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National Electrical Code Articles and Information

National Electrical Code Top Ten Tips: Article 310 -- Conductors for General Wiring

by Mark Lamendola

Please note, we do quote from copyrighted material. While the NFPA does allow such quotes, it does so only for the purposes of education regarding the National Electrical Code. This article is not a substitute for the NEC.

These are the 10 310 items we deem most important, based on the pervasiveness of confusion and the potential costs of same.

Article 310 is arguably the most misunderstood and misapplied area of the National Electrical Code. The same claim can be made for Article 250 (grounding) and Article 430 (motors). Our intent here is to help clear up some of the mystery, but a true understanding requires dedicated study. Do keep in mind ampacity is the amperage capacity of a conductoróthe lower the ampacity, the larger the conductor must be to handle a given current (though this is in a step fashion, as conductors come in standard sizes).

  1. 310.4 explains the requirements for running conductors in parallel.
  2. Table 310.5 gives the minimum size for current-carrying conductors, with 10 exceptions noted. This table is based on voltage ranges, only. You can expect ampacity issues to require a larger conductor than the size shown.
  3. 310.8 lists which conductor insulation types you may use for locations other than dry locations. When using the conductor tables for ampacity-based conductor sizing, you need to limit your selections per this listing. You must also consider 310.9 and 310.10 when using those tables. Table 310.13 also contains the information of these three articles, in more detail and in a tabular format. Itís the largest table in the NEC.
  4. Table 310.13 is a treasure-trove of information. You must use this table to select the cable type(s) appropriate for your particular installation. Itís typical to make a "short list" of those types that meet the installation needs and then select based on price, workability of the insulation, and availability of the wire in sufficient quantities for your project. The circular mils information allows you to calculate wire fill, which is another consideration to add to the previous one. A thinner jacket allows you to put more wires in a smaller space, and this aspect may produce a significant cost variance in wireway, supports, enclosures, and labor.
  5. 310.15 addresses ampacity in great detail. You must cross-reference this information to Table 310.5, as 310.15 does not account for voltage. Nor does 310.15 account for voltage dropóthe wire size the NEC recommends might be too small for efficient operation. Also, you can get more than one ampacity from the tables or your calculations. In such cases, (A)(2) instructs you to use the lower of two or the lowest of more than two. That is, you must assume less ampacity than the more favorable calculation(s) show. You must adjust (derate) based on the number of current-carrying conductors in a given wireway, as well. 310.15(B)(2) explains this and the exceptions. Table 310.15(B)(2)(a) shows these in a very clear format. Another important point is you donít count a grounding or bonding conductor when using this table. Table 310.15(B)(6) shows conductor types and sizes for dwelling services and feeders.
  6. Table 310.16 applies to situations where you have three or less current-carrying conductors in a single wireway. This is typical for services and feeders, but not very typical for branch circuits. You must select from the column that shows the cable (identified by the insulating material letter designation) you intend to use, and choose between copper and aluminum.
  7. Table 310.17 applies to situations where you have single-insulated conductors (that doesnít mean a single conductoróit means the conductor isnít insulated twice, as would be the case if itís in an insulated sheath with other conductors) in free air. This is typical for branch circuits. You must select from the column that shows the cable (identified by the insulating material letter designation) you intend to use, and choose between copper and aluminum.
  8. Tables in Article 311 all have temperature correction factors.
  9. NEC Article 311 Tables are a great source of confusion, but need not be. What really helps is understanding their arrangement and purposes. These tables are arranged in logical order, assuming you are using typical wiring methods for each application.
  1. Tables 310.21 through 310.86 are special circumstance tables. They typically apply to services and generally allow for more ampacity in a given cable. Itís usually worthwhile to read through the descriptions to see if one of these applies to your situation.

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How the NEC is arranged

  1. The first four Chapters of the NEC apply to all installations.
  2. Article 90 precedes Chapter One, and establishes the authority of the NEC.
  3. Article 80 follows the body of the NEC; it exists as Annex H. It provides the requirements for administration.
  4. Chapters 5, 6, and 7 are the "special" chapters, covering special: occupancies, equipment, and conditions (in that order).
  5. Chapter 8 provides the requirements for communications systems.
  6. Chapter 9 provides tables.
  7. The appendices provide mostly reference information.
  8. Appendix D contains examples that every NEC user should study.

Try your NEC moxy:

  • Do you know the difference between bonding and grounding? Hint: Look in the NEC, Article 100.
  • Does the NEC refer to grounding incorrectly in any of its articles? Yes! So be careful to apply the Article 100 definitions. Don't ground where you should bond.
  • When doing motor load calculations, which Article covers hermetic motors? Answer: While Article 440 covers the application of hermetic motors, it does so only by amending Article 430 because hermetic motors are a special case of motors. For motor load calculations, refer to Article 430.
  • Does the NEC provide a voltage drop requirement? Yes! It does so in a special case, which is Article 648 Sensitive Electronic Equipment. But for general applications, it does not provide a requirement; it merely provides a recommendation in a couple of FPNs.
  • Take our Code Quizzes.

Remember other applicable codes, rules, standards, and references:

  • OSHA's electrical worker safety rules.
  • IEEE standards.
  • NETA standards.
  • NFPA standards.
  • International Codes (if applicable to the installation).
  • State Codes (if the state has them).
  • Local ordinances and permit requirements.
  • Local fire codes.
  • Manufacturer requirements or guidelines.
  • Customer security requirements.
  • Industry standards.
  • Your company's own internal standards, practices, and procedures.
  • Engineering drawing notes.


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