I have a 60 watt temperature regulated induction heater that heats liquid flowing through steel tubing with an outside diameter of a quarter inch. In the prototype I wound automotive zip cord around the tubing (with the ends of the conductors cross-connected in the electrical equivalent of a center-tapped coil).
If I bring this to market it will sell as a kit and will need easy installation, so in the final product I would most likely use coiled retractile power cable. It won't require tape or shrink wrap to keep it in place; during installation the customer will simply spiral the coiled cord onto the tubing, after which it will never budge.
The requirements: a quarter inch inside diameter or slightly less, two conductors rated for at least five amps, oil resistance, and a temperature rating of 85C or more. Easy enough to find unshielded SJEOO(W) or SJOO(W) coiled power cable, but hard to find in the right size. A lot of the websites I've looked at don't specify diameter -- and where they do, the cables come half inch I.D. and bigger. I found one supplier that carries a small cable that has an inside diameter about an eighth inch. I thought I'd get some just to try it so I e-mailed the company and asked if I could buy a few feet but they said "We don't sell bulk wire". Does anybody have a source for cable in the particular size I need?