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# Wire gauge for combined 120/240 volts

#1
05-01-17, 07:31 AM
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Join Date: Apr 2017
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Wire gauge for combined 120/240 volts

Hi everyone, I've come to these forums for research and reference before but never to post.

I need to run electrical wiring from a 200 amp breaker box to a barn and a well that is about 450' away. At the end of this id like to have 240 V for a 1hp shallow well jet pump and also 120 V for running mainly lights, battery chargers, an impact wrench and occasionally an air compressor (10.5 amps) which should be the biggest draw and is what I will base my calculations on.

So according to Southwire Voltage Drop Calculator I will need the following:

Water Pump: 1 HP Sta-Rite pump draws 7.4 amps (at end of 450 feet) at 240 volts with 3% drop = 10ga wire. This matches up with what is in the Stall-Rite Manual.

Air Compressor: 10.5 amps (at end of 450 feet) at 120 volts with 3% drop = 4ga wire.

Now all that makes sense to me. but I want to use the 3 wire (black, red, white + ground) direct burial wire for this project. Do I need to use the 4ga wire for the entire project?

If I use the 10ga wire wire it should be fine for my pump but then if I tap off one of the hot wires and the neutral to create a 120V outlet at my barn my wire gauge is much too small for this since according to a voltage drop calculator I should be using 4ga wire.

Is my best solution to just wire the entire thing for 120V and pay for 2ga wire?

one more question. on the voltage drop calculator is has a section for whether it is direct burial or in conduit. How does this matter? I plan on using direct burial wire and then sliding PVC over it fore extra protection.

Thanks for the help, Dan

#2
05-01-17, 07:50 AM
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Join Date: Mar 2006
Location: USA
Posts: 33,575
The loads are not calculated separately. You add the loads that may be on at the same time to figure the breaker size and voltage drop.

So you have a ~8 amp load + ~11 amp load + 20 amp for general purpose use = 39 amps so a 50 amp breaker. Figure potential voltage drop using a seat of the pants guess of 40 amps so #3 or #2 copper.
I plan on using direct burial wire and then sliding PVC over it fore extra protection.
Bad plan. Cable is difficult to pull in conduit. Better to use individual conductors. If you use cable you might consider aluminum mobile home cable. 2-2-2-4 MH cable would probably be adequate.
If I use the 10ga wire wire it should be fine for my pump but then if I tap off one of the hot wires and the neutral to create a 120V outlet at my barn
No, You will need to install a subpanel. The wire size is to small, you can only have one power feed to a detached structure, and maximum breaker for a 120 volt general purpose circuit is 20 amps.

Note: #2 aluminum would give a voltage drop of 4.7% at 40 amps but that would be within tolerances. Your actual voltage at any time would more likely be less than 40a so even less of a drop.

Last edited by ray2047; 05-01-17 at 09:38 AM. Reason: Correct typo as mentioned by Pat.
#3
05-01-17, 09:30 AM
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Join Date: Feb 2016
Location: United States, Virginia
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Did you mean 4.7% voltage drop, not 4.7 volts?

Last edited by ray2047; 05-01-17 at 10:52 AM.
#4
05-01-17, 10:52 AM
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Join Date: Mar 2006
Location: USA
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Yep, that was a typo. Thanks for the catch.

#5
05-02-17, 10:16 PM
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Join Date: Apr 2017
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Hello ray2047. I really appreciate your response! I didn’t think of using a sub panel but it seems like such a great solution!

So the voltage drop calculation for this sub panel would be calculated based on 240 volts since the sub panel is powered by 240 volts. Now the amps at end of cable is calculated adding the amperage of the 120V load and the 240V load (plus an extra 20 amps general purpose buffer). Did I get all this correct?

Assuming this is correct I put all this into the southwire voltage drop calculator:

- Aluminum wire
- 240 volts
- max voltage drop of 5% (is this acceptable for electric motors)
- 450 foot length of cable
- 40 amps at end of cable.

Out of this it calculates #2 aluminum wire as you had suggested.

Now from my newly installed sub panel I install my 120v breaker (for the air compressor) and 240v breaker (for the well pump) and wire everything up.

Would it sound reasonable to use aluminum #2 THHN/THWN wire buried in conduit for this project? Im thinking I’ll run 3 wires (black, red and white) and then sink a new grounding rod next to the sub panel.

Any suggestions on what conduit size to use. It is pretty much a straight line.

Thank you so much for all your help!

Dan

#6
05-02-17, 10:27 PM
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Join Date: Mar 2006
Location: USA
Posts: 33,575
Im thinking I’ll run 3 wires (black, red and white) and then sink a new grounding rod next to the sub panel.
A ground rod isn't a substitute for an EGC. You still need the ground wire for the equipment ground. Minimum conduit 1¼".

#7
05-03-17, 02:06 PM
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Join Date: Apr 2017
Location: USA
Posts: 4
Hi ray2047, I'm looking at this Southwire 2-2-2-4 Black Stranded AL USE MHF Cable found here:

Southwire 500 ft. 2-2-2-4 Black Stranded AL USE MHF Cable-28712801 - The Home Depot

Does this look acceptable? Im going to skip the conduit and just direct bury it at 24" deep since it says approved for direct burial. I will use my existing ground at the power pole as you suggest.

I appreciate all the help!

Dan

#8
05-03-17, 03:16 PM
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Location: United States, Virginia
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To be clear, if installing a subpanel at your barn you will need a grounding electrode system (rods) at the barn in addition to the equipment ground conductor that runs from the main service panel to the barn's subpanel.

#9
05-03-17, 06:37 PM
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Does this look acceptable?
Yes, that would be fine. You will still need conduit for there the cable rises above 24" below ground and up out of the ground.

#10
05-06-17, 05:53 AM
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Join Date: Apr 2017
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Thanks to everyone for all the help and clarifications. I will be sure to drive down an additional grounding rod and put the ends of the wires coming up out of the trench in conduit.

Thanks again!

Dan