Actually,
There are "proper" knives for stripping cables and large wires.
Being a Liney, I own one of these things, how it differs to your stock standard knife, is that it has a T-shaped piece welded to the end of the blade, not only so that it only cuts the insulation (and not the conductor material), but if you slip with it, you aren't going to get the end of the knife stuck into a part of your body, where it could do some really hideous damage.
My one sort of looks something like this:
LINK 
