Electrical Testing
Tip of the Week: The Value of Annex C in NFPA 70 (NEC)

Tip of the Week: The Value of Annex C in NFPA 70 (NEC)

This annex consists of fill tables for conduit and tubing.

After the body of the NEC, concluding with Chapter 9, you’ll find a series of “Informative Annexes.” In previous issues, we looked at Annexes A and B. Now it’s time to see what’s in Annex C.

As with Annex B, Annex C consists of tables. But whereas Annex B consists of ampacity tables, Annex C consists of fill tables for conduit and tubing. If your conductors or fixtures wires are all the same size, you can use one of these tables to see how many of a given conductor of a given size you can run in a given size of conduit or tubing.

There are 24 tables. Tables C.1 through C.12 (with no additional nomenclature) are based on Chapter 9, Tables 4 and 5. What about the other 12 tables? Each is a variation of Tables C.1 through C.12. The differences:

  • They are based on Chapter 9, Tables 4 and 5(A).
  • Each has a suffix of (A), thus Table C.1(A), C.2(A), C.3(A), and so on.
  • The conductors for the (A) tables are “conductors of the compact type.”

What is a “compact type” of conductor? If we look at the notes under Table C1, we read that table is for concentric stranded conductors only. This gives us a clue that compact conductors are not concentric in their stranding. But what are they? The note also says to use Table C.1(A). The note under Table C.1(A) says that compact stranding is “...the result of a manufacturing process where the stranded conductor is compressed to the extent that the interstices (voids between the wires) are virtually eliminated.”

You’ll find the same set of notes for each of the other Table pairs (e.g., C.2 and C.2(A), C.3 and C.3(A), and so on).

If you compare Table C.1 and C.2 values, you’ll notice that there’s not much difference until the trade size gets to be fairly large. Basic geometry explains this, and so does basic algebra. A small reduction in the conductor size doesn’t save enough volume in the tubing relative to the tubing volume to permit adding one more conductor. Put another way, 10% of four isn’t going to equal an entire unit but 10% of ten will (using those numbers only to illustrate the point, not to refer to the actual effects at scale).

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