Annex C of the NEC is 58 pages long. That's even longer, by a significant percentage, than Art. 430. It's about twice the size of Art. 250. Why would the NEC devote so much page “real estate” to this resource? Is there a compelling reason? Yes. Having this information handy (in the back of the NEC) can save you time and prevent expensive rework. Annex C provides fill tables for two common families of raceway: conduit and tubing.
- Conduit is a wiring method covered by Art. 342 through Art. 356. Each of the nine articles addresses one of the nine types of conduit.
- Tubing is a wiring method covered by Art. 358 through Art. 362. Each of the three articles addresses one of the three types of tubing.
Annex C covers the tubing types first, in C.1, C.2, and C.3. You'll also notice C.1(A), C.2(A), and C.3(A). The A designation is for compact conductors.
As you might expect, the other nine subparts correspond to the nine conduit articles in Chapter 3. Each of these also has an A designation.
The Annex C tables are for conductors of the same size. This is more widely applicable than it might seem. Similar circuits (e.g., lighting) grouped in raceway (e.g., for backbones) tend to have same-size conductors.
Where minor variations occur, you can adapt by running (typically) one less circuit in the raceway, using the next size raceway, or using separate raceway for the larger conductors.
As with other NEC limits, engineering considerations may call for exceeding NEC requirements. For example, it may be prudent to use less fill for high harmonic loads.