withstanding heat

You won't be able to use standard PCV (or similar) insulated and sheathed cable as it will just melt at these temperatures. Covering the cable it in asbestos rope won't help (the individual wires inside the cable will still melt and short together).

Separating the individual conductors inside the cable and insulating them separately will help, but the best way is to actually use wire that is manufactured with insulation designed to handle 250 deg C (such as the following).

http://www.radix-wire.com/250.htm

Try a google search (or similar) for "high temperature cable" and "high temperature wire".

Use copper sheathed copper cored mineral insulated cable sometimes know as 'pyrotenics' type cable. Manufactured with various conductor sizes and number of cores.

You can try FIBRE GLASS WRAPPING

Do you still use Asbestos?

Do solve this problem decently: you need a heat resistant cable. (not fire resistant!!)

Options are MI power cable (PYROTENAX is a known make)

and teflon (PTFE) insulated power cables.

There will be other possibilities but make sure you buy one which is designed to work at least 20°C higher than the maximal temperature you expect it to see in it's life.