voltage drop underground

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Stevenfyeager

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United States, Indiana
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electrical contractor
I have to provide power for lights and a few outlets to a pole barn 130 feet from a house. Due to rock and other obstacles I would like to keep the trench shallow, a couple of circuits, the 12 inch to top of conduit as in the table, 120 volts, protected by GFI. I was thinking #12 UF in pvc, but looking at voltage drop calculators, that may be too small of wire. I was thinking that usually up to 200 feet is when voltage drop is not an issue.
 
Stevenfyeager said:
I have to provide power for lights and a few outlets to a pole barn 130 feet from a house. Due to rock and other obstacles I would like to keep the trench shallow, a couple of circuits, the 12 inch to top of conduit as in the table, 120 volts, protected by GFI. I was thinking #12 UF in pvc, but looking at voltage drop calculators, that may be too small of wire. I was thinking that usually up to 200 feet is when voltage drop is not an issue.
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The length could be a few feet or a few miles before voltage drop becomes an issue.

The load current is just as important as the length.

That said, I would not run less than 10 AWG for this circuit perhaps even 8 AWG if the load is going to be close to 20 amps.
 
iwire said:
The length could be a few feet or a few miles before voltage drop becomes an issue.

The load current is just as important as the length.

That said, I would not run less than 10 AWG for this circuit perhaps even 8 AWG if the load is going to be close to 20 amps.
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There are 3 overhead door openers, each 1/2 hp, and 2 boat battery chargers, invisible fence, and lighting.
 
Stevenfyeager said:
There are 3 overhead door openers, each 1/2 hp, and 2 boat battery chargers, invisible fence, and lighting.
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Voltage drop is hard to nail down due to the fact the current flow has a huge impact on it.

You will pretty much have to decide how much current in amps is going to be used at any one time.

Here are some examples of how the current changes things.

In each example I used 12 AWG at 120 volts with a max of 3% drop

With just 1 amp of load

A maximum distance of 1036.774 feet will limit the voltage drop to 3% or less with a #12 Copper conductor delivering 1.0 amps on a 120 volt system.
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With 5 amps of load

A maximum distance of 207.355 feet will limit the voltage drop to 3% or less with a #12 Copper conductor delivering 5.0 amps on a 120 volt system.
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Now with 10 amps of load, now you can't even make it to the building.

A maximum distance of 103.677 feet will limit the voltage drop to 3% or less with a #12 Copper conductor delivering 10.0 amps on a 120 volt system.
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15 amps of load

A maximum distance of 69.118 feet will limit the voltage drop to 3% or less with a #12 Copper conductor delivering 15.0 amps on a 120 volt system.
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20 amps ...

A maximum distance of 51.839 feet will limit the voltage drop to 3% or less with a #12 Copper conductor delivering 20.0 amps on a 120 volt system.
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Go here and you can play around with voltage drop http://www.southwire.com/support/voltage-drop-calculator.htm
 
While I was goofing around with the voltage drop calculator I tried using 10 AWG and it was marginal.

8 AWG looks like a better choice assuming a 15 amp load at times.

A maximum distance of 173.772 feet will limit the voltage drop to 3% or less with a #8 Copper conductor delivering 15.0 amps on a 120 volt system.
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Stevenfyeager said:
Thank you for that advice, and so early on a Sat morning! Where was I remenbering 200 feet from?)
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As a quick and dirty rule of thumb we move up one size per 100' of home run length for branch circuits. For us it is often a required note to follow on the electrical plans we work from.

We do a lot of large, flat buildings, this one https://goo.gl/maps/UoxovP4vZZJ2 we used 4 AWG copper for 20 amp 120 volt roof top unit service outlets.
 
I pretty much always run a 3 wire to an outbuilding. One more wire gets you twice the power available. Plus, neutral currents cancel in a 3 wire and improve voltage drop.
 
