Section 230.42/Minimum Size of Service-Entrance Conductors
NEC 230.42 establishes minimum service-entrance conductor sizing based on connected load calculations and the rating of the service disconnecting means.
Service-entrance conductors must have an ampacity not less than the maximum load to be served, calculated per Article 220, and not less than the rating of the service disconnecting means. For a single-phase, 120/240V, 3-wire residential service, the minimum conductor size is determined by the calculated load or the disconnect rating, whichever is larger. Conductors must also not be smaller than specified in Table 310.16 for the corresponding ampacity. Section 230.42(B) sets absolute minimums: for single-family dwellings, the minimum service size is 100 amperes. For all other installations, the minimum ungrounded conductor size is 8 AWG copper or 6 AWG aluminum.
Why this section exists
Service-entrance conductors carry the entire building electrical load from the utility connection to the main disconnect. Undersized conductors create fire risk from overheating and voltage drop that damages equipment. The minimum sizing requirements ensure that conductors are matched to the actual calculated load and the protective device rating, with absolute minimums for dwelling units that reflect modern electrical demands including HVAC, cooking, and electric vehicle charging.
What plan reviewers look for
Plan reviewers verify the service load calculation per Article 220 and confirm that the specified service-entrance conductor size has ampacity equal to or greater than the calculated load (with the 125% continuous load factor applied where applicable). They check that the conductor ampacity matches or exceeds the rating of the main disconnect. For residential services, they verify the 100-ampere minimum is met. They also check conductor temperature ratings against Table 310.16 and terminal temperature limits per Section 110.14.