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
- 310.4 explains the requirements for running conductors
- 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.
- 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
- 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.
- 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.
- 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.
- 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
- Tables in Article 311 all have temperature correction factors.
- 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.
- Table 310.16 is typically for service entrances.
- Table 310.17 is typically for branch circuits.
- NEW Table 310.16 and Table 310.17 are your basic ampacity
- Table 310.18 is Table 310.16, but for higher temperatures.
- Table 310.19 is Table 310.17, but for higher temperatures.
- Table 320 is Table Table 310.17, but for cables supported
on a messenger.
- 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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Learn more about:
How the NEC is arranged
- The first four Chapters of the NEC apply to all installations.
- Article 90 precedes Chapter One, and establishes the authority of the NEC.
- Article 80 follows the body of the NEC; it exists as Annex H. It provides the requirements for administration.
- Chapters 5, 6, and 7 are the "special" chapters, covering special: occupancies, equipment, and conditions (in that order).
- Chapter 8 provides the requirements for communications systems.
- Chapter 9 provides tables.
- The appendices provide mostly reference information.
- 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.