You can only imbue armor resists to what the base for the item would be, without exceptional bonuses, +15.
If I remember correctly, a Bascinet is 7/2/2/2/2 resists normally.
So, you could imbue physical to 22, any of the others to 17.
That's why, if you are going to imbue resists on an armor piece, you want to start with an armor piece that has high stats in the other resists from the exceptional bonuses, then imbue points into the low ones.
For example, if you want an item that has high fire & poison resists, you want to look for one that has a runic bonus to neither, or perhaps one of them, with the EX bonus scattered in the ones ou aren't going to change the value of.