Extension speaker cabinet wiring. Do I need speaker cable or instrument cable?

[Zappacat]

I just built a speaker extension cabinet for a friend of mine.  12" cannabis rex for a Fender Super Champ XD.  I'm mounting a 1/4" mono jack on the external cabinet so he can unplug the speaker out coming from the champ and use a 1/4" mono male to male cable to connect the external cabinet to the champ XD.  Does the cable need to be SPEAKER cable or will a regular instrument cable work just as well?  Thanks in advance.

[Enzo]

Speakers ALWAYS need to be connected with speaker cable.

The wire inside an instrument cable is teeny tiny stuff.