We noted in Part 2 of this series that if you’re working with 800A or less, you can use the next higher standard OCPD if you meet three conditions [240.4(B)]. What if you need that OCPD to be more than 800A? Any time the OCPD is rated over 800A, the ampacity of the conductors it’s protecting must be at least the rating of the OCPD [240.4(C)].
For small conductors (18AWG to 10AWG), the size and material of the conductor limit the size of the OCPD. For example, the maximum OCPD for 14 AWG copper is 15A, but for 12 AWG copper it’s 20A. If that 12 AWG conductor were aluminum or copper clad aluminum, its OCPD would be limited to 15A. But replace that 12 AWG copper conductor with a slightly more expensive 10 AWG conductor and you can run a 30A circuit.
If you’re running small conductors look them up in Sec. 240.4(D)(1) through (7) of the NEC. Contracting firms can eliminate repeated calculations by using the Sec. 240.4(D) values as “calculation unneeded” standards.
