My idea is this (i haven't played factorio much so numbers will be probably way off).
I know about 3 types of power lines (haven't played much).
My suggestion: basic wooden power line - 1 visible cable (as it is now) 1 or 5MW power limit for cable.
7x7 power line will have 2 visible cables when connected to each other and 5 or 25MW power limit.
Biggest power cables will have 3 visible cables when connected and 25 or 100MW power limit.
What do you say?

Sorry if it's already in game or suggested, i didn't find it when i was searching if it was already suggested or existed in game.