Know the requirements and exceptions to 230.41.
It seems obvious that service entrance conductors must be insulated, but the NEC states it anyhow [230.41]. This seems to contradict the construction requirements for Type SE (Service Entrance) and Type USE (Underground Service Entrance) cables [338.100]. But you can resolve this apparent conflict by reading the five exceptions to 230.41 and by reading the definitions of Type SE and Type USE cable [338.2].
After you determine the loads per the applicable part(s) of Art. 220, you can determine the minimum ampacity from 310.15. But note that you may have to exceed that 310.15 value because the ampacity can never be less than any of the sum of any of the noncontinuous loads plus [230.42(A)]:
- 125% of the continuous loads. Exception: Grounded conductors not connected to an overcurrent protection device (OCPD) can be sized without the 125% rule.
- The continuous load after the application of any adjustment or correction factors. This is new with the 2014 NEC and you may have noticed it doesn’t include the 125% rule.
- The continuous load, if the service entrance conductors terminate in an OCPD where both the OCPD and its assembly are listed for 100% of their rating.
Nor can they be less than the rating of the service disconnect specified in 230.79. Size grounded conductors per 250.24(C).