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Yet another long run voltage calcluation question

#1
06-19-22, 08:18 AM
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Yet another long run voltage calcluation question

I am planning to run power from the house to a pole barn about 400 feet away.
Figuring 450 foot total conductor length.

Is it feasible to use transformers to step up voltage at the house and step down voltage at the barn to reduce wire size requirements?

The main panel is in a new garage attached to the house. When it was built I had the electrician put a 2" PVC LB to the outside, not realizing at the time how large a high amp service to the barn would need to be

I am hoping to run enough power to be able to run big tools, like a 5HP compressor, a welder etc. I was originally thinking I would need 150 Amp service.
The calculator I used indicates I would need 500kcmil wire for a 150 amp service and 3% drop over the distance. To run 3 conductors would need 4 inch conduit.
I am not sure how I would get 4 inch conduit connected to my main panel let alone run and connect the 500kcmil cable.

If I run 100 amp, I can use 4/0 copper but still need 2-1/2 or 3" conduit depending on number of conductors.

With the current 2" conduit through the exterior wall of the garage, it looks like I would be limited to a 70 Amp service over 2/0 copper. I can put three conductors into 2" conduit

I looked into having the power company put a second meter direct to the barn but they told me it would have to be billed as a business account which is more expensive.

Electrician quoted almost 10K just to install the 150 amp service, exclusive of trenching or boring cost.

#2
06-19-22, 10:02 AM
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You need to do a load calculation of what would be the most likely highest total amp load at one time. I venture to guess you don't need 150A.
Per conduit fill you can put four #4/0 XHHW-2 conductors in 2". But I dare say that's a hell of a pull at 400ft.

#3
06-19-22, 10:47 AM
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Yes indeed it would. I could run larger conduit to barn and just have the very short run of 2" from ground level to the box. 4/0 gets me to 100 Amps with 2.79% voltage drop.

I assume I am entering the values correctly in the calculator. Using PVC conduit, .85 power factor, 240V, single phase, single set of conductors, one way distance of 450 feet

#4
06-19-22, 11:07 AM
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Just some general things.... you need to upsize wire size due to length but you can tap a smaller size at the ends to fit in devices. It only needs to be small enough to fit in the breaker. Reduces the size of the conduit needed to enter the building too. Requires a splice box to change sizes.

At the horizontal run ends..... use galvanized or IMC sweeps as PVC can melt with excessive pulls.

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#5
06-19-22, 11:21 AM
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If this is a standard barn service with only a couple of items running at one time I doubt you will even come close to 100 amps load. Voltage is determined by the load on the system, not the available load. Your calculation using 100 amp for the load would be the maximum that the load can be to keep the voltage drop to less than 3%. If the voltage drop gets higher than that nothing really happens, just motors work a little harder.

Since this will be a sub-panel and not a separate service you will need to run a ground wire as well. So you will need 3 @ 4/0 XHHW (hot, hot, neutral) and 1/0 XHHW for the ground wire. (Ground must be increased in size proportional to the other conductors)

The other issue you may run into is getting a breaker to feed the barn. In most cases, the largest plug-in breaker you will find is a 100 or 125 amp tow pole breaker.

I know you said a separate service to the barn would be more expensive but have you seen the price of wire right now? 4/0 XHHW is about \$2 per foot and 3/0 copper is almost \$5 per foot.

Lastly, you may want to consider doing direct burial for such a long run. I typically like to use conduit in the ground, especially with aluminum wires, but unless you have something to help pull in the wire that is going to be a tough pull.

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#6
06-19-22, 11:35 AM
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As Tolyn mentioned, direct bury is an option. 4/0 MHF is something to consider.

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#7
06-19-22, 02:00 PM
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Thanks very much you have all given me something to think about.
The main panel has an existing 150 two pole feeding the house. Garage was an add on.
However, looking at low end welder loads and 60 gal compressor loads, a 100 amp service would probably be more than enough. Barn is going to be my workshop for heavy dirty work so I don't want to find myself constrained by power limitations.

The intervening space between the house and the barn is fairly heavily wooded. I think trenching would be pretty tough and not a very straight line. I was thinking of directional boring from the barn back to the house. Could probably have the borer pull the direct burial cable on the backhaul instead of conduit.

I noticed nobody mentioned the feasibility of using transformers, not a common solution?

#8
06-19-22, 02:50 PM
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Transformers are an option but would likely not be cost-effective. You will pay many thousands more for the transformers than upsizing the wires.

I have never had a directional boring crew pull in the wire. I always have them pull in a 2" tube and I run my stuff in that. They will add a pull rope if that is something you need.

Unless you are adding some kind of electric heat you should be fine with 100 amps. As I mentioned it is the actual load you have to calculate for so if you have a 150 amp two pole available, I see no reason you can't use that.