Patriot heavy-duty U porcelain insulator (10 per box)

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Porcelain is what you use when the job is too much for plastic. On a system running a high output energizer, a plastic insulator can begin to track over the years, letting current creep across its surface into the post, and once that starts the whole run reads low and nothing you do at the energizer end fixes it. Porcelain does not track, does not go brittle in the sun, and does not soften. The U pattern carries a strained wire in a rounded groove, spreading the load rather than putting a sharp bend in the wire at the one point where it is under the most tension. That is why it belongs at corners, ends and gateways rather than out on the line. Ten to a box. This is deliberately not the insulator for every post on the fence. Use the cheap plastic on your straights and put porcelain where a failure means restringing a whole line: the main corners, the terminations, and the anchor points on any run with real energy behind it. They will still be there when the wire has been replaced twice.