Do I need to replace this wire?
I want to replace and old style screw in fuse box with a new 100a service panel to a detached garage. The Main feeder wire is probably from the 50's - 60's it is not romex. It is two large gauge wires wrapped with an outer ground wire and then encased in some type of plastic casing. The wire is copper but appears siver until you scratch it. So there are a total of 3 wires.
To replace all if it with thhn would be really expensive. What are my options here? Can I use the existing wire or does it need to be upgraded to work with the new service panel?