What size wire?

Old 09-29-02, 05:30 PM
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What size wire?

We're installing an automatic gate, and we need to run electrical wire about 700 ft. The motor is 1/2 HP, 5.4A@115V. Does anybody know what size wire to use? We were going to use a direct burial type wire. Thanks!
Old 09-29-02, 06:27 PM
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#6 awg copper will give 2.9% voltage drop.
Can you get the motor at 240V? If so you can go with #8 copper.
Old 09-29-02, 08:19 PM
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wire size needed

cm=K x I x D divided by VD

cm= 25.8(approx.k of 1 ph. cu) x 9.8 (amps for 1/2 hp 115v motor per table 430-148) x 700(feet) =176988

176988 divided by 3.6(recomended 3% voltage drop) = 49163cm

This would require #3 copper

Did you consider a solar powered controller?
Old 09-30-02, 11:33 AM
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Since you have two let me give you a third answer.

cm or circular mils is the outer diameter of the wire

I am using the following equation to provide you with an answer.
Since we are finding a wire size we can use an approximate value for K which for copper is 12.9
cm= 2xKxIxD divided by the accepted allowable voltage drop for a branch circuit of 3%

2x12.9x9.8x700 divided by 3.45 volts.= 51,300 cm

2 is the distance out and back
12.9 is the approximate value for K
9.8 is ampacity according to table 430-148 of the NEC which is what you must go by unless you are sizing heaters for a controller.
700 being your distance
3.45 is 3% of 115 volts

a #3 copper conductor has a circular mil area of 52,620 so this wire will accomodate your needs.
Texsparky is correct about the number 3. Is there any way that you can use a 240 volt motor? Doing so would cut the amperage in half and make it so that you can use a smaller wire size.

This installation will be a major expense!!!!!!!! You will need to install the wire in conduit and I am assuming that this will be an underground run. 700 feet of trench 18 inches deep won't come cheap!!
Not sure about the solar issue but it may be worth looking into..
Good Luck!!
Old 09-30-02, 08:25 PM
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You may be able to find some info on solar here

Old 10-01-02, 05:30 PM
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Something is being misted:

After some research I believe you have a DC motor. It is very common for residential gates.
Is there is some type of control box to make this gate properly work. Where is it going to be mounted? What amperage does it call for? Does this control box possible use DC voltage to run the motor, to composite for the distance? Does the directions give you a wire size for the distance?

Last edited by aphares; 10-01-02 at 07:55 PM.
Old 10-02-02, 07:25 PM
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what size wire?

According to the manufacturer's specs, it's 115VAC. Specs read as follows:

1/2 HP 117VAC, 60Hz, 5.4A @ 115VAC

I was able to locate the manufacturer's recommended wire size, and they call for 8AWG up to 460FT. I was able to find an alternate route that got the distance down to about 515FT. The installer said in his experience, it should be fine because it only runs for 12 seconds at a time.

So, based on the installer's input, I was going to use 8AWG direct burial wire. I found 8-3 wire in 500 ft spools, but I have a coupl of questions. Is it really OK to bury the wire without any conduit for protection? Also, since I'm over the distance recommended by the mfr, would I be better off combining 2 of the 3 wires for the hot line, since I don't need all three?

I appreciate all you expert's inputs!

Old 10-02-02, 08:13 PM
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Do you think that you could get the installer to warranty it?If the manufacturer recommends #8 only up to 460 ft ,then you need to at least go to #6 for 515 ft.or you risk the warranty.

You can't parallel conductors smaller than 1/0 for this application and if you could, it would have to be both the hot and the neutral

You will find 6-2 w/ground is probably cheaper than the 8/3 w/grd.your talking about using.
It's ok to direct bury the wire if it is 24" deep
Old 10-02-02, 09:08 PM
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Conductors size:

Based on the odd amperage, and horsepower, this motor was matching a table in the NEC 430.147, which is why I was suggesting it to be DC, But not the case. Sorry.

Texsparky is correct you cannot parallel conductors under a 1/0.

Handyron also suggested a # 6 conductor, and using the nameplate rating of the motor, I agree with his calc.

At this distance and installing the cable underground, it really is something you only want to do once. If you have a good source of electricity at the house then you want the same source, or voltage supplying the Gate. What happens when you decrease the wire size is you choke the voltage from getting to the other end. this ends up shorting the life of the motor, or over heating the wires. In turn causing you to have to replace one or the other for trying to save a dollar now. Which I understand, but at this distance a trench and wire is not cheap. Texsparky is also correct about the 24 just be sure the cable is type UF (underground feeder), or Direct burial.

Last edited by aphares; 10-02-02 at 09:56 PM.
Old 10-02-02, 09:49 PM
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4-4-6 alum would be a lot cheaper.
Old 10-04-02, 01:12 PM
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Consider using a 1kva 220/110 volt step-down transformer which would be located at the motor.And install a grounding rod at the motor enclosure to maintain the eqiuptment in a Grounded condition should the Equiptment Grounding Conductor in the cable open.
Old 10-04-02, 07:33 PM
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If your lot is so big that the gate is 500+ feet away you don't need to to worry about the few extra dollars running # 6 or even # 4 wire will cost you. The most expensive part is the labor to install it.

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