erc-encoding-coding-alist

erc-encoding-coding-alist is a variable defined in `erc-backend.el'.
Its value is

nil

Documentation:
Alist of target regexp and coding-system pairs to use.
This overrides `erc-server-coding-system' depending on the
current target as returned by `erc-default-target'.

Example: If you know that the channel #linux-ru uses the coding-system
`cyrillic-koi8', then add '("#linux-ru" . cyrillic-koi8) to the
alist.

You can customize this variable.