That depends on the type of joint/ size of weld.
You can weld 3 inch plate with 120 amps- as long as you have beveled edges at the joint and make multiple passes.
Sure, it'll take a while, but it can be welded, and welded properly.
How much can 180 amps weld in a single pass ?
I suppose you could weld 1/2 plate, using 5/32 (or maybe 3/16) rod, at 180 amps-using no bevels and just a single pass, but even then your resulting weld won't carry the same load as the base material.