Understanding the NEC


Finding information in the NEC is very difficult if you don't understand the layout of its contents. In this exercise, we will explore some of the problems and challenges you will face while using the Code Book. Please use your Code Book for this exercise.

Example 1.

Find the ampacity of a No. 10 THHN. The key word is ampacity. Look up ampacities in the NEC index and you should see different subheadings under the boldface word ampacity. The first sub heading is Conductors 310-15, Tables 310-16 through 310-19, B-310-1 through B-310-10, 310-61 through 310-84, and 374-6.

The first (310-15) and last (374-6) listings are section numbers. The B-310-1 through B-310-10 are tables, but the B indicates that they are found in Appendix B at the back of the NEC. If you turn to the beginning of Appendix B in the back of your Code Book, the very first line tells you that the appendix is not part of the Code and that it is included only for informational purposes.

Back to the index. If we turn to Section 310-15 of the NEC, it tells us that the ampacity can be determined by one of two methods. In almost all cases we will use the method listed in subsection (a), which refers to Table 310-16. Find Table 310-16 in your Code Book, the first column (down) is the conductor size in American Wire Gauge or kcmil. Go down until you find the number 10. Now move across to the right to the THHN column (third column), you will find that No. 10 THHN has an ampacity of 40 amperes.

This ampacity is used only when there are no more than three conductors in the raceway or earth, based on an ambient temperature of 86°F (conditions listed at the top of Table 310-16). What is the No. 10 THHN ampacity if there are 6 conductors in the raceway? What if the ambient temperature is 106°F? What is ambient temperature anyway? It seems we're getting more questions than answers!

Example 2.

What is the maximum size circuit breaker (overcurrent protection device) that can be used to protect a No. 10 THHN conductor?

The Index has no reference to size, and the subheading for circuit breakers refers you to sections that will not give you the answer. Try looking up the word conductors, nothing there about breakers. Looks as if we are at a dead end. The problem is not the NEC or it's index; it's our lack of knowledge of the NEC terms. If you don't know that a circuit breaker is an overcurrent protection device (Article 240) you will go in circles trying to use the index. The key words are conductor overcurrent protection. Go to the subheading overcurrent protection under conductors. The 240-3 and 240-4 listings mean Sections 240-3 and 240-4.

Go to Section 240-3 in the NEC, now we seem to be getting somewhere. Section 240-3 contains the requirement of conductor overcurrent protection. But, all this section says is that conductors must be protected at their ampacity as specified in Section 310-15. If you go to Section 310-15, you will find a FPN (Fine Print Note) that refers you to Table 310-16.

The ampacity of No. 10 THHN is 40 amperes. Can we really put a 40- breaker on No. 10 THHN rated 40 amperes? No, the 40 amperes listed in the table has an obelisk † symbol next to 40. The obelisk refers to the note at the bottom of Table 310-16, which indicates that the maximum overcurrent protection for No. 10 conductors is 30 amperes. The No. 10 THHN conductor has an ampacity of 40 amperes, but the maximum overcurrent protection device (circuit breaker) permitted on the conductor is 30 amperes.

Example 3.

You are wiring a motor control circuit with No. 18 fixture wire (TF, TFF, TFFN). How do you know the ampacity of No. 18 TF? Why isn't the ampacity of No. 18 TF, TFF or TFFN listed in Table 310-16? Again, it seems that we're getting more questions than answers.

If you don't know that TF stands for fixture wire, you would not be able to find the answer. Now, go back to ampacities in the index. There is a subheading fixture wires, and it lists Table 402-3. If you look at Section 402-3, you will find all kinds of information; but, it doesn't mention anything about ampacity. So, where is the ampacity? Look at Section 402-5, which is immediately after Table 402-3. Why didn't the index simply say Table 402-5 instead of Table 402-3? I don't know. There are some things about the NEC that you just have to know, and the only way to learn is through practice and experience.

Many people say the Code takes you in circles; it sometimes does, but most often, you take yourself in circles. The problem generally isn't the Code, but your inexperience in using and understanding the NEC. Becoming proficient with the NEC is a matter of practice.

This book should help you understand the NEC terms, the structure of the NEC, how to use the NEC, and what the NEC means. This can get you off to a great start, but you need to practice what you are learning to become truly proficient.

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