Branch Circuit Continuous Load Example
What size branch-circuit overcurrent protection device and conductor (Type NM-B) is required for a 80A continuous load (60?C Rated Conductors).
Step 1 - Size the overcurrent protection device in accordance with 210.20(A) - The branch-circuit overcurrent protection device must be sized not less than 125% of 80A.
80A x 125% = 100A [240.6(A)]
Step 2 - Select the conductor to comply with 210.19(A), which requires the branch-circuit conductor to be sized no less than 125% of the continuous load, 80A x 125% = 100A. The conductor is selected to the 60?C temperature rating per 334.80 and 110.14(C) in accordance with Table 310.15(B)(16). In this case, 1 AWG having a rating of 110A at 60?C is suitable.
Step 3 - We must ensure the conductor is protected against overcurrent in accordance with the requirements of 210.19(A)(1) for the conductors and sized per the requirements in 210.12(B) which directs you to 240.4 and the next size up rule would apply to the OCPD as the conditions of 240.4(B) are met.
Above is a revamped Mike Holt Example to reflect your example....
and their is NEVER any love in this forum....lol:jawdrop:
The next problem you run into is the scope of conductor sizes for Type NM Cable- See UL 719 below.....
5 Conductors
5.1 Only soft-annealed copper, copper-clad aluminum, or an acceptable aluminum alloy shall be used for
the conductor or conductors in a cable. Soft-annealed copper shall comply with the Standard Specification
for Soft or Annealed Copper Wire, ASTM B 3. Solid aluminum conductors in size 12 ? 8 AWG shall comply
with the requirements for aluminum-wire stock (aluminum-alloy conductor material). All other aluminum
conductors shall comply with Requirements for Aluminum Conductors of an 8000 Series Alloy, Section 10
in UL 1581. Copper-clad aluminum conductors shall comply with Requirements for Copper-Clad
Aluminum Conductors, Section 11 in UL 1581. In a given cable, all conductors shall be of the same metal.
5.2
A copper circuit conductor shall not be smaller than 14 AWG and shall not be larger than 2 AWG. An
aluminum or copper-clad aluminum circuit conductor shall not be smaller than 12 AWG and shall not be
larger than 2 AWG.
5.3 An 8 AWG or larger circuit conductor and a 6 AWG or larger grounding conductor shall be stranded.
Conductors shall comply with the requirements in Clause 4.1 of the Standard for Thermoplastic-Insulated
Wires and Cables, UL 83 except as modified in this Section.
