Voltage drop and wire size

[Treemonkey1000]

There are better links but this is a quick and dirty one that you can plug some numbers into

Voltage Drop Calculator

 

[Al Carr]

I'm new,
and I have to disagree with "n8586m's" recommended wire size of #12 for a 300' run.

And I'll tell you why,

300' from the house someone is going to need a drill, saw, battery charger or something that draws higher current than those 4 lights, it's just going to be handy there, so Bob (or who ever is you favorite name for these people that takes things on thereselves) sorry Bob) desides to put a recepticle in. Bob always does this.
And Bob is everywhere. May not be today or tomorrow but
it will happen.

Run #10 and give him 4 lights and a recepticle and he will be happy.

Again, sorry Bob

 

[JiminGa]

one should also note that some state's requirements are more restrictive towards drop than NEC standards. check your local codes

 

[AndyMA]

It really boils down to how much drop is acceptable. Is the person willing to accept a 5% drop or a 6 or more percent drop? NEC suggests a 5% or less drop, and that is a good rule of thumb. I have found that in some cases a drop larger than 5% can produce acceptable results.

There is actually quite a bit more involved in conductor selection that just voltage drop. Anytime you have a drop you also have heat generated. This raises the temp of both the conductor and insulation. Deponding on the type of insulation and how it is used even a modest voltage drop can overheat a conductor and it's insulation. A conductor in free air behaves quite differently than one in romex, or one in various sizes of conduit, etc.

Andy (an EE who's designed in lot's of conductors)

 

[ray66v]

There is actually quite a bit more involved in conductor selection that just voltage drop. Anytime you have a drop you also have heat generated. This raises the temp of both the conductor and insulation. Deponding on the type of insulation and how it is used even a modest voltage drop can overheat a conductor and it's insulation. A conductor in free air behaves quite differently than one in romex, or one in various sizes of conduit, etc.

Andy (an EE who's designed in lot's of conductors)

Yes Andy, I am aware that a drop can reach a point of overheat. And then of course that would be unacceptable.
I believe the fact that the codes allow for some flexibility on these matters is because each situation is different. (Point I was trying to make).
I prefer to do things to a point that I can be confident there will be not problems. However, out here in the country, I have seen things done that I would not do, and they have worked fine.
I wish we could all have the benefit of an EE to evaluate our particular situation. Although, I have found that asking one of the many different engineers I know, a simple question, usually results in an answer that would make a Politician proud.

 

[n8586m]

If you are going to change the load by adding a receptacle, then you have to re-do the calculations. If you are going to add a 15 amp circuit at the end of the 300 foot run, the circular mill calculation comes out to 19,350 at a 5% voltage drop. No. 8 wire is 16,510 which would be too small, so you would need No. 6, which is 26,240. The point is, you just can't make a guess and hope to be right, you have to do the actual calculations.

The run will probably be under ground either as direct burial or in conduit. In either case, the ambient temperature will not exceed 60 degrees C, so no consideration for insulation other than standard thhn or romex will be required.

 

[ray66v]

If you are going to change the load by adding a receptacle, then you have to re-do the calculations. If you are going to add a 15 amp circuit at the end of the 300 foot run, the circular mill calculation comes out to 19,350 at a 5% voltage drop. No. 8 wire is 16,510 which would be too small, so you would need No. 6, which is 26,240.

The price difference between the #12 that will work for lighting, from the original calculation. and the #6 that you would need to have the 15 amp receptacle? #6 is approximately 4.5 times as expensive. Multiplied over 300", plus the larger conduit, equals a lot of money. Like around $500 more, better be sure you want that outlet.

 

[swampvol]

What a lot of people don't consider, engineers included, is when determining voltage drop you need to know what the voltage is, under load, at the tap especially when supplying motors.

Don't assume that because you're tapping from your panel box that you're guaranteed 120/240 volts at your starting point. Too many times I have seen a 3000 sq.ft house with a 200 amp service being fed with a 100 ft. drop of #1 aluminum wire from the POCO. The transformer that feeds your house is the starting point.

 

[swampvol]

Also, when supplying a motor that comes from the factory set at 120, convert it to 240 if possible, keeps the voltage drop down and the motor runs cooler. Most well pumps are multi-tapped.