Temperature profile of wire near a resistive heating element

Question

I am trying to understand what the temperature profile would be near a 30W resistive heater that is PID controlled to heat a surface to 250C. Correct me if I am wrong, but most wire insulation is designed with much lower temperatures in mind like 70-90C so I cannot connect a regular 2 conductor 18 AWG wire to this heater without melting the jacket.

But how far from the resistive heating element should is reasonable to assume normal wire temperatures?

I understand copper is great at conducting heat but with my 3 meter cable I imagine that the non heater end of the cable is at normal temperature.

How far away from the heater should one install special wiring?

Edit: Looking at the product there is a Molex connector just a few inches away from the heater cartridge. I am fairly certain that if the wire was still hot at this point it would melt the plastic connector. Therefore is likely no need for the second portion of the wiring to be high temperature wiring. Correct me if I am wrong though, but it puzzles me why the manufacturer would continue the high temperature wiring after the connector, except out of extreme caution.

Answer 1

Nylon PVC and a few others are not rated very high. Teflon is rated for higher temperatures but very close to 250°C. There are also glass sleeved wire insulation that have higher ratings. The datasheet for the wire should list the temperature rating.

Source: http://polyfluoroltd.blogspot.com/2011/04/seven-sides-of-ptfe-or-why-ptfe-is-way_18.html

It's really hard to say how far you'll need the insulation length to go, one of the issues being that the wire temp will be determined by the wire length, the insulation thickness, the insulation thermal conductivity and the ambient temperature around the insulation. It turns into a really big thermal engineering problem that aren't readily solved, and in my experience it usually takes as much time to do a small experiment to find out the temperature than it does to run simulations or do hand robust hand calculations.

That being said:
1 m of 18 AWG (with an 0.82 mm² or 8.2e-7 m² cross sectional area) wire will conduct about 0.065 W of heat, that isn't going to heat up much on the other end of the wire. (If you could imagine what 100 mW heater would do for heat rise.)

Source: https://thermtest.com/thermal-resources/conduction-calculator

In the high temperature designs I've seen, terminal blocks are used that are heat sunk to a lower temperature terminal block. The terminal block is mounted outside of the hot area of a device.