There's a current issue with Optifine and Sphax, well actually it would be the same for any and all texture packs but Sphax is the most complete out of all of them and because of that atm it's specific to Sphax.
The problem is Optifine seems to limit a single folder to 1024 textures (or something like that), when you hit more textures it crashes the game or if you are in game it will refuse to load the TP. Sp614x (Optifine modder) knows about this, Sphax knows about this, atm there's no fix other than to remove Tinkers Construct textures and any other textures from the Sphax pack which have more than 1024 files or don't use Optifine.