Article 210 of the 2014 NEC provides rules for minimum conductor sizes [210.19]. For circuits over 600V, the rules are pretty simple:
- For unsupervised installations, the ampacity must be at least 125% of the designed potential load of utilization equipment that will operate simultaneously.
- For supervised installations, qualified persons under engineering supervision can determine the size 210.19(B)]. But only if:
- Conditions of design and installation are provided under engineering supervision.
- Persons with documented training and experience in over 600V systems provide maintenance, monitoring, and servicing.
For circuits under 600V, the rules are more extensive.
Before those are even presented, we encounter not one, but four Informational Notes. One of these is the famous note on voltage drop, which many people misunderstand to be an NEC requirement. The fact these Informational Notes are before the text rather than after it is noteworthy. The other three refer you to other requirements in the NEC; keep these in mind when reading the branch circuit requirements.
Branch-circuit conductors must have an ampacity at least as large as the load to be served [210.19(A)(1)]. This differs from that of feeder conductors, which must have an ampacity of the noncontinous load plus 125% of the continous load. But if the load to be served is larger (after application of adjustment factors), use that as the load for ampacity purposes.