If theres no pain and nerve isnt damaged, do i need a root canal if my cavitys big and the dentist needs to drill past the nerve to clean the tooth?

Most likely yes: If your dentist needs to drill into the pulp chamber where the nerve is located to remove the decay s/he will introduce bacteria from the carious lesion into the nerve and blood vessels. Just because you are not in pain does not mean you do not need root canal therapy. Not all medical conditions have pain associated with them (high blood pressure, diabetes, and some skin cancers). Need to treat.