Cow said:
... neutral currents cancel in a 3 wire and improve voltage drop.
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That's true for continuous loads but won't be any help when one of the garage-door-opener motors is starting and the loads on the other leg are turned off.

And if the motor's on one leg and the lights are on the other, the lights will see a voltage rise when the motor runs.

A 120-volt, half-horsepower motor will draw less than 10 amps when running, but might draw 3-5 times that much when starting. 12AWG will have an impedance of almost half an ohm, which will cause a lot more than 3% drop during that transient moment. Do a voltage-drop calculation based on starting current (locked-rotor amps) and decide whether that's acceptable.
 
Stevenfyeager said:
I have to provide power for lights and a few outlets to a pole barn 130 feet from a house. Due to rock and other obstacles I would like to keep the trench shallow, a couple of circuits, the 12 inch to top of conduit as in the table, 120 volts, protected by GFI. I was thinking #12 UF in pvc, but looking at voltage drop calculators, that may be too small of wire. I was thinking that usually up to 200 feet is when voltage drop is not an issue.
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Another issue that will come up with your plan: you can't run multiple branch circuits or feeders to a separate building (see 225.30) You will need to run a multiwire branch circuit or a feeder to the separate building. This will disqualify the ability to use the 120 volt 20 amp GFCI protected circuit buried at only 12 inches deep.

You could use RMC if you wanted to remain shallow though.

I would just pull THWN through your raceway instead of UF - it will pull much easier.
 
Personally, I would be concerned that the owner does not really understand the limits the smallest possible feed to the building will impose in the future. Try to get him to realize that he will not be able to run a large air compressor, or welder or electric vehicle charger, or maybe even a table saw or space heater etc. with the lights on. This may not seem like a problem at this time, but he might be living with an underfed building for a long time. It would suck for him if he has to dig it back up in a couple of years.

Also, with the cost of trenching etc being so high I would consider throwing some direct burial cat5e or something in the hole. You never know if he will have future security video needs, etc. It is cheap to take care of it when you are already trenching.
 
You didn't say how big the building is. Just remember that it may be another 100 feet of cable distance from one corner to the farthest corner of a 30' x 60'.

Big conduit is way cheaper than big wire, and could give some future-proofing if the owner feels it is justified after you explain the limitations imposed by the distance and small wire.
 
ActionDave said:
That's a little overkill for a garage door opener don't you think?
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Doing a calculation is overkill? Let's not turn this into another engineer vs. electrician thread.

I'm not suggesting installing cable that will carry locked-rotor amperage with 3% drop. I'm suggesting running the worst-case numbers and predicting what will happen before installing anything.
 
drcampbell said:
Doing a calculation is overkill? Let's not turn this into another engineer vs. electrician thread.

I'm not suggesting installing cable that will carry locked-rotor amperage with 3% drop. I'm suggesting running the worst-case numbers and predicting what will happen before installing anything.
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Locked rotor/motor starting current surge can and will cause voltage dips and even higher voltage surges after recovering from the dip and needs consideration at times.

Some sensitive equipment like medical imaging equipment does want you to account for such transient conditions in selecting conductor sizes.

X ray machines with exposure times and peak current draw in the millisecond ranges they still want pretty good sized conductors supplying this equipment to prevent voltage drop from being an issue with the electronic controls and accessories, as well as a stable enough source there is no significant overvoltage from recovering from the surge. But this all goes well beyond the average needs of a shed a hundred feet or so from someone's house - usually.
 
Unless there is a tap changing regulator in the path there should not be any overshoot on recovery from the surge.
The other side of an MWBC, feeder or even service will often see a voltage increase during the surge.
 
GoldDigger said:
Unless there is a tap changing regulator in the path there should not be any overshoot on recovery from the surge.
The other side of an MWBC, feeder or even service will often see a voltage increase during the surge.
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Yes that is probably correct for utility supplied system and the minimal load we have here compared to the source, local generation may overshoot upon recovery from a surge.
 
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