I need a root canal on top tooth. one of the premolars( 3rd or 5 tooth from the middle). i hate injections in my mouth. i can handle usual injection in jaw or gum line but i'm wondering if i'll need a hard palate injection. what determines the need?

For root canal you only need the local injection (into the gum), no need for nerve root injection.

If the "usual" infiltration does not seep deep enough into the bone to anesthetise the palatal nerves innervating that tooth, you'll need a palatal injection. As I previously answered, it only lasts a second or two and it is really not that bad. I doubt you would want the procedure performed if it is needed.