Electrically, what causes a fuse to melt?

Electrically - what causes a fuse to melt instead of break. I've heard it is loose connections, which to me, would be adding resistance to the circuit ... correct? I assume the circuit is just dissipating extra heat, but not extra current ... is this right?

The element in a fuse is just a wire... same as the 4 gauge wire it is protecting. It is a smaller gauge of wire, sure, but of a known melting point and resistance. This is why the elements in fuses are all different size, and the smaller the wire, the higher the temperature for a given amount of current running through it. A 100A fuse, for example might offer a resistance of .033 ohms, but at 100A running through it, that is a dissipation of 3.3 watts. 3.3 watts with no heatsink, in a glass tube, with no air convection, and it will eventually get hot enough to melt... That's pretty much it, really...It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

I gotcha. I've seen whole fuseholders melt, could something this simple cause it? Customers always ask me why, and I cant come up with the best reason. They always just assume they should up their fuse size.

If the circuit's resistance doubles, it can only carry half the power, right? so is the other half of the power dissipated as heat?

um...they melt because most these days are made of crappy plastic that melts...If you get a man's fuse holder like this the holder wouldn't even break a sweat lol http://www.bluesea.com/product.asp?Product_Id=23471&d_Id=&l1=&l2=

ive never seen an ANL fuse holder melt, ive only seen the agu and maxi fuses/holders melt.

I've need my friends fuse holder melt...woulda burned to hell but he noticed smoke coming from his hood...burned a ridge in his air filter box too lol