using the wrong tables . . . 1,200 ft of 10 gauge produces 1.19868 ohms of resistance . . . which is a little less than 3 volts of voltage drop @ 2 amps. . . . 6 gauge has 0.47 ohms of resistance . . . about 1 volt of drop @ 2 amps . . . granted, @ 20 amps the 6 gauge would still be supplying 115 volts . . . but that would be designing for a never to be used condition.

Do direct me to the correct resistance table!

Larry
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Old 03-29-2010, 11:46 AM   #4
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1,200 ft -underground wire size


With #10 I am seeing a 5.8v drop @120v for 2a
With #8 a 3.6v drop
With #6 a 2.3v drop

Are you sure you have 125v as the source ?
That's on the high side

Distance is 1200' - since power must go to the device & back to complete the circuit

http://www.csgnetwork.com/voltagedropcalc.html

This one doesn't tell you the drop, just the wire size to use
http://www.elec-toolbox.com/calculators/voltdrop.htm
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Old 03-29-2010, 12:06 PM   #5
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1,200 ft -underground wire size


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Originally Posted by Scuba_Dave View Post
With #10 I am seeing a 5.8v drop @120v for 2a
With #8 a 3.6v drop
With #6 a 2.3v drop

Are you sure you have 125v as the source ?
That's on the high side

Distance is 1200' - since power must go to the device & back to complete the circuit

http://www.csgnetwork.com/voltagedropcalc.html

This one doesn't tell you the drop, just the wire size to use
http://www.elec-toolbox.com/calculators/voltdrop.htm
Hi Scuba-Dave,

2 x 1,200' . . . so 2,400 ft of resistance . . . yep, using the wrong table (smiling) . . .excellent point!

Yes, 125 volts is a little high . . . but the electric company said it was to allow for long wire runs in the house and still have 120 volts at the outlets . . . we have a 1,000 amp transformer for the house . . . with four 200 amp panels . . . the exterior power runs on it's own panel (not too much of a load) . . . so outlets near the panel end up at the high end of the band, with 125 volts.

Not that I should always count on 125 volts . . . but, being we have it . . . figure I should take advantage of it.

Back to the powering the gate . . . Would then appear we do need to at least use 8 gauge? . . .THHN? solid? 18" deep?

Thank you,

Larry
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Old 03-29-2010, 12:13 PM   #6
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1,200 ft -underground wire size


The #8 would give you a 3% drop so that seems acceptable
2a is only 240 watts - make sure that is enough power
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Old 03-29-2010, 12:37 PM   #7
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1,200 ft -underground wire size


is there any charger available that would use 240 volts rather than 120? For a 2 amp load with an acceptable voltage drop, that would allow you to use #10 cu rather than #6 cu.

btw: there are 2500' rolls of wire commonly available. If you make splices in the run, you will have to provide junction boxes for the splices. It is contrary to code to have a splice in a wire just simply in the conduit.

If you could use 240 volt, you could purchase a 2500' roll of some color and be able to make the full run (2 conductors) from that one roll (presuming your measurements are accurate).

don't forget the EGC (ground wire)
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Old 03-29-2010, 01:35 PM   #8
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1,200 ft -underground wire size


Hi Scuba_Dave,

Appreciate the help!

Yes, I was surprised at the low power requirements . . . The gate opener has a 12 volt battery and a tickle charger equivalent to 5 watts . . . The Fiber Optic transmitter is designed to be powered by current over ethernet (once again a very low power consumption) . . . was thinking of just running a real long DC line to handle both . . . but, by the time I figured in the excavation & better low voltage wire . . . . I was almost up to the cost / service of AC . . . and 1,200' is a bit long for a DC line.

Appreciate the help!

Larry
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Old 03-29-2010, 01:37 PM   #9
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1,200 ft -underground wire size


Hi Nap,

Great points!

Will look at the 240 volt idea . . . quite interesting.

Will also ask about getting 2,500 ft rolls . . . Have a lot of wiring to do . . . so, will definitely use it up.

Thank you,

Larry
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Old 03-29-2010, 11:52 PM   #10
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1,200 ft -underground wire size


Accourding to my calucatour that is specificly used in electrical useage.

1200 feet @ 120 volt @ 2 amp load you will need 16mm { #6 AWG }

now let me change the voltage to 240 and see how it come up.

1200 feet @ 240 volts @ 2 amp you will need 6.0mm { #10 AWG }

and voltage drop for both are at 2%.

But for myself ya never know you may want to add some luminaires at the gate so that something you may want to add to it so let me come up with other figures.

like example

Charger 2 amp
Luminaires 2 amp { 240 watts }
so that total of 4 amp

again let me run the same thing you will see the differnce

1200 feet @120 Volts @4 amp you will need 25mm {#4 AWG }
1200 feet @ 240 volts @4 amp you will need 16mm { #6 AWG }

That basied on 2% voltage drop

Really basically you can have up to 5% voltage drop before you will have issue with the system.

Btw some case I will install 120/480 transformer and that useally dealt with voltage drop very easy.

I just got done install the trasfomer 240/480 for water pump for my customer and the distance is shy over a click { kilometer } { 3280 feet }

Merci,Marc
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Old 03-30-2010, 01:00 AM   #11
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1,200 ft -underground wire size


Voltage drop formula (Vd=2xLxKxI/D) L=LENGHT, K=CONSTANCE, I=CURRENT, D=CROSS SECTION OF CONDUCTOR IN CICULAR MIL. Constance for copper is 10.8 and aluminum is 17. You want to keep the voltage drop at about 3% but never more than 5%. By my calculations, #8=3.1% drop and #6=1.97%. either conductor will work. That is calculated with a 2 amps load.
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Old 03-30-2010, 02:02 PM   #12
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1,200 ft -underground wire size


Quote:
Originally Posted by bhound84 View Post
Voltage drop formula (Vd=2xLxKxI/D) L=LENGHT, K=CONSTANCE, I=CURRENT, D=CROSS SECTION OF CONDUCTOR IN CICULAR MIL. Constance for copper is 10.8 and aluminum is 17. You want to keep the voltage drop at about 3% but never more than 5%. By my calculations, #8=3.1% drop and #6=1.97%. either conductor will work. That is calculated with a 2 amps load.
why make life tough?


http://www.elec-toolbox.com/calculators/voltdrop.htm
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Old 03-30-2010, 10:58 PM   #13
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1,200 ft -underground wire size


The Code does not require any adjustments for voltage drop. It is mentioned only as a fine print note, and those are not enforceable.

With the type of load you are using, I believe that it is overkill to install anything larger than the #10 you were originally planning on using.

If that provides unsatisfactory results, you could then install a small booster transformer as needed without having to replace the wires.

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