How the electric fence calculator works
Pick the animal, then give the fence line as a length, side by side, on the map or in acres. The animal sets the height, the number of hot wires and the post spacing. Sheep and deer fences alternate hot and ground wires, so those presets also fill in the ground wires.
The energizer is sized from the miles of hot wire, not the miles of fence. The NRCS Iowa job sheet asks for one joule of output for each mile of wire and counts every strand, and Virginia Tech gives the same one joule per mile. UGA adds 25% where weeds and brush touch the wire. Ground wires carry no charge, so they are left out.
Joules = fence length in miles × hot wires (× 1.25 with weeds)
The ground rods come from the joules. NRCS Iowa buries 3 ft of ground rod for each joule, with rods at least 10 ft apart. Virginia Tech uses at least 3 rods 6 ft deep up to 15 joules, 5 rods up to 25 joules and 7 up to 35 joules. The calculator shows both and uses the larger count, never fewer than 3.
Ground rod length = 3 ft × joules; rods = that length ÷ rod length, rounded up
A worked example: half a mile of cattle fence
A 2,640 ft cattle fence with two corners and a 14 ft gate, two hot wires at 22 and 32 in, line posts every 50 ft:
- 2,626 ft of fence times 2 wires is 5,252 ft, 0.99 miles of hot wire
- Energizer: at least 0.99 joules, or 1.24 joules if weeds reach the wire
- Ground: NRCS Iowa's 3 ft per joule is a single rod, Virginia Tech's minimum is 3, so 3 rods 10 ft apart, a ground field 20 ft long
- 5,777 ft of wire with 10% for ties and splices, 5 rolls of 1,320 ft
- 53 T-posts, 110 insulators, and 28 wood posts for the corners, the gate, the braces and 2 pull posts
The two rules part ways on big chargers. A 20 joule energizer needs 60 ft of rod by NRCS Iowa, which is 10 rods of 6 ft or 8 rods of 8 ft, while Virginia Tech asks for 5. Enter your charger's joules in the optional field to size the ground to the box you own.
How much voltage does each animal need?
The sources disagree, and the gap is wide. Extension sources mostly give the least shock that still works; manufacturer charts give a higher target.
| Animal | NRCS Iowa (minimum) | UGA | Virginia Tech | Speedrite | Powerflex |
|---|---|---|---|---|---|
| Cattle | 1,000 V | 700 V short hair, about 2,000 V long hair | 5,000 V | 2,000 to 3,000 V | 4,000 to 5,000 V |
| Horses | 1,000 V | 2,000 to 3,000 V | |||
| Sheep | 2,000 V | about 2,000 V | 7,000 V | 4,000 to 5,000 V | 7,000 to 9,000 V |
| Goats | 2,500 to 3,000 V | about 2,000 V | 4,000 to 5,000 V | 7,000 to 9,000 V | |
| Hogs | 2,000 V | ||||
| Deer | 2,500 to 3,000 V | 4,000 to 5,000 V |
Montana State asks for at least 4,000 V on a deer fence. UGA ties its higher figure to hair: about 2,000 V for long-haired cattle, sheep and goats. Treat the NRCS Iowa figures as the floor.
How high should the wires be?
Put one hot wire at nose height of the animal you want to hold, as NRCS Iowa puts it. Some layouts from the sources:
- Cows: one wire at 30 in; cows with calves at 17 and 38 in (Kentucky)
- Cow and calf cross fence: 2 wires at 22 and 32 in (University of Missouri)
- Cattle or horse perimeter: 5 wires at 10, 20, 30, 40 and 50 in (NRCS Iowa, Missouri)
- Sheep: 7 or 8 wires to about 48 in, bottom wire hot at 6 in (Oklahoma State)
- Hogs: 3 low wires between 6 and 24 in (WSU)
- Deer: about 6 ft with 8 wires, every other wire a ground (Montana State)
The drawing next to the results spaces the wires evenly up to the top wire; move them to these heights when you build.
How far apart are electric fence posts?
Much farther than on a barbed fence. NRCS Iowa allows posts up to 100 ft apart for 1 or 2 wires, up to 50 ft for 3 or 4, and posts and stays every 30 ft above 4 wires. The University of Missouri notes that a single high tensile wire can span 120 ft, but 40 to 60 ft is more realistic. Kentucky gives a practical limit of about 1,200 ft for a run of low-density polywire.
Safety around the fence
Virginia Tech puts the energizer's ground rods at least 33 ft from the ground rod of your home's electrical service or any other utility ground, so fence current stays out of the house wiring. Power the fence only from an energizer built for the job, installed as its maker describes, and never wire a fence to household current. Where neighbors, walkers or children can reach the wire along a road, path or property line, hang warning signs on it so nobody touches it by surprise. Driving ground rods and posts means digging, so call 811 a few working days ahead to have buried lines marked.
Insulators and the rest of the list
The insulator count is our estimate: one per hot wire on every line post, plus 10%. None of the sources we read gives a per-post figure. Corners, ends and gate posts also need end-strain insulators, which the list does not count.
Powerflex rates one joule for about 3 miles of average fence, so the one joule per mile rule used here leaves room to spare. For the full fence bill with other wire types, see the farm fence calculator, count wire for a non-electric fence with the barbed wire calculator, or find the fence line for a small paddock on how much fence for 1 acre. The fence calculator covers every other fence type.