Electric fence design

Snow is melting (except when it’s falling), spring is in the air, and we might go down to Maine one of these next couple weekends to do some tractor maintenance and prep work for the fence. Up to now we’ve cleared to the fence line, pulled the stumps, and left four well-placed trees to serve as corner posts. The basic fencing concept is to make a stout electric fence with as little permanent infrastructure as possible; given that the orchard may expand in coming years, it would be great to be able to reconfigure it with relatively little work.

One of the things that irk me about fences and trees is when you see a nice big tree with fence wire buried deep in the bark; it lets disease into the bark, it’s dangerous to fell it, no sawyer will touch it, and it’s more trouble than its worth even to turn the butt into firewood – it is destined to lay around for decades. So, my concept goes like this: Lay a treated 8′ 4×4 up against each corner tree, plumb it up, and fasten it to the tree with three large galvanized lag bolts. Depending on the cant of the trunk, it may have to be spaced away from the tree at one end or the other, but the bolts will be big enough to take some shear. As the tree grows, it will close the gap between the bark and the 4x, but so long as you remember to back off the lags a couple turns every few years, the tree will be fine, and even if you abandoned it entirely it would have to grow over a foot in diameter before the lags were unrecoverable. Anyway, the next step is to install corner insulators on the 4x4s at intervals suggested by Montana State, and using insulators like these.

I like the idea of being able to slack the wires and gather them up at the top in order to pass tractors and the like underneath, so rather than make the conductor topologically captive inside the loop of wire that supports the corner insulator, I’m toying with the idea of chopping the heads off of 5/16 lag bolts, bending them 90 degrees, and screwing them part way into the 4×4 (wish I had access to that hydraulic ironworker ;). Now the wire can be removed from the insulator and tied up. Threaded galvanized L bolts are available (like for foundations) but I haven’t seen L lag bolts – surely they’re available somewhere, but probably not at Roger’s Hardware.

So, now we have corner posts in place. The next big puzzle is the gate. The main concerns are that it be tall and wide enough to allow whatever farm equipment we might want to bring in to pass, and strong enough to take the combined tension of all the fence conductors. I think the thing to do is to make it in the form of an inverted U of 6×6. The top timber serves to carry the tension from one side to the other. Since the bedrock is only about 2′ down, we can dig down and pin the lower ends of the posts into the rock for a really secure installation if we want. One side gets insulated terminations, the other side gets the ratchet tensioners. The doors to the gates will swing outward, and there probably need to be some diagonal 4×4 braces from the gate posts to the ground to take the lateral loads from the swinging of the gate.

Finally, there’s the issue of the midspan supports. The point of high-tension electric fence is to minimize the need for posts, but since the ground is not perfectly flat we will want smaller posts along the 120-180′ runs to keep the wires spaced apart and conformal to the ground. I was thinking of driving PT 2x4s into the ground and putting plain old straight-run fence insulators on them. These might be supported at the top by a stout (3/16″?) piece of galvanized aircraft cable running around the perimeter between corner posts and winched tight.


One Response to “Electric fence design”

  1. Keith Says:

    Lag bolts or lag screws šŸ˜‰ FWIW, I have never seen l L-lags either, aside from gate hinge parts like:

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