erc-startup-file-list

erc-startup-file-list is a variable defined in `erc.el'.
Its value is

("~/.emacs.d/.ercrc.el" "~/.emacs.d/.ercrc" "~/.ercrc.el" "~/.ercrc" ".ercrc.el" ".ercrc")

Documentation:
List of files to try for a startup script.
The first existent and readable one will get executed.

If the filename ends with `.el' it is presumed to be an Emacs Lisp
script and it gets (load)ed. Otherwise it is treated as a bunch of
regular IRC commands.

You can customize this variable.