Planning an Electric Fence: Wire, Insulators and Energiser Size

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An electric fence is a shopping list plus one big decision: the energiser. This guide gets you to a sensible list and a starting rating to take to the shop or the maker's chart.

1. Step 1: the fence length

• For a rectangular paddock: length = 2 × (length + width).
• For any other shape: walk or measure the whole line. For an open line, measure it end to end.

2. Step 2: the live wire

Multiply the fence length by the number of live (hot) wires or tapes, then add a bit extra for ties, gate leads, joins and sag. The calculator adds 5% as a starting point.

Live wire = fence length × number of live wires × (1 + extra).

Then divide by the roll length on the label, and round up.

3. Step 3: posts and insulators

• Posts carrying insulators: fence length ÷ post spacing, rounded up. For an open line, add one more for the far end.
• Insulators: one per live wire per post.

The post spacing depends on the stock, the ground and the wire or tape you use, so take it from your plan or the maker's guide. The calculator starts at 10 m.

4. Step 4: the energiser rating

Energisers are rated by the maker in kilometres of fence and in joules. The kilometre rating on the box is usually for a clean test fence. Real fences lose a lot of the pulse to weeds, long grass and dry ground, so they need more.

The calculator gives you a starting point in two ways:

• Kilometres to look for: the total live wire in km, plus headroom. The calculator starts at 50% so the fence still bites with weeds, dry ground or later extensions.
• A rough joules figure: that rating divided by a rule of thumb. Common rules range from about 3 km per joule with heavy weeds to about 10 km per joule on a clean fence. The calculator starts at 5.

These are rules of thumb, not a sizing method. Choose the energiser from the maker's ratings for your fence, your stock and your conditions.

5. Worked example: a 200 m × 100 m paddock

Using the calculator's starting figures: two live wires, posts every 10 m, 5% extra wire, 500 m rolls, 50% headroom and 5 km per joule.

Take "rated 1.9 km or more, around 0.4 joules" to the maker's chart as a starting point. Then go up for weeds, long runs or hard-to-hold stock.

6. Earthing

Earthing matters as much as the energiser. A big energiser on a poor earth gives a weak fence. Follow the energiser maker's earthing instructions for the number, length and spacing of earth stakes, especially in dry or sandy ground.

7. Safety and the rules

• Use an approved energiser.
• Put warning signs on the fence where the public may come near it, such as along roads and walkways, and follow your state's rules.
• Never use barbed wire as a live wire.
• Mains energisers plug into a standard 240V AC outlet. Any new 240V AC wiring or outdoor outlets for them must be done by a licensed electrician.
• Solar or battery energisers: follow the maker's instructions for the panel, battery and mounting.

8. Common mistakes

• Buying an energiser on the box's kilometre rating for a weedy fence.
• Skimping on earth stakes.
• Forgetting the extra wire for gate leads and joins.
• Counting insulators per post instead of per wire per post.
• Leaving out warning signs along a road or track.

9. Quick summary

• Live wire = length × live wires × a little extra.
• Insulators = posts × live wires.
• Look for an energiser rated well above your wire length, because weeds drain the pulse.
• Earthing is as important as the energiser.
• Approved energiser, warning signs, no barbed wire live, and a licensed electrician for any new 240V AC outlet.

Try it with your own paddock: Electric Fence Calculator. For posts, strainers and plain or barbed wire, see How to Estimate Fencing Materials.

General information only. Follow the energiser maker's instructions and your state's rules.