Although Annex B is not part of the NEC requirements, it does provide information that can help you with NEC compliance. The Chapter 3 ampacity tables don't address every type of installation. If yours isn't covered by a table, how can you get the correct minimum ampacity?

If the problem is simply that you have more than three conductors, the answer is to use those Chapter 3 tables and just derate the ampacities per Table 310.15(B)(2)(a). For other applications not covered by these tables, you'll need to determine ampacity under "engineering supervision." The NEC helps clarify what that entails in Annex B.

The bulk of Annex B consists of tables that provide "typical ampacities for conductors rated 0V through 2000V"…duct bank tables, and…"conductors rated up to 0V to 5,000V."

Preceding these tables, you will find about 1ࡩ pages that explain how to determine ampacity, using the subsequent tables. Those pages refer to four standards, which the engineer determining ampacity should consult. Two of them focus on ampacity calculations, and one contains ampacity tables. The fourth is a soil resistivity reference, which helps with buried applications.

If you look at Tables B.310.5 through B.310.7, you will see "Rho" repeated over and over. While this may remind you of a well-known nursery rhyme, it has nothing to do with boats going gently down the stream. Rho is the thermal resistivity value, which is a measure of the heat transfer capability of a substance by conduction. For polyvinyl chloride (PVC), a common insulation material, Rho is 650. It’s 500 for rubber and 450 for polyethylene.