Size and rating of Service-Entrance Conductors, NEC 230-42

(a) General. Service-entrance conductors shall be of sufficient size to carry the loads as computed in Article 220. Ampacity shall be determined from Section 310-15. 

Exception: The maximum allowable current of approved busways shall be that value for which the busway has been listed or labeled. 

(b) Ungrounded Conductors. Ungrounded conductors shall have an ampacity of not less than: 

(1) 100 amperes, 3-wire for a service to a one-family dwelling with six or more 2-wire branch circuits.

(2) 100 amperes, 3-wire for a service to a one-family dwelling with an initial net computed load of 10 kVA or more. 

(3) 60 amperes for other loads. 

Exception No. 1: For loads consisting of not more than two 2-wire branch circuits, No. 8 copper or No. 6 aluminum or copper-clad aluminum.

Exception No. 2: By special permission, for loads limited by demand or by the source of supply, No. 8 copper or No. 6 aluminum or copper-clad aluminum.

Exception No. 3: For limited loads of a single branch circuit, No. 12 copper or No. 10 aluminum or copper-clad aluminum, but in no case smaller than the branch-circuit conductors.