Ok, this is interesting…

When you wear no or light armour, you gain the armour’s bonus to your Armour Class (AC), along with either your Dexterity or Intelligence modifier. So, a Wizard with no armour with a 20 Intelligence has a 15 AC to begin with.

When you wear heavy armour, you gain the armour’s bonus to your Armour Class, but you get no ability modifier.

Shields also add to AC (though they won’t have enhancement bonuses to further increase it).

Armour types:
Light
Cloth – +0 AC (but may be enchanted)
Leather – +2 AC
Hide – +3 AC
Heavy
Chain +6 AC
Scale +7 AC
Plate +8 AC
Shields
Light Shield +1 AC
Heavy Shield +2 AC