# COMMUNITY FORUM

royb

05:32AM | 12/20/02
Member Since: 12/19/02
Wanting to run a service to the barn from the house. The house has 400 amp service, barn is 500 ft away. If I have done my homework, I will need #2 for 50 amp service in barn, with a #4 Neutral and #4 ground (I think I could go smaller on the Neutral and ground, but feel better with larger given the distance).

The question, the easiest path is "L" shape to avoid water and sewer lines (and trees). I would like to run it underground in a straight run 400' to a disconect on a pole then go 100' overhead to the barn to eliminate a trun in the conduit. Is there any reason to reduce the conductor size when going from the disconnect to the barn or should it all be #2?

Also, when going overhead do you still have the Neutral and two conductors and use the support wire for the ground?

thanks,

Roy

dana1028

04:33PM | 12/20/02
Member Since: 08/30/02
Roy -

I use Mike Holt's formulas for sizing conductors (to adjust for voltage drop over long distances [www.mikeholt.com]).

The standard formula for sizing a conductor to compensate for distance and stay within the generally accepted 3% voltage drop limitation is:

CM = 2 x K x I x D/volts dropped

CM = circular mils (area of a conductor in circular mils taken from NEC Chap 9, Table 8).

2 (2 conductors)
K = the constant K value is 12.9 ohms for copper
I = amperes (50 in your case)
D = distance
volts dropped = 3% x 240 = 7.2 volts
thus....

2x12.9x50x500/7.2 = 89,583 circular mils...the smallest conductor that size is 1/0!

I've done this using 3 different formulas and all the answers indicate 1/0 conductor size to keep the voltage drop within limits.

IF my arithmetic is correct, then your #4 ground (equipment ground) is the minimum size also (250-122(b) requires the equipment ground to be proportionately oversized ..."where conductors are adjusted in size to compensate for voltage drop, equipment grounding conductors, where installed, shall be adjusted proportionately according to circular mil area)....I won't get into the arithmetic for that but my calculations indicate #4 is needed.

...I suspect your neutral is way too small...won't get into that formula.

royb

12:27PM | 12/22/02
Member Since: 12/19/02
I should have stated that I did the calculation based on 5% which would give 53,750 and #2 is large enough. With that said, I do not know what is allowable as far as voltage drop and I also do not NEED 50 amps but I knew #4 would not be enough. It looks like I could go to 30 amps and use #2 or go with the 1/0. Thanks for pointing it out. Now for the rest of the questions?

And yes I am trying to get an electrician to do it, but with the Holidays, and the building demands most don't want to fool with something this small. I really just want to be ready with a reasonable plan when I do get one out here so I don't take up too much of their time.

Roy

tdhorne

05:27AM | 12/26/02
Member Since: 09/01/02
quote:
Also, when going overhead do you still have the Neutral and two conductors and use the support wire for the ground?

Yes, that is definitely best practice. That type of overhead wiring is called quadplex. It is also a good idea to install a grounding electrode at the yard pole to provide a degree of lightning protection to the overhead conductors. The Grounding Electrode Conductor (EGC) should be run up the pole and connected to the Equipment Grounding Conductor (EGC) at the drip loop.

Since you are running conduit to the yard pole you might want to consider running a bare number two copper conductor in that trench to serve as a supplemental grounding electrode. If you dig the trench to a depth of three feet instead of the code minimum one and one half foot depth it will make such an electrode much more effective. This will serve as a very low impedance electrode for both your home and your barn and render the entire electrical system far more surge and spike resistive. The electrode conductor is run from the bonded buss bar in your homes service equipment enclosure to the bonded buss bar of the EGC drip loop on the pole. This technique should not be used if the ground through which the conduit is run is pasture or other live stock holding area because the animals waste would be too corrosive to the bare copper.

You may want to consider running a second conduit at the two foot level if you will need communications or alarm wiring to the barn in the future. Remember to install warning tape one foot above the conduit during back filling.
--
Tom

tdhorne

05:47AM | 12/26/02
Member Since: 09/01/02
Be advised that you are permitted to use rigid metal conduit elbows (Sweeps) in the underground conduit run to avoid the possibility of cutting through the plastic elbows during the pulling of conductors as long as the elbows are buried at least eighteen inches to the top of the elbow. This is another good reason to make the trench deeper than the eighteen inch minimum permitted by the code for PVC conduit.
--
Tom

250-86. Other Conductor Enclosures and Raceways
Except as permitted by Section 250-112(i), metal enclosures and raceways for other than service conductors shall be grounded.
Exception No. 3: A metal elbow that is installed in an underground installation of rigid nonmetallic conduit and is isolated from possible contact by a minimum cover of 18 in. (457 mm) to any part of the elbow shall not be required to be grounded.

bigfoot

12:40PM | 12/26/02
Member Since: 12/21/